CVE-2025-53539 — fastapi-guard
Fix: rennf93/fastapi-guard@d9d50e8CVE-2025-53539 is a CWE-1333 vulnerability in fastapi-guard. A fix is available for fastapi-guard — see the affected versions and patch details below.
ReDoS in fastapi-guard's penetration attempts detector
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
Exploitation and automatability from CISA’s SSVC triage for CVE-2025-53539.
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.
Real-World Exposure
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
fastapi-guard detects penetration attempts by using regex patterns to scan incoming requests. However, some of the regex patterns used in detection are extremely inefficient and can cause polynomial complexity backtracks when handling specially crafted inputs.
It is not as severe as exponential complexity ReDoS, but still downgrades performance and allows DoS exploits. An attacker can trigger high cpu usage and make a service unresponsive for hours by sending a single request in size of KBs.
PoC
payload = lambda n: '<'*n+ ' '*n+ 'style=' + '"'*n + ' '*n+ 'url('*n # complexity: O(n^5)
print(requests.post("http://172.24.1.3:8000/", data=payload(50)).elapsed) # 0:00:03.771120
print(requests.post("http://172.24.1.3:8000/", data=payload(100)).elapsed) # 0:01:17.952637
print(requests.post("http://172.24.1.3:8000/", data=payload(200)).elapsed) # timeout (>15min)
Single-threaded uvicorn workers can not handle any other concurrent requests during the elapsed time.
Impact
Penetration detection is enabled by default. Services that use fastapi-guard middleware without explicitly setting enable_penetration_detection=False are vulnerable to DoS.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | fastapi-guard | all versions | 3.0.1pip install --upgrade 'fastapi-guard==3.0.1' |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for fastapi-guard, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update fastapi-guard to 3.0.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2025-53539 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 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2025-53539 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2025-53539. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2025-53539 in your dependencies?
O3 Security finds CVE-2025-53539 across PyPI dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.