CVE-2026-34520
Aiohttp ≤ 3.13.4
Raw vector
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:U/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:XSummary
CVE-2026-34520 is a low-severity HTTP Request/Response Splitting (CWE-113) vulnerability in Aiohttp Aiohttp. Its CVSS base score is 2.7 (Low).
Operationally, exploitation aligns with the MITRE ATT&CK technique Content Injection (T1659); ranked at the 38th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.
The strongest mitigations our analysis identified map to SI-10 (Information Input Validation) and SI-15 (Information Output Filtering) — see the control section below for these in your framework.
Deeper analysis AI-assisted summary
Synthesised by an AI model from the NVD description and linked references — a reading aid, not an authoritative source.
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.
OWASP Top 10 for Web (2025)
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-18046
Vulnerability Data
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.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise Techniques
CVEs Like This One
Affected Assets
Mitigating Controls
Control response
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V3.4.2V4.1.3V1.3.6V4.2.3
Mitigating Controls (NIST 800-53 r5) AI
Input validation stops unneutralized CR/LF characters from being accepted and later emitted in HTTP headers.
Output filtering can neutralize CRLF sequences before they reach HTTP response headers.
Mitigating Controls (NIST CSF 2.0) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.
Secure SDLC practices directly require input sanitization and header handling that prevent CRLF injection.
Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.
Security testing in development and acceptance can detect CRLF injection flaws before deployment.
Secure development lifecycle mandates input validation and output encoding that directly prevents CRLF injection into HTTP headers.
Application security requirements explicitly call for controls against injection flaws including HTTP header manipulation.
Secure architecture principles reduce the likelihood of header-splitting vulnerabilities through proper component isolation.
Secure coding standards require neutralization of CRLF sequences before inclusion in HTTP headers.
Logging of HTTP traffic can record header-splitting attempts, aiding detection and forensics.