Critical severity · published Apr 1, 2026
CVE-2026-34520
AIOHTTP: C parser (llhttp) accepts null bytes and control characters in response header values - header injection / security bypass
9.1
CVSS
0.5%
EPSS
No
On KEV
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.
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Confidentiality
None
Integrity
High
Availability
High
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.5% · higher than 40% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
AI
aiohttp · aio-libs
< 3.13.4 · source: vendor, NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-34520
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-34520
Patch
https://github.com/aio-libs/aiohttp/commit/9370b9714a7a56003cacd31a9b4ae16eab109ba4
Release Notes
https://github.com/aio-libs/aiohttp/releases/tag/v3.13.4
Patch
https://github.com/aio-libs/aiohttp/security/advisories/GHSA-63hf-3vf5-4wqf
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