CVE-2026-34519Network attack vector

Header injection via unchecked carriage returns

Published Apr 1, 2026 · Updated Apr 1, 2026

HTTP response splitting in aio-libs aiohttp before 3.13.4 allows remote attackers to inject response headers through a crafted reason phrase. StreamResponse._set_status rejects line feeds but not carriage returns in an application-supplied reason, and the HTTP writer serializes that reason into the status line without neutralizing carriage returns. Exploitation requires an application to place remote input in the reason parameter; injected bytes can alter the response, including adding a Set-Cookie header, but do not directly expose server data or execute code.

CVSS severity5.3
Medium
EPSS probability0.29%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

HTTP response splitting in aio-libs aiohttp before 3.13.4 allows remote attackers to inject response headers through a crafted reason phrase. StreamResponse._set_status rejects line feeds but not carriage returns in an application-supplied reason, and the HTTP writer serializes that reason into the status line without neutralizing carriage returns. Exploitation requires an application to place remote input in the reason parameter; injected bytes can alter the response, including adding a Set-Cookie header, but do not directly expose server data or execute code.

The record

CVE
CVE-2026-34519
Published
Apr 1, 2026
Updated
Apr 1, 2026
Vendor
aio-libs
Product
aiohttp
Classifications
CWE-113, T1565.002
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 1, 2026CVE publishedPublication date reported by the CVE source.
  2. Apr 1, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 3.13.4

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

aio-libs · aiohttpversion=< 3.13.4

Published CVSS scores

2.7GitHub, Inc.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
5.3NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-113Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting')
T1565.002Transmitted Data Manipulation

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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