CVE-2026-34520
published 2026-04-01CVE-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…
PriorityP353critical9.1CVSS 3.1
AVNACLPRNUINSUCNIHAH
EPSS
0.46%
37.2th percentile
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.
Affected
4 ranges
| Vendor | Product | Version range | Fixed in |
|---|---|---|---|
| aio-libs | aiohttp | < 3.13.4 | 3.13.4 |
| aiohttp | aiohttp | < 3.13.4 | 3.13.4 |
| aiohttp | aiohttp | >= 0 < 3.13.4 | 3.13.4 |
| debian | python-aiohttp | — | — |
CVSS provenance
nvdv3.19.1CRITICALCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H
nvdv4.02.7LOWCVSS: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:X
osv2.7LOW
vendor_debian2.7LOW
vendor_redhat2.7LOW
Stop checking back — get the weekly exploitation signal.
Every Monday: what got weaponized or added to CISA KEV in the last seven days — each CVE cross-linked to its PoC, Nuclei template, and detection rule. Free, one email a week, unsubscribe in one click.
VulDB
aio-libs aiohttp up to 3.13.3 Control Character response splitting (GHSA-63hf-3vf5-4wqf / Nessus ID 318080)
vuldb·2026-06-01·CVSS 2.7
CVE-2026-34520 [LOW] aio-libs aiohttp up to 3.13.3 Control Character response splitting (GHSA-63hf-3vf5-4wqf / Nessus ID 318080)
A vulnerability marked as problematic has been reported in aio-libs aiohttp up to 3.13.3. Affected is an unknown function of the component Control Character Handler. Performing a manipulation results in http response splitting.
This vulnerability is identified as CVE-2026-34520. The attack can be initiated remotely. There is not any exploit available.
It is suggested to upgrade the affected component.
OSV
CVE-2026-34520: AIOHTTP is an asynchronous HTTP client/server framework for asyncio and Python
osv·2026-04-01·CVSS 2.7
CVE-2026-34520 [LOW] CVE-2026-34520: AIOHTTP is an asynchronous HTTP client/server framework for asyncio and Python
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.
OSV
AIOHTTP's C parser (llhttp) accepts null bytes and control characters in response header values - header injection/security bypass
osv·2026-04-01
CVE-2026-34520 [LOW] AIOHTTP's C parser (llhttp) accepts null bytes and control characters in response header values - header injection/security bypass
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 is response 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
GHSA
AIOHTTP's C parser (llhttp) accepts null bytes and control characters in response header values - header injection/security bypass
ghsa·2026-04-01
CVE-2026-34520 [LOW] CWE-113 AIOHTTP's C parser (llhttp) accepts null bytes and control characters in response header values - header injection/security bypass
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 is response 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
Red Hat
aiohttp: AIOHTTP: Header injection vulnerability due to improper character handling
vendor_redhat·2026-04-01·CVSS 2.7
CVE-2026-34520 [LOW] CWE-1286 aiohttp: AIOHTTP: Header injection vulnerability due to improper character handling
aiohttp: AIOHTTP: Header injection vulnerability due to improper character handling
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.
A flaw was found in AIOHTTP, an asynchronous HTTP client/server framework for Python. The default C parser incorrectly processed null bytes and control characters present in HTTP response headers. This vulnerability could allow a remote attacker to inject malicious data into these headers, potentially leading to unexpected application behavior or security bypasses.
Mitigation: Mitigation for this issue is either not available or the currently avail
Debian
CVE-2026-34520: python-aiohttp - AIOHTTP is an asynchronous HTTP client/server framework for asyncio and Python. ...
vendor_debian·2026·CVSS 2.7
CVE-2026-34520 [LOW] CVE-2026-34520: python-aiohttp - AIOHTTP is an asynchronous HTTP client/server framework for asyncio and Python. ...
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.
Scope: local
bookworm: open
bullseye: open
forky: open
sid: open
trixie: open
No detection rules found.
No public exploits indexed.
Wiz
CVE-2026-34520 Impact, Exploitability, and Mitigation Steps | Wiz
blogs_wiz·CVSS 2.7
CVE-2026-34520 [LOW] CVE-2026-34520 Impact, Exploitability, and Mitigation Steps | Wiz
## CVE-2026-34520 :
Wolfi vulnerability analysis and mitigation
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.
Source : NVD
## 2.7
Score
Published April 1, 2026
Severity LOW
CNA Score 2.7
Affected Technologies
Wolfi
Chainguard
Has Public Exploit No
Has CISA KEV Exploit No
CISA KEV Release Date N/A
CISA KEV Due Date N/A
Exploitation Probability Percentile (EPSS) 17.3
Exploitation Probability (EPSS) 0.1
Affected packages and libraries
checkov
datahub-ingestion-fips
Sources
NVD
Chainguard Has Fix Added at: Apr 02, 2026
Debian 11, 12, 13, 14 Severity CRITICA
Bugzilla
CVE-2026-34520 aiohttp: AIOHTTP: Header injection vulnerability due to improper character handling
bugzilla·2026-04-01·CVSS 2.7
CVE-2026-34520 [LOW] CVE-2026-34520 aiohttp: AIOHTTP: Header injection vulnerability due to improper character handling
CVE-2026-34520 aiohttp: AIOHTTP: Header injection vulnerability due to improper character handling
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.
2026-04-01
Published