CVE-2026-34520 AIOHTTP's C parser (llhttp) accepts null bytes and control characters in response header values - header injection/security bypass
The aiohttp C parser accepts null bytes and control characters in response headers, allowing header injection and security bypasses.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-34520?
CVE-2026-34520 is a low-severity 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, the C parser (the default for most installs) accepted null bytes and control characters in response headers. This issue has been patched in version 3.13.4.
Weakness: CWE-113
Which versions are affected by CVE-2026-34520?
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-2026-34520?
Upgrade aiohttp to 3.13.4 or later.
Is CVE-2026-34520 being exploited?
There is no known exploitation of CVE-2026-34520 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-81779, CLEANSTART-2026-DB06757, CLEANSTART-2026-ED48015, CLSA-2026-1776818192, CLSA-2026-1777509590, DEBIAN-CVE-2026-34520 and 12 more.