CVE-2026-53540: python-multipart Negative Content-Length Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-v9pg-7xvm-68hf",
  "modified" : "2026-06-15T20:23:45Z",
  "published" : "2026-06-15T20:23:45Z",
  "aliases" : [ "CVE-2026-53540" ],
  "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.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "python-multipart"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "0.0.31"
      } ]
    } ]
  } ],
  "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"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-1284" ],
    "severity" : "LOW",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-06-15T20:23:45Z",
    "nvd_published_at" : null
  }
}