CVE-2026-34525: AIOHTTP: Duplicate Host header accepted

Severity: medium (CVSS 5.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-04-01 · updated 2026-10-26
Weakness: CWE-20

Description

AIOHTTP is an asynchronous HTTP client/server framework for asyncio and Python. Prior to version 3.13.4, multiple Host headers were allowed in aiohttp. This issue has been patched in version 3.13.4.

Affected products

aio-libs aiohttp: before 3.13.4 — fixed in 3.13.4
Pypi Aiohttp: before 3.13.4; 0.1, 0.10.0, 0.10.1, 0.10.2, 0.11.0, 0.12.0 (+298) — fixed in 3.13.4

How to fix

Upgrade to a fixed version: aio-libs aiohttp 3.13.4; Pypi Aiohttp 3.13.4. Check the vendor advisory for mitigations.

References

https://github.com/aio-libs/aiohttp/commit/53e2e6fc58b89c6185be7820bd2c9f40216b3000
https://github.com/aio-libs/aiohttp/commit/e00ca3cca92c465c7913c4beb763a72da9ed8349
https://github.com/aio-libs/aiohttp/releases/tag/v3.13.4
https://github.com/aio-libs/aiohttp/security/advisories/GHSA-c427-h43c-vf67

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