CVE-2026-34516: Aiohttp AIOHTTP Has A Multipart Header Vulnerability

MODERATE
Advisory Identifier: GHSA-m5qp-6w8w-w647
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CVSS 7.5
CWE-770
Published
Advisory Summary

CVE-2026-34516 is a MODERATE (CVSS 7.5) aiohttp has a multipart header vulnerability (CWE-770: Allocation of Resources Without Limits or Throttling) affecting Aiohttp libraries in the PyPI ecosystem. The issue involves aiohttp has a multipart header size bypass. Remediation is available by upgrading affected packages to version 3.13.4.

CVE-2026-34516 at a Glance

CVE Identifier CVE-2026-34516
GHSA Identifier GHSA-m5qp-6w8w-w647
Severity MODERATE
CVSS v3.1 Base Score 7.5
CWE Classification CWE-770: Allocation of Resources Without Limits or Throttling
Affected Product Aiohttp
Package Ecosystem PyPI
Published Date
Last Updated

Security Impact

CVSS: 7.5

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality None (C:N)
Integrity None (I:N)
Availability High (A:H)

How to Fix CVE-2026-34516

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-2026-34516, 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-770: Allocation of Resources Without Limits or Throttling, where the product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated. 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 A response with an excessive number of multipart headers may be allowed to use more memory than intended, potentially allowing a DoS vulnerability. ### Impact Multipart headers were not subject to the same size restrictions in place for normal headers, potentially allowing substantially more data to be loaded into memory than intended. However, other restrictions in place limit the impact of this vulnerability. ----- Patch: https://github.com/aio-libs/aiohttp/commit/8a74257b3804c9aac0bf644af93070f68f6c5a6f

CVE-2026-34516 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-m5qp-6w8w-w647
NVD — CVE-2026-34516 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-34516
Security Fix Commit FIX COMMIT
https://github.com/aio-libs/aiohttp/commit/8a74257b3804c9aac0bf644af93070f68f6c5a6f
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-2026-34516?
CVE-2026-34516 is a security vulnerability in Aiohttp involving AIOHTTP has a Multipart Header Size Bypass under CWE-770: Allocation of Resources Without Limits or Throttling.
Which Aiohttp versions are affected?
Affected versions include releases prior to 3.13.4 across affected packages: aiohttp.
What version fixes CVE-2026-34516?
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 is the CVSS score of CVE-2026-34516?
CVE-2026-34516 has a CVSS v3.1 base score of 7.5 (MODERATE) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.
What CWE is associated with CVE-2026-34516?
CVE-2026-34516 is classified under CWE-770: Allocation of Resources Without Limits or Throttling (The product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated.).
How can I check PyPI dependencies for CVE-2026-34516?
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-m5qp-6w8w-w647",
  "modified" : "2026-04-06T23:12:04Z",
  "published" : "2026-04-01T21:43:07Z",
  "aliases" : [ "CVE-2026-34516" ],
  "summary" : "AIOHTTP has a Multipart Header Size Bypass",
  "details" : "### Summary\n\nA response with an excessive number of multipart headers may be allowed to use more memory than intended, potentially allowing a DoS vulnerability.\n\n### Impact\n\nMultipart headers were not subject to the same size restrictions in place for normal headers, potentially allowing substantially more data to be loaded into memory than intended. However, other restrictions in place limit the impact of this vulnerability.\n\n-----\n\nPatch: https://github.com/aio-libs/aiohttp/commit/8a74257b3804c9aac0bf644af93070f68f6c5a6f",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
  }, {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/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-m5qp-6w8w-w647"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-34516"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/aio-libs/aiohttp/commit/8a74257b3804c9aac0bf644af93070f68f6c5a6f"
  }, {
    "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-770" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-04-01T21:43:07Z",
    "nvd_published_at" : "2026-04-01T21:16:59Z"
  }
}