CVE-2026-59881 AIOHTTP: WebSocket client accepts compressed frames without negotiated permessage-deflate
A malicious server can cause excessive CPU and memory consumption in the aiohttp WebSocket client by sending compressed frames without proper negotiation.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-59881?
CVE-2026-59881 is a medium-severity denial of service vulnerability in aio-libs aiohttp, published on 30 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
AIOHTTP is an asynchronous HTTP client/server framework for asyncio and Python. Prior to 3.14.2, the WebSocket client accepts and decompresses frames with the RSV1 bit set even when the permessage-deflate extension was not negotiated, allowing a malicious server to cause unexpected CPU and memory consumption. This issue is fixed in version 3.14.2.
Weakness: CWE-20
Which versions are affected by CVE-2026-59881?
3 open source packages ship an affected version, including aiohttp.
| Package | Affected | Fixed in |
|---|---|---|
aiohttp PyPI | All versions | 3.14.2 |
aiohttp PyPI | All versions | 3.14.2 |
aiohttp PyPI | All versions | 3.14.2 |
How do I fix CVE-2026-59881?
Upgrade aiohttp to 3.14.2 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-59881 being exploited?
There is no known exploitation of CVE-2026-59881 in the wild and no public exploit code.
Sources
Also known as AZL-94397, CLEANSTART-2026-UK19001, CLSA-2026-1788442611, DEBIAN-CVE-2026-59881, ECHO-331a-abdb-db3b, GHSA-mq44-7p77-q5h7 and 33 more.