{"id":"PYSEC-2026-3039","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.","aliases":["CVE-2026-42561","GHSA-pp6c-gr5w-3c5g"],"modified":"2026-07-13T16:51:16.759429947Z","published":"2026-07-13T15:15:39.936658Z","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"},{"type":"PACKAGE","url":"https://pypi.org/project/python-multipart"},{"type":"ADVISORY","url":"https://github.com/advisories/GHSA-pp6c-gr5w-3c5g"}],"affected":[{"package":{"name":"python-multipart","ecosystem":"PyPI","purl":"pkg:pypi/python-multipart"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"0.0.27"}]}],"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","0.0.18","0.0.19","0.0.2","0.0.20","0.0.21","0.0.22","0.0.23","0.0.24","0.0.25","0.0.26","0.0.3","0.0.4","0.0.5","0.0.6","0.0.7","0.0.8","0.0.9"],"database_specific":{"source":"https://github.com/pypa/advisory-database/blob/main/vulns/python-multipart/PYSEC-2026-3039.yaml"}}],"schema_version":"1.7.5","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"}]}