CVE-2026-34525: AIOHTTP: Duplicate Host header accepted
Summary
Multiple Host headers were allowed in aiohttp.
Impact
Mostly this doesn't affect aiohttp security itself, but if a reverse proxy is applying security rules depending on the target Host, it is theoretically possible that the proxy and aiohttp could process different host names, possibly resulting in bypassing a security check on the proxy and getting a request processed by aiohttp in a privileged sub app when using Application.adddomain().
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Patch: https://github.com/aio-libs/aiohttp/commit/e00ca3cca92c465c7913c4beb763a72da9ed8349 Patch: https://github.com/aio-libs/aiohttp/commit/53e2e6fc58b89c6185be7820bd2c9f40216b3000
Other sources
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.
— MITRE
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
pip/aiohttpto a version that resolves this vulnerability.Fixed in 3.13.4 - Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Patch 53e2e6fc58b89c6185be7820bd2c9f40216b3000 - Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Patch e00ca3cca92c465c7913c4beb763a72da9ed8349