AIOHTTP: Too many URL-encoded form fields block the event loop

published 2026-11-08 5d ago modified 2026-11-13 4h ago also EUVD-2026-125758 , GHSA-xgw8-8ccx-rmqh

Unreviewed: the maintainers' own advisory

Published by the maintainers on their GitHub repository aio-libs/aiohttp. GitHub has not reviewed it into the GitHub Advisory Database, so the affected packages and versions are the maintainers' word. Read the full advisory on GitHub.

cvss: 8.7; version 4.0, CNA
cisa kev: Not listed; not in CISA's catalog
epss: —; no EPSS score yet
public exploits: 0; none known

AIOHTTP is an asynchronous HTTP client/server framework for asyncio and Python. Prior to 3.14.4, Request.post() does not limit the number of fields in a URL-encoded form body, and inserting many repeated keys into MultiDict takes quadratic time on the event loop. A remote attacker can send a form POST within client_max_size to a handler that calls Request.post() and block the event loop. This issue is fixed in version 3.14.4.

Patch / fix

fixed in 1 links 5

pypi aiohttp 3.14.4

github.com/aio-libs/aiohttp@615fec3 commit
github.com/aio-libs/aiohttp@8b01005 commit
github.com/aio-libs/aiohttp#13738 pull request
github.com/aio-libs/aiohttp#13741 pull request
github.com/aio-libs/aiohttp/releases/tag/v3.14.4 release

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CVSS scores

assigned by	version	score	vector
CNA	4.0	8.7	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
CNA	4.0	8.7	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:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

Affected packages

ecosystem	package	affected	fixed in
pypi	aiohttp	<=3.14.3	3.14.4

Affected products

aio-libs aiohttp

Classification

Weakness

CWE-407 Inefficient Algorithmic Complexity

References

Sources

CVE List CVE-2026-128126
NVD CVE-2026-128126
Maintainer advisory GHSA-xgw8-8ccx-rmqh

Fencer Intel keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

The maintainers of aio-libs/aiohttp published their own advisory on GitHub. Reviewed sources take precedence over it.