CVE-2026-34520 PUBLISHED

AIOHTTP: C parser (llhttp) accepts null bytes and control characters in response header values - header injection / security bypass

Assigner: GitHub_M
Reserved: 30.03.2026 Published: 01.04.2026 Updated: 01.04.2026

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.

Metrics

CVSS 4.0

CVSS Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:U
CVSS Score: 2.7

Exploitability Metrics		Vulnerable System Impact Metrics		Subsequent System Impact Metrics
Attack Vector	Network	Confidentiality	None	Confidentiality	None
Attack Complexity	Low	Integrity	Low	Integrity	None
Attack Requirements	None	Availability	None	Availability	None
Privileges Required	None
User Interaction	None

CVSS 4.0

Product Status

Vendor	aio-libs
Product	aiohttp
Versions	Version < 3.13.4 is affected

References

Problem Types

CWE-113: Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting') CWE