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Difficult to exploit vulnerability allows unauthenticated attacker with logon to the infrastructure where Java SE executes to compromise Java SE. Successful attacks of this vulnerability can result in unauthorized read access to a subset of Java SE accessible data.

Difficult to exploit vulnerability allows unauthenticated attacker with network access via multiple protocols to compromise Java SE. Successful attacks of this vulnerability can result in unauthorized ability to cause a partial denial of service (partial DOS) of Java SE.

Last updated 2 June 2026

Summary

parseform() 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

parseform() 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(contentlength - bytesread, chunksize). The header value was parsed to an integer without checking its sign, so a Content-Length of -1 made this expression negative, and inputstream.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 parseform() 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 MultipartParser directly from the ASGI receive() stream and do not call parseform(). Known parseform() 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.

The realistic exposure is limited to bespoke WSGI or http.server handlers that forward raw client headers into parseform(). 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.

Summary

QuerystringParser treated ; as a field separator in application/x-www-form-urlencoded bodies, in addition to &. The WHATWG URL standard, modern browsers, and Python's urllib.parse (since the CVE-2021-23336 fix) treat only & as a separator. This creates a parser differential: the same bytes are tokenized into different fields than a WHATWG compliant intermediary would produce, allowing an attacker to smuggle extra form fields past an upstream body inspecting component.

Details

In pythonmultipart/multipart.py, the FIELDNAME and FIELDDATA states located the next separator by scanning for & and, failing that, for ;:

python seppos = data.find(b"&", i) if seppos == -1: seppos = data.find(b";", i)

As a result, ; acted as a field boundary. Because the fallback only triggered when no & remained in the current chunk, tokenization also depended on unrelated bytes later in the buffer and on how the body was split across write() calls. This is the same class of issue as CVE-2021-23336 in CPython's urllib.parse.

For example, a body inspecting WAF or gateway that follows the WHATWG rule (only & separates fields) receives:

role=user&x=;role=admin

The upstream parses two fields, role=user and x=";role=admin", sees a benign role=user, and forwards the request. QuerystringParser parsed the same bytes as three fields: role="user", x="", and role="admin". The application (for example via Starlette/FastAPI request.form(), where the last value wins) then received role=admin, a value the upstream validator never saw.

The parser is reachable through the public QuerystringParser class, the high level FormParser, createformparser, and parseform APIs, and Starlette/FastAPI request.form() for url encoded bodies.

Impact

Interpretation conflict / HTTP parameter pollution. An attacker can smuggle extra or overriding form fields past an upstream component that applies the WHATWG separator rule, reaching the backend with parameters the intermediary did not observe.

Mitigation

Upgrade to python-multipart 0.0.30 or later, which treats only & as a field separator per the WHATWG URL standard. ; is parsed as ordinary field data, matching urllib.parse, browsers, and other compliant parsers.

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