AIOHTTP has late size enforcement for non-file multipart fields causes memory DoS

MEDIUM 5.3CVE-2026-34517 · GHSA-3wq7-rqq7-wx6j · PYSEC-2026-2099 · Published 01/04/2026 · updated 26/10/2026

Package	Affected versions	Patched versions
aiohttp PyPI	`< 3.13.4`	`3.13.4`
aiohttp aiohttp	`< 3.13.4`	`3.13.4`

Description

Summary

For some multipart form fields, aiohttp read the entire field into memory before checking client_max_size.

Impact

If an application uses Request.post() an attacker can send a specially crafted multipart request to force significant temporary memory allocation even when the request is ultimately rejected.

Patch: https://github.com/aio-libs/aiohttp/commit/cbb774f38330563422ca0c413a71021d7b944145

NVD description

AIOHTTP is an asynchronous HTTP client/server framework for asyncio and Python. Prior to version 3.13.4, for some multipart form fields, aiohttp read the entire field into memory before checking client_max_size. This issue has been patched in version 3.13.4.

References

aio-libs/aiohttp security advisory · WEB
nvd.nist.gov/vuln/detail/CVE-2026-34517 · ADVISORY
aio-libs/aiohttp commit cbb774f · WEB
aio-libs/aiohttp · PACKAGE
aio-libs/aiohttp release v3.13.4 · WEB

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