How severe is CVE-2026-34520?

No CVSS score has been assigned to CVE-2026-34520 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-34520?

CVE-2026-34520 affects the following packages: aiohttp (PyPI). Ecosystems affected: PyPI.

How do I fix CVE-2026-34520?

Update aiohttp to 3.13.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-34520 is resolved across your whole dependency graph.

How do I detect CVE-2026-34520 in my PyPI dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for aiohttp, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-34520 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Is CVE-2026-34520 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-34520 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-34520?

CVE-2026-34520 has an EPSS (Exploit Prediction Scoring System) score of 0.5%, placing it in the 42th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-34520?

CVE-2026-34520 is classified as CWE-113 (CWE-113). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-34520 published, and has it been updated?

CVE-2026-34520 was published on April 1, 2026 and was last updated on October 28, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐍 PyPI

Not in CISA KEV

CVE-2026-34520 — aiohttp

Fix: aio-libs/aiohttp@9370b97

CVE-2026-34520 is a CWE-113 vulnerability in aiohttp. A fix is available for aiohttp — see the affected versions and patch details below.

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

Also known asGHSA-63hf-3vf5-4wqfPYSEC-2026-2102

Published

Apr 1, 2026

Updated

Oct 28, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Nov 6, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-34520.

EPSS Exploitation Probability

via FIRST.org ↗

0.5%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs42th percentile — riskier than 42% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

Real-World Exposure

1 pkg affected

🐍aiohttp

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

Description

Summary

The C parser (the default for most installs) accepted null bytes and control characters in request headers.

Impact

An attacker could send header values that are interpreted differently than expected due to the presence of control characters. For example, request.url.origin() may return a different value than the raw Host header, or what a reverse proxy interpreted it as., potentially resulting in some kind of security bypass.

Patch: https://github.com/aio-libs/aiohttp/commit/9370b9714a7a56003cacd31a9b4ae16eab109ba4

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`aiohttp`	all versions	3.13.4`pip install --upgrade 'aiohttp==3.13.4'`

Affected Products

1 product · 1 configurations

Application

aiohttpaiohttp

< 3.13.4

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for aiohttp, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update aiohttp to 3.13.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-34520 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatLow

Workaround published by Red Hat

Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base or stability.

Source: Red Hat security advisory for CVE-2026-34520 (CC BY 4.0)

Frequently Asked Questions

### Summary The C parser (the default for most installs) accepted null bytes and control characters in request headers. ### Impact An attacker could send header values that are interpreted differently than expected due to the presence of control characters. For example, `request.url.origin()` may return a different value than the raw Host header, or what a reverse proxy interpreted it as., potentially resulting in some kind of security bypass. ----- Patch: https://github.com/aio-libs/aiohttp/commit/9370b9714a7a56003cacd31a9b4ae16eab109ba4

O3 Security · Impact-Aware SCA

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