AIOHTTP's C parser (llhttp) accepts null bytes and control characters in response header values - header injection/security bypass
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 software
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| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Python (PyPI) | aiohttp | < 3.13.4 | 3.13.4 |
References
- https://github.com/aio-libs/aiohttp/releases/tag/v3.13.4
- https://github.com/aio-libs/aiohttp/commit/9370b9714a7a56003cacd31a9b4ae16eab109ba4
- https://github.com/aio-libs/aiohttp/security/advisories/GHSA-63hf-3vf5-4wqf
- https://nvd.nist.gov/vuln/detail/CVE-2026-34520
- https://github.com/aio-libs/aiohttp
- https://github.com/pypa/advisory-database/tree/main/vulns/aiohttp/PYSEC-2026-2102.yaml
- CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H- Weakness (CWE)
CWE-113- Platform
- Python (PyPI)
- Package
aiohttp- Disclosed
- 01.04.2026
- Updated
- 11.10.2026 15:17
Sources and licenses
OSV / GitHub Advisory Database - PYSEC-2026-2102 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-63hf-3vf5-4wqf - CC BY 4.0
Vulnerability history: aiohttp
All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 55
- First → latest
- 02.2021 10.2026
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- High 7
- Medium 29
- Low 19
Disclosures per year
- 2021: 1
- 2022: 0
- 2023: 6
- 2024: 7
- 2025: 1
- 2026: 40
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AIOHTTP's C parser (llhttp) accepts null bytes and control characters in response header values - header injection/security bypass this entry
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