CVE-2025-69227
6.6
Infinite Loops & Vanishing Asserts: The AIOHTTP DoS
Alon Barad
Software EngineerJan 6, 2026·7 min read·9 visits
PoC Available
Executive Summary (TL;DR)
If you run aiohttp with Python's `-O` flag, you aren't just optimizing code—you're deleting security checks. CVE-2025-69227 allows an attacker to send a specific multipart POST request that traps the server in an infinite loop, permanently hanging the event loop and killing availability. The fix? Stop using `assert` for control flow.
A critical Denial of Service vulnerability in aiohttp caused by the improper use of assert statements for input validation. When run in optimized mode, these checks vanish, allowing attackers to trigger infinite loops via malformed multipart requests.
Official Patches
Fix Analysis (1)
Technical Appendix
CVSS Score
6.6/ 10
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:UAffected Systems
Python web applications using aiohttpMicroservices handling multipart/form-data uploadsAI/ML model serving endpoints (often use aiohttp)Proxy servers built on aiohttp
Affected Versions Detail
| Product | Affected Versions | Fixed Version |
|---|---|---|
aiohttp aio-libs | <= 3.13.2 | 3.13.3 |
| Attribute | Detail |
|---|---|
| CWE ID | CWE-835 |
| Attack Vector | Network |
| CVSS Score | 6.6 (Medium) |
| Impact | Denial of Service (High Availability Impact) |
| Vulnerability | Infinite Loop via Stripped Asserts |
| Prerequisite | Python Optimization (-O) Enabled |
MITRE ATT&CK Mapping
CWE-835
Infinite Loop
Loop with Unreachable Exit Condition ('Infinite Loop')
Known Exploits & Detection
Vulnerability Timeline
Patch bc1319ec committed
2026-01-03
GHSA-jj3x-wxrx-4x23 published
2026-01-05
CVE-2025-69227 published
2026-01-06