Python-Multipart is a streaming multipart parser for Python. Prior to 0.0.31, 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. This vulnerability is fixed in 0.0.31.
PUBLISHED5.2CWE-1284
Python-Multipart: Negative Content-Length in parse_form buffers the entire body in memory
Problem type
Affected products
Kludex
python-multipart
< 0.0.31 - AFFECTED
References
GitHub Security Advisories
GHSA-v9pg-7xvm-68hf
python-multipart: Negative Content-Length in parse_form buffers the entire body in memory
https://github.com/advisories/GHSA-v9pg-7xvm-68hfSummary
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.
Details
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.
Impact
This 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:
- Starlette and FastAPI drive
MultipartParserdirectly from the ASGIreceive()stream and do not callparse_form(). - Known
parse_form()consumers either do not forwardContent-Lengthto it, recompute it from the already-read body, or run behind a layer (such as Werkzeug) that normalizes a negativeContent-Lengthto0.
The 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.
Mitigation
Upgrade to version 0.0.31 or later, which rejects a negative Content-Length with a ValueError before reading the stream.
JSON source
https://cveawg.mitre.org/api/cve/CVE-2026-53540Click to expand
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