PYSEC-2026-1851

Vulnerability from pysec - Published: 2026-07-07 14:34 - Updated: 2026-07-07 17:25
VLAI
Details

Summary

When parsing form data, python-multipart skips line breaks (CR \r or LF \n) in front of the first boundary and any tailing bytes after the last boundary. This happens one byte at a time and emits a log event each time, which may cause excessive logging for certain inputs.

An attacker could abuse this by sending a malicious request with lots of data before the first or after the last boundary, causing high CPU load and stalling the processing thread for a significant amount of time. In case of ASGI application, this could stall the event loop and prevent other requests from being processed, resulting in a denial of service (DoS).

Impact

Applications that use python-multipart to parse form data (or use frameworks that do so) are affected.

Original Report

This security issue was reported by: - GitHub security advisory in Starlette on October 30 by @Startr4ck - Email to python-multipart maintainer on October 3 by @mnqazi

Severity
7.5 (High)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
8.7 (High)
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
Impacted products
Name purl
python-multipart pkg:pypi/python-multipart
Aliases
To clipboard 

{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "python-multipart",
        "purl": "pkg:pypi/python-multipart"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "0.0.18"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "0.0.1",
        "0.0.10",
        "0.0.11",
        "0.0.12",
        "0.0.13",
        "0.0.14",
        "0.0.15",
        "0.0.16",
        "0.0.17",
        "0.0.2",
        "0.0.3",
        "0.0.4",
        "0.0.5",
        "0.0.6",
        "0.0.7",
        "0.0.8",
        "0.0.9"
      ]
    }
  ],
  "aliases": [
    "CVE-2024-53981",
    "GHSA-59g5-xgcq-4qw3"
  ],
  "details": "### Summary\n\nWhen parsing form data, `python-multipart` skips line breaks (CR `\\r` or LF `\\n`) in front of the first boundary and any tailing bytes after the last boundary. This happens one byte at a time and emits a log event each time, which may cause excessive logging for certain inputs.\n\nAn attacker could abuse this by sending a malicious request with lots of data before the first or after the last boundary, causing high CPU load and stalling the processing thread for a significant amount of time. In case of ASGI application, this could stall the event loop and prevent other requests from being processed, resulting in a denial of service (DoS).\n\n### Impact\n\nApplications that use `python-multipart` to parse form data (or use frameworks that do so) are affected. \n\n### Original Report\n\nThis security issue was reported by:\n- GitHub security advisory in Starlette on October 30 by @Startr4ck\n- Email to `python-multipart` maintainer on October 3 by @mnqazi",
  "id": "PYSEC-2026-1851",
  "modified": "2026-07-07T17:25:14.632117Z",
  "published": "2026-07-07T14:34:46.120148Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/Kludex/python-multipart/security/advisories/GHSA-59g5-xgcq-4qw3"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2024-53981"
    },
    {
      "type": "WEB",
      "url": "https://github.com/Kludex/python-multipart/commit/c4fe4d3cebc08c660e57dd709af1ffa7059b3177"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/Kludex/python-multipart"
    },
    {
      "type": "PACKAGE",
      "url": "https://pypi.org/project/python-multipart"
    },
    {
      "type": "ADVISORY",
      "url": "https://github.com/advisories/GHSA-59g5-xgcq-4qw3"
    }
  ],
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
      "type": "CVSS_V3"
    },
    {
      "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",
      "type": "CVSS_V4"
    }
  ],
  "summary": "Denial of service (DoS) via deformation `multipart/form-data` boundary"
}



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