CVE-2026-42561: Python multipart Denial of Service Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-pp6c-gr5w-3c5g",
  "modified" : "2026-05-14T20:42:16Z",
  "published" : "2026-05-06T21:56:14Z",
  "aliases" : [ "CVE-2026-42561" ],
  "summary" : "python-multipart has Denial of Service via unbounded multipart part headers",
  "details" : "### Summary\n\n`python-multipart` has a denial of service vulnerability in multipart part header parsing. When parsing `multipart/form-data`, `MultipartParser` previously had no limit on the number of part headers or the size of an individual part header. An attacker could send a request with either many repeated headers without terminating the header block or a single very large header value, causing excessive CPU work before request rejection or completion.\n\n### Impact\n\nApplications that parse attacker-controlled `multipart/form-data` with affected versions of `python-multipart` can experience CPU exhaustion. ASGI applications using Starlette, FastAPI, or other frameworks that invoke `python-multipart` may have worker or event-loop delays while processing malicious upload requests.\n\n### Details\n\nThe affected parser states are `HEADER_FIELD_START`, `HEADER_FIELD`, `HEADER_VALUE_START`, `HEADER_VALUE`, and `HEADER_VALUE_ALMOST_DONE`. The issue can be triggered by:\n\n- A multipart part with an oversized individual header value.\n- A multipart part with many repeated header lines or an unterminated header block.\n\nBoth variants are addressed by enforcing default parser limits for maximum header count and maximum header size.\n\n### Mitigation\n\nUpgrade to `python-multipart` `0.0.27` or later.\n\nIf upgrading is not immediately possible, reduce exposure by enforcing request body size limits at the server, proxy, or framework layer. This is only a mitigation; affected versions of `python-multipart` still parse multipart part headers without the default header count and header size limits.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "python-multipart"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "0.0.27"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/Kludex/python-multipart/security/advisories/GHSA-pp6c-gr5w-3c5g"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-42561"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/Kludex/python-multipart"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-770" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-05-06T21:56:14Z",
    "nvd_published_at" : "2026-05-13T21:16:47Z"
  }
}