CVE-2025-53539
📋 TL;DR
CVE-2025-53539 is a denial-of-service vulnerability in FastAPI Guard's penetration detection system where inefficient regex patterns can cause polynomial complexity backtracking when processing specially crafted inputs. This affects all FastAPI applications using vulnerable versions of fastapi-guard middleware. Attackers can cause resource exhaustion and service disruption.
💻 Affected Systems
- fastapi-guard
📦 What is this software?
Fastapi Guard by Fastapi Guard
⚠️ Risk & Real-World Impact
Worst Case
Complete service unavailability due to CPU/memory exhaustion, potentially affecting all users of the application.
Likely Case
Degraded performance or temporary service disruption for affected endpoints, requiring restart of the FastAPI application.
If Mitigated
Minimal impact with proper rate limiting, request filtering, and updated regex patterns in place.
🎯 Exploit Status
Exploitation requires sending specially crafted HTTP requests to trigger inefficient regex patterns.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 3.0.1
Vendor Advisory: https://github.com/rennf93/fastapi-guard/security/advisories/GHSA-j47q-rc62-w448
Restart Required: Yes
Instructions:
1. Update fastapi-guard: pip install fastapi-guard==3.0.1
2. Restart your FastAPI application
3. Verify the update with: pip show fastapi-guard
🔧 Temporary Workarounds
Disable penetration detection
allTemporarily disable the vulnerable regex-based penetration detection feature
Modify FastAPI configuration to set penetration_detection=False in fastapi-guard middleware
Implement request filtering
linuxAdd WAF or reverse proxy with request filtering before requests reach FastAPI
Configure nginx/apache to filter suspicious request patterns
🧯 If You Can't Patch
- Implement strict rate limiting to prevent mass exploitation attempts
- Deploy web application firewall (WAF) with regex attack protection rules
🔍 How to Verify
Check if Vulnerable:
Check fastapi-guard version: pip show fastapi-guard | grep VersionCheck Version:
pip show fastapi-guard | grep VersionVerify Fix Applied:
Verify version is 3.0.1 or higher and test with sample malicious patterns📡 Detection & Monitoring
Log Indicators:
- High CPU usage spikes
- Increased request processing times
- Application crashes or restarts
- Regex timeout errors in logs
Network Indicators:
- Unusual request patterns with complex strings
- Repeated requests to same endpoints with varying payloads
SIEM Query:
source="fastapi.logs" AND ("CPU spike" OR "timeout" OR "regex") AND status=500