{"id":"PYSEC-2026-3040","summary":"python-multipart: Negative Content-Length in parse_form buffers the entire body in memory","details":"### Summary\n\n`parse_form()` did not validate the `Content-Length` header before using it to bound its chunked read of the request body. A negative `Content-Length` turned the bounded read into a read-until-EOF, so the entire body was loaded into memory in a single read instead of in fixed-size chunks.\n\n### Details\n\n`parse_form()` reads the input stream in chunks, never reading more than the remaining `Content-Length` at a time. The per-chunk size is computed as `min(content_length - bytes_read, chunk_size)`. The header value was parsed to an integer without checking its sign, so a `Content-Length` of `-1` made this expression negative, and `input_stream.read(-1)` reads until end of stream. The intended bounded, chunked read therefore collapsed into a single unbounded read of the whole stream. The amount read is still bounded by what the client actually sends.\n\n### Impact\n\nThis only affects code that calls `parse_form()` directly with a `Content-Length` header taken from attacker-controlled input and without normalizing a negative value first. No known package is affected:\n\n* Starlette and FastAPI drive `MultipartParser` directly from the ASGI `receive()` stream and do not call `parse_form()`.\n* Known `parse_form()` consumers either do not forward `Content-Length` to it, recompute it from the already-read body, or run behind a layer (such as Werkzeug) that normalizes a negative `Content-Length` to `0`.\n\nThe realistic exposure is limited to bespoke WSGI or `http.server` handlers that forward raw client headers into `parse_form()`. In that case a crafted request buffers the body in memory at once, degrading availability under concurrent requests rather than causing a complete denial of service.\n\n### Mitigation\n\nUpgrade to version `0.0.31` or later, which rejects a negative `Content-Length` with a `ValueError` before reading the stream.","aliases":["CVE-2026-53540","GHSA-v9pg-7xvm-68hf"],"modified":"2026-07-13T16:33:29.088234122Z","published":"2026-07-13T15:46:17.309964Z","references":[{"type":"WEB","url":"https://github.com/Kludex/python-multipart/security/advisories/GHSA-v9pg-7xvm-68hf"},{"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-v9pg-7xvm-68hf"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-53540"}],"affected":[{"package":{"name":"python-multipart","ecosystem":"PyPI","purl":"pkg:pypi/python-multipart"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"0.0.31"}]}],"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","0.0.27","0.0.28","0.0.29","0.0.3","0.0.30","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-2026-3040.yaml"}}],"schema_version":"1.7.5","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L"}]}