CVE-2026-34519: Aiohttp AIOHTTP Has HTTP Response Splitting Vulnerability

LOW
Advisory Identifier: GHSA-mwh4-6h8g-pg8w
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CWE-113
Published
Advisory Summary

CVE-2026-34519 is a LOW aiohttp has http response splitting vulnerability (CWE-113: Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting')) affecting Aiohttp libraries in the PyPI ecosystem. The issue involves aiohttp has http response splitting via \r in reason phrase. Remediation is available by upgrading affected packages to version 3.13.4.

CVE-2026-34519 at a Glance

CVE Identifier CVE-2026-34519
GHSA Identifier GHSA-mwh4-6h8g-pg8w
Severity LOW
CVSS v3.1 Base Score N/A
CWE Classification CWE-113: Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting')
Affected Product Aiohttp
Package Ecosystem PyPI
Published Date
Last Updated

How to Fix CVE-2026-34519

To remediate this vulnerability, upgrade all affected Aiohttp dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

aiohttp fixed: 3.13.4 
pip install aiohttp>=3.13.4
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

pip show aiohttp

How to Check if You Are Affected

To determine whether your Python environment is affected by CVE-2026-34519, inspect your installed packages using pip:

pip show aiohttp

Analyze the terminal output to locate any version of Aiohttp in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 3.13.4), your application is affected.

Note: The pip show aiohttp command checks currently installed packages in your active Python environment.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-113: Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting'), where the product receives data from an http agent/component (e.g., web server, proxy, browser, etc.), but it does not neutralize or incorrectly neutralizes cr and lf characters before the data is included in outgoing http headers. In Aiohttp, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

### Summary An attacker who controls the `reason` parameter when creating a `Response` may be able to inject extra headers or similar exploits. ### Impact In the unlikely situation that an application allows untrusted data to be used in the response's `reason` parameter, then an attacker could manipulate the response to send something different from what the developer intended. ----- Patch: https://github.com/aio-libs/aiohttp/commit/53b35a2f8869c37a133e60bf1a82a1c01642ba2b

CVE-2026-34519 Affected Aiohttp Versions

1 component(s)
Fixed Versions:
3.13.4
Product / Package Ecosystem Affected Versions Fixed Version
aiohttp
pypi/aiohttp
PyPI
< 3.13.4
3.13.4

References

GitHub Advisory / Wiki ADVISORY
https://github.com/aio-libs/aiohttp/security/advisories/GHSA-mwh4-6h8g-pg8w
NVD — CVE-2026-34519 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-34519
Security Fix Commit FIX COMMIT
https://github.com/aio-libs/aiohttp/commit/53b35a2f8869c37a133e60bf1a82a1c01642ba2b
Aiohttp Source Repository REPOSITORY
https://github.com/aio-libs/aiohttp
Aiohttp Release RELEASE
https://github.com/aio-libs/aiohttp/releases/tag/v3.13.4

Frequently Asked Questions

What is CVE-2026-34519?
CVE-2026-34519 is a security vulnerability in Aiohttp involving AIOHTTP has HTTP response splitting via \r in reason phrase under CWE-113: Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting').
Which Aiohttp versions are affected?
Affected versions include releases prior to 3.13.4 across affected packages: aiohttp.
What version fixes CVE-2026-34519?
Upgrading to 3.13.4 (or the latest patched release for your respective branch) resolves the vulnerability.
Is aiohttp affected?
Yes, aiohttp is affected in releases prior to 3.13.4. Upgrade your dependency to 3.13.4 or later.
What CWE is associated with CVE-2026-34519?
CVE-2026-34519 is classified under CWE-113: Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting') (The product receives data from an HTTP agent/component (e.g., web server, proxy, browser, etc.), but it does not neutralize or incorrectly neutralizes CR and LF characters before the data is included in outgoing HTTP headers.).
How can I check PyPI dependencies for CVE-2026-34519?
Run the dependency tree command: 'pip show aiohttp'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-mwh4-6h8g-pg8w",
  "modified" : "2026-04-01T21:48:24Z",
  "published" : "2026-04-01T21:48:24Z",
  "aliases" : [ "CVE-2026-34519" ],
  "summary" : "AIOHTTP has HTTP response splitting via \\r in reason phrase",
  "details" : "### Summary\n\nAn attacker who controls the `reason` parameter when creating a `Response` may be able to inject extra headers or similar exploits.\n\n### Impact\n\nIn the unlikely situation that an application allows untrusted data to be used in the response's `reason` parameter, then an attacker could manipulate the response to send something different from what the developer intended.\n\n-----\n\nPatch: https://github.com/aio-libs/aiohttp/commit/53b35a2f8869c37a133e60bf1a82a1c01642ba2b",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "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"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "aiohttp"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "3.13.4"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 3.13.3"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/aio-libs/aiohttp/security/advisories/GHSA-mwh4-6h8g-pg8w"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-34519"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/aio-libs/aiohttp/commit/53b35a2f8869c37a133e60bf1a82a1c01642ba2b"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/aio-libs/aiohttp"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/aio-libs/aiohttp/releases/tag/v3.13.4"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-113" ],
    "severity" : "LOW",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-04-01T21:48:24Z",
    "nvd_published_at" : "2026-04-01T21:17:00Z"
  }
}