CVE-2025-54365 — fastapi-guard patch contains bypassable RegEx

Affected: rennf93 fastapi-guard

fastapi-guard is a security library for FastAPI that provides middleware to control IPs, log requests, detect penetration attempts and more. In version 3.0.1, the regular expression patched to mitigate the ReDoS vulnerability by limiting the length of string fails to catch inputs that exceed this limit. This type of patch fails to detect cases in which the string representing the attributes of a <script> tag exceeds 100 characters. As a result, most of the regex patterns present in version 3.0.1 can be bypassed. This is fixed in version 3.0.2.

GHSA-rrf6-pxg8-684g — FastAPI Guard has a regex bypass

Affected: PyPI: fastapi-guard

### 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 <script> tag exceeds 100 characters. Therefore, most of the regex patterns present in version 3.0.1 can be bypassed.

### PoC

1. clone the fastapi-guard repository 2. Navigate to the examples directory and modify the main.py source code. Change the HTTP method for the root route from GET to POST. <img width="1013" height="554" alt="image" src="https://github.com/user-attachments/assets/cf93ea37-2fd7-4251-abb6-b55f88685f54" /> 3. 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.

```python import requests

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

obvious_payload = { "obvious" : "<script>alert(1);</script>" } response = requests.post(url=URL, json=obvious_payload) print(f"[+] response of first re…

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CVE-2025-54365 — fastapi-guard patch contains bypassable RegEx

GHSA-rrf6-pxg8-684g — FastAPI Guard has a regex bypass