What is CVE-2026-34520?

CVE-2026-34520 is a critical-severity vulnerability in Aiohttp, classified under HTTP Response Splitting. CVSS score: 9.1/10. Published 2026-04-01.

How severe is CVE-2026-34520?

Critical severity. CVSS v3 base score is 9.1 out of 10.

Vulnerability in Aiohttp

CVE-2026-34520

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.

Published 2026-04-01 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 9.1 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H.

EPSS exploit prediction

EPSS: 0.007 (50.3th percentile), scored 2026-09-30.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 50th percentile — 50.3% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.005 · newest: 0.007 · change: +0.002

Affected products

Aiohttp
Aio-Libs Aiohttp — versions < 3.13.4

Weakness classification (CWE)

CWE-113 — HTTP Response Splitting

References

github.com/aio-libs/aiohttp/security/advisories/GHSA-63hf-3vf5-4wqf (Patch, Vendor Advisory)
github.com/aio-libs/aiohttp/commit/9370b9714a7a56003cacd31a9b4ae16eab109ba4 (Patch)
github.com/aio-libs/aiohttp/releases/tag/v3.13.4 (Release Notes)

Frequently asked questions

What is CVE-2026-34520?: CVE-2026-34520 is a critical-severity vulnerability in Aiohttp, classified under HTTP Response Splitting. CVSS score: 9.1/10. Published 2026-04-01.
How severe is CVE-2026-34520?: Critical severity. CVSS v3 base score is 9.1 out of 10.