CVE-2026-34520: AIOHTTP is an asynchronous HTTP client/server framework for asyncio and Python

Q: How do I fix CVE-2026-34520?

aiohttp: upgrade to 3.13.4. Exact commands for each OS are on this page.

Q: Is CVE-2026-34520 being exploited?

CVE-2026-34520 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2026-34520 affect?

CVE-2026-34520 affects aiohttp. Severity is critical (CVSS 9.1). 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 patche

Q: How urgent is CVE-2026-34520?

Priority 47/100. Critical severity; Remotely exploitable, no login, no user action

This month

CRITICAL 9.1REMOTEUPGRADE

CVE-2026-34520 is a critical-severity flaw (CVSS 9.1) affecting aiohttp. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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.

Why this priority

Critical severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	9.1 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.5% (top 60%)
CISA exploited list	No
Weakness	CWE-113
Published	2026-04-01
Last modified	2026-10-26

Publisher (CNA): GitHub, Inc. · Vendor: aiohttp

Last updated 2026-10-26 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-04-01 21:17 UTCPublished CVE record published
2026-10-26 21:10 UTCUpdated Record last modified

aiohttp aiohttp UPGRADE

Affected: < 3.13.4

Fixed in: 3.13.4

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy aiohttp || rpm -q aiohttp || aiohttp --version

2. UPGRADE to 3.13.4 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade aiohttp

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade aiohttp

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-34520?

aiohttp: upgrade to 3.13.4. Exact commands for each OS are on this page.

Is CVE-2026-34520 being exploited?

CVE-2026-34520 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-34520 affect?

CVE-2026-34520 affects aiohttp. Severity is critical (CVSS 9.1). 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 patche

How urgent is CVE-2026-34520?

Priority 47/100. Critical severity; Remotely exploitable, no login, no user action

Software pages

Python vulnerabilities

Related CVEs

CVE-2026-34514 MEDIUM 5.3 UPGRADE AIOHTTP is an asynchronous HTTP client/server framework for asyncio and Python
CVE-2026-34517 MEDIUM 5.3 UPGRADE AIOHTTP is an asynchronous HTTP client/server framework for asyncio and Python
CVE-2026-34518 MEDIUM 5.3 UPGRADE AIOHTTP is an asynchronous HTTP client/server framework for asyncio and Python
CVE-2026-34519 MEDIUM 5.3 UPGRADE AIOHTTP is an asynchronous HTTP client/server framework for asyncio and Python
CVE-2026-34525 MEDIUM 5.3 UPGRADE AIOHTTP is an asynchronous HTTP client/server framework for asyncio and Python

References

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