CVE-2026-34517 AIOHTTP: Late size enforcement for non-file multipart fields causes memory DoS
Attackers can cause memory exhaustion and denial of service by sending large multipart form fields that bypass size limits before being loaded into memory.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-34517?
CVE-2026-34517 is a low-severity denial of service vulnerability in aio-libs aiohttp, published on 1 Apr 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 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.
Weakness: CWE-770
Which versions are affected by CVE-2026-34517?
3 open source packages ship an affected version, including aiohttp. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
aiohttp PyPI | All versions | 3.13.4 |
aiohttp PyPI | All versions | 3.13.4 |
aiohttp PyPI | All versions | 3.13.4 |
Products include Aiohttp Aiohttp.
How do I fix CVE-2026-34517?
Upgrade aiohttp to 3.13.4 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-2026-34517 being exploited?
There is no known exploitation of CVE-2026-34517 in the wild and no public exploit code.
Other vulnerabilities in aio-libs aiohttp
- CVE-2024-52303 aiohttp memory leak when middleware is enabled when requesting a resource with a non-allowed method
- CVE-2025-69223 AIOHTTP's HTTP Parser auto_decompress feature is vulnerable to zip bomb
- CVE-2024-30251 Denial of service when trying to parse malformed POST requests in aiohttp
- CVE-2023-49081 aiohttp's ClientSession is vulnerable to CRLF injection via version
- CVE-2026-22815 AIOHTTP: Uncapped memory usage possible through aiohttp allowing unlimited trailer headers
Sources
Also known as AZL-81770, CLEANSTART-2026-ER42066, CLEANSTART-2026-FL05460, CLSA-2026-1780274965, DEBIAN-CVE-2026-34517, ECHO-185f-c78c-4e82 and 11 more.