Description

FastAPI Guard has a regex bypass

Summary

The regular expression patched to mitigate the ReDoS vulnerability by limiting the length of string fails to catch inputs that exceed this limit.

Details

In version 3.0.1, you can find a commit like the one in the link below, which was made to prevent ReDoS. https://github.com/rennf93/fastapi-guard/commit/d9d50e8130b7b434cdc1b001b8cfd03a06729f7f

This commit mitigates the vulnerability by limiting the length of the input string, as shown in the example below. r"<script[^>]*>[^<]*<\\/script\\s*>" -> <script[^>]{0,100}>[^<]{0,1000}<\\/script\\s{0,10}>

This type of patch fails to catch cases where the string representing the attributes of a

PoC

clone the fastapi-guard repository

Navigate to the examples directory and modify the main.py source code. Change the HTTP method for the root route from GET to POST.

After that, set up the example app environment by running the docker-compose up command. Then, run the Python code below to verify that the two requests return different results.

import requests

URL = "<http://localhost:8000>"

obvious_payload = {
    "obvious" : "<script>alert(1);</script>"
}
response = requests.post(url=URL, json=obvious_payload)...
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Impact

Due to this vulnerability, most of the regex patterns can potentially be bypassed, making the application vulnerable to attacks such as XSS and SQL Injection.

Mitigation

Aliases

References