CVE-2026-34520

Critical

Published: 01 April 2026

Published

01 April 2026

Modified

16 April 2026

KEV Added

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Patch

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CVSS Score 9.1 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H

EPSS Score 0.0006 19.3th percentile

Risk Priority 18 60% EPSS · 20% KEV · 20% CVSS

Summary

CVE-2026-34520 is a critical-severity HTTP Request/Response Splitting (CWE-113) vulnerability in Aiohttp Aiohttp. Its CVSS base score is 9.1 (Critical).

Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked at the 19.3th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.

The strongest mitigations our analysis identified are NIST 800-53 SI-2 (Flaw Remediation) and SC-7 (Boundary Protection).

Threat & Defense at a Glance

What attackers do: exploitation maps to Exploit Public-Facing Application (T1190). What defenders deploy: see the NIST 800-53 controls recommended below.

Threat & Defense Details

Mitigating Controls (NIST 800-53 r5)AI

SI-2 Flaw Remediation good match

prevent

Directly requires timely identification, reporting, and correction of flaws like the AIOHTTP C parser's acceptance of null bytes and control characters in response headers.

SI-10 Information Input Validation partial match

prevent

Mandates validation of information inputs such as HTTP response headers to reject invalid characters like null bytes and control characters before processing.

SC-7 Boundary Protection partial match

prevent

Enables boundary protection mechanisms like web application firewalls to inspect and block network traffic containing malformed HTTP response headers.

MITRE ATT&CK Enterprise TechniquesAI

T1190 Exploit Public-Facing Application Initial Access

Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.

attack.mitre.org →

Why these techniques?

The vulnerability is a remotely exploitable flaw in the AIOHTTP web framework allowing manipulation via HTTP response headers, directly enabling exploitation of public-facing applications.

Confidence: MEDIUM · MITRE ATT&CK Enterprise v18.1

NVD 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.

Deeper analysisAI

CVE-2026-34520 is a vulnerability in AIOHTTP, an asynchronous HTTP client/server framework for asyncio and Python. Prior to version 3.13.4, the default C parser accepted null bytes and control characters in response headers, as associated with CWE-113. The vulnerability was published on 2026-04-01 and carries a CVSS v3.1 base score of 9.1 (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H).

Remote attackers can exploit this vulnerability over the network with low attack complexity, requiring no privileges, user interaction, or scope changes. Exploitation disrupts integrity (high) and availability (high) without affecting confidentiality, stemming from improper handling of disallowed characters in HTTP response headers.

The vulnerability has been patched in AIOHTTP version 3.13.4. Mitigation involves upgrading to this version or later. Relevant resources include the security advisory at https://github.com/aio-libs/aiohttp/security/advisories/GHSA-63hf-3vf5-4wqf, release notes at https://github.com/aio-libs/aiohttp/releases/tag/v3.13.4, and the patching commit at https://github.com/aio-libs/aiohttp/commit/9370b9714a7a56003cacd31a9b4ae16eab109ba4.