CVE-2025-53020 - Apache httpd HTTP/2 Memory Exhaustion PoC

A proof of concept demonstrating CVE-2025-53020, a memory exhaustion vulnerability in Apache httpd's HTTP/2 implementation I found in mid-2025.

📖 Deep dive: galbarnahum.com/posts/apache-httpd-cve-2025-53020

Vulnerability Overview

Apache httpd is vulnerable to a denial-of-service attack via HTTP/2 header processing. The vulnerability exists due to unnecessary memory duplication when handling repeated header names in HTTP/2 requests.

Attack Mechanism

1. HPACK Exploitation: The attack leverages HTTP/2's HPACK header compression by first sending an initial request to populate the server's HPACK dynamic table with a long header name
2. Memory Amplification: Subsequent requests reference this header name multiple times, but the server allocates new memory for each occurrence instead of reusing the existing entry
3. Resource Exhaustion: By sending batches of requests with thousands of header repetitions, an attacker can rapidly exhaust server memory

Impact

• Denial of Service (DoS)
• Server memory exhaustion
• Potential service unavailability

Requirements

• Python 3.7+
• h2 - HTTP/2 protocol library
• tqdm - Progress bar for attack visualization

Installation

pip install h2 tqdm

Usage

Basic Usage

python poc.py --url https://target.example.com/

Command Line Arguments

Argument	Default	Description
--url	required	Target URL (e.g., https://example.com/path)
--header-name-length	4064	Length of the header name (max: 4064)
--header-reps	2063	Number of header repetitions per request (recommended max: 2063)
--requests-per-batch	1	Number of requests per batch
--batches	100	Number of batches to send
--delay	0	Delay between batches in seconds
--num-headers-to-repeat	1	Number of different header names to create (uses chars a, b, c, etc.)

Examples

Standard attack against HTTPS target:

python poc.py --url https://target.com/api/endpoint

Attack with custom header settings:

python poc.py --url http://localhost:8080/test --header-reps 1000 --batches 200

Attack with multiple distinct header names:

python poc.py --url https://target.com --num-headers-to-repeat 3 --header-name-length 3000

Slower attack with delay between batches:

python poc.py --url https://target.com --delay 0.5 --batches 500

Technical Details

HPACK Encoder Patch

The PoC includes a patch for the h2 library's HPACK encoder to correctly handle empty header values. This fixes a bug where the encoder incorrectly indexes headers with empty string values (treating "" as falsy instead of checking for None).

Constraints

• Header name length: Maximum 4064 bytes (limited by HTTP/2 frame constraints)
• Header repetitions: Recommended maximum 2063 per request
• Multiple header names: When using --num-headers-to-repeat > 1, header name length should be < 4000

Affected Versions

Apache httpd versions vulnerable to CVE-2025-53020. Check the official Apache security advisories for specific version information and patches.

Disclaimer

⚠️ This tool is provided for educational and authorized security testing purposes only.

• Only use this tool against systems you own or have explicit written permission to test
• Unauthorized access to computer systems is illegal
• The authors are not responsible for any misuse or damage caused by this tool
• Always follow responsible disclosure practices

License

This project is for security research purposes. Use responsibly and ethically.