# CVE-2026-128126: AIOHTTP: Too many URL-encoded form fields block the event loop

Status: published. Published: 2026-11-13. Updated: 2026-11-13.

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.

## Remediation

No remediation reported in collected sources

## Affected products and versions

- aio-libs / aiohttp: < 3.14.4 (affected). Source: CNA: GitHub_M

## Severity and prioritization

- CVSS 4.0: 8.7 (high); 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; source: CNA: GitHub_M

- CISA KEV: No listing in collected sources

- EPSS: Not available

- SSVC: Not available

- Weaknesses: CWE-407

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://github.com/aio-libs/aiohttp/security/advisories/GHSA-xgw8-8ccx-rmqh (CNA: GitHub_M; x_refsource_CONFIRM)

- https://github.com/aio-libs/aiohttp/pull/13738 (CNA: GitHub_M; x_refsource_MISC)

- https://github.com/aio-libs/aiohttp/pull/13741 (CNA: GitHub_M; x_refsource_MISC)

- https://github.com/aio-libs/aiohttp/commit/615fec3d4f088f30d2ee72cad760152e92aa1c53 (CNA: GitHub_M; x_refsource_MISC)

- https://github.com/aio-libs/aiohttp/commit/8b0100572936df28977b9de8a6cc7e464c0b521e (CNA: GitHub_M; x_refsource_MISC)

- https://github.com/aio-libs/aiohttp/releases/tag/v3.14.4 (CNA: GitHub_M; x_refsource_MISC)

## Provenance

- CNA: GitHub_M: https://www.cve.org/CVERecord?id=CVE-2026-128126; retrieved 2026-11-13; source updated 2026-11-13

Revision: f269382b923b28acd3f83cc93fe26a2b2fa6a4abd4f1039ce6f7479d574ddf69

Canonical: https://y2.dev/cve/CVE-2026-128126

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