How severe is CVE-2026-34520?

CVE-2026-34520 has a CVSS 3.x base score of 9.1, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity high, and availability high.

Is CVE-2026-34520 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (37th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-34520?

CVE-2026-34520 affects Aiohttp. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-34520?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.

Does CVE-2026-34520 have an EU (EUVD) identifier?

Yes. CVE-2026-34520 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-18046.

When was CVE-2026-34520 published?

CVE-2026-34520 was published on 2026-04-01 and last updated on 2026-06-17.

CVE-2026-34520

CVE-2026-34520 is a critical-severity vulnerability in Aiohttp with a CVSS 3.x base score of 9.1. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-113.

Key facts

Severity: Critical (CVSS 3.x base score 9.1)
CVSS v4: 2.7
EPSS exploit prediction: 0% (37th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-18046
Weakness: CWE-113
Affected product: Aiohttp
Published: 2026-04-01
Last modified: 2026-06-17

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.

Frequently asked questions

What is 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.
How severe is CVE-2026-34520?: CVE-2026-34520 has a CVSS 3.x base score of 9.1, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity high, and availability high.
Is CVE-2026-34520 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (37th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-34520?: CVE-2026-34520 affects Aiohttp. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-34520?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.
Does CVE-2026-34520 have an EU (EUVD) identifier?: Yes. CVE-2026-34520 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-18046.
When was CVE-2026-34520 published?: CVE-2026-34520 was published on 2026-04-01 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:aiohttp:aiohttp:*:*:*:*:*:*:*:*

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