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Vulnerability · Published Apr 1, 2026 · Assigned by GitHub_M

CVE-2026-34520AIOHTTP: C parser (llhttp) accepts null bytes and control characters in response header values - header injection / security bypass

Critical 9.1

In plain English

A critical-severity security flaw in aiohttp from aio-libs. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.5% (higher than 42% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)9.1Critical
Exploit odds (30 days)0.5%top 58.5% of all CVEs
Exploited in the wildNo reportsCISA SSVC: none
Attack toolingNoneno public tooling known

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.

Affected packages & fixed versions

EcosystemPackageFixed inAdvisory
Alpine:v3.23py3-aiohttp3.13.5-r0ALPINE-CVE-2026-34520
Alpine:v3.24py3-aiohttp3.13.5-r0ALPINE-CVE-2026-34520
Debian:12python-aiohttp3.8.4-1+deb12u2DEBIAN-CVE-2026-34520
Debian:13python-aiohttpno fix listedDEBIAN-CVE-2026-34520
Debian:14python-aiohttp3.13.5-1DEBIAN-CVE-2026-34520
PyPIaiohttp3.13.4GHSA-63hf-3vf5-4wqf
PyPIaiohttp3.13.4PYSEC-2026-2102
Ubuntu:25.10python-aiohttpno fix listedUBUNTU-CVE-2026-34520
Ubuntu:26.10python-aiohttpno fix listedUBUNTU-CVE-2026-34520

Affected products (as reported)

VendorProductVersions
aio-libs aiohttp < 3.13.4

Weakness: CWE-113 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H

References