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 patch…

EPSS: 0.005 (37.2th percentile) — read the EPSS interpretation.

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.

Affected products

Aiohttp
Aio-libs Aiohttp — versions < 3.13.4

Weakness classification (CWE)

CWE-113 — HTTP Response Splitting

References

security-advisories@github.com (x_refsource_CONFIRM, Patch, Vendor Advisory)
security-advisories@github.com (Patch, x_refsource_MISC)
security-advisories@github.com (x_refsource_MISC, 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.