CVE-2025-53539: Fastapi Guard DOS Vulnerability

CVE-2025-53539 Overview

CVE-2025-53539 is an Algorithmic Complexity Attack vulnerability affecting FastAPI Guard, a security library for FastAPI that provides middleware to control IPs, log requests, and detect penetration attempts. The library's penetration attempt detection mechanism uses regular expressions 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. This vulnerability is classified as CWE-1333 (Inefficient Regular Expression Complexity).

Critical Impact

Remote attackers can craft malicious HTTP requests that trigger catastrophic regex backtracking, causing denial of service conditions in FastAPI applications relying on fastapi-guard for security middleware.

Affected Products

• FastAPI Guard (fastapi_guard) versions prior to 3.0.1
• Applications using fastapi-guard penetration detection middleware
• FastAPI web applications with fastapi-guard security middleware enabled

Discovery Timeline

• 2025-07-07 - CVE CVE-2025-53539 published to NVD
• 2025-12-31 - Last updated in NVD database

Technical Details for CVE-2025-53539

Vulnerability Analysis

This vulnerability is a classic Regular Expression Denial of Service (ReDoS) issue stemming from poorly constructed regex patterns within the fastapi-guard library. The penetration detection component of fastapi-guard inspects incoming HTTP requests using regex-based pattern matching to identify malicious payloads. The problematic patterns contain constructs that lead to exponential or polynomial time complexity when processing certain input strings.

When an attacker submits a carefully crafted request containing input designed to exploit the inefficient regex, the pattern matching engine enters a state of catastrophic backtracking. This occurs because the regex engine explores an enormous number of possible matches before ultimately failing, consuming excessive CPU resources during the process. Since this affects security middleware that processes every incoming request, even a single malicious request can tie up server resources.

The vulnerability exists in the cloud handler component where regex patterns are used to parse HTML responses and extract URLs. The original pattern used greedy quantifiers (.*?) without proper bounds, allowing unbounded matching that can be exploited.

Root Cause

The root cause is the use of inefficient regular expression patterns that contain unbounded quantifiers and overlapping alternatives. Specifically, the vulnerable regex patterns use constructs like .*? (non-greedy match of any character) without length restrictions, which can lead to polynomial-time complexity when the regex engine must explore numerous possible match combinations during backtracking. The fix implements bounded quantifiers (.{1,500}?) to limit the maximum match length and prevent excessive backtracking.

Attack Vector

The attack is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by sending HTTP requests containing specially crafted strings designed to trigger worst-case regex matching behavior. The malicious input is processed by the fastapi-guard middleware before reaching the application logic, making all protected endpoints potential attack surfaces. Since the vulnerable component is security middleware, it paradoxically becomes the vector for denial of service attacks against the applications it is meant to protect.

# Vulnerable vs Fixed Regex Pattern
# Source: https://github.com/rennf93/fastapi-guard/commit/d9d50e8130b7b434cdc1b001b8cfd03a06729f7f

# VULNERABLE - Unbounded non-greedy quantifier allows catastrophic backtracking
# pattern = r'href=["\'](https://download\.microsoft\.com/' r'.*?\.json)["\']'

# FIXED - Bounded quantifier limits match length to prevent ReDoS
pattern = r'href=["\'](https://download\.microsoft\.com/.{1,500}?\.json)["\']'
match = re.search(pattern, decoded_html)

Source: GitHub Commit

Detection Methods for CVE-2025-53539

Indicators of Compromise

• Unusually high CPU utilization on FastAPI application servers without corresponding increase in legitimate traffic
• Slow or unresponsive API endpoints protected by fastapi-guard middleware
• HTTP requests containing abnormally long or repetitive string patterns in headers, query parameters, or body content
• Log entries showing extended request processing times for specific malformed requests

Detection Strategies

• Monitor request processing latency for FastAPI applications using fastapi-guard; sudden spikes may indicate ReDoS exploitation attempts
• Implement request timeout mechanisms to identify and terminate requests that exceed normal processing duration
• Review application performance metrics for CPU-bound anomalies correlated with incoming HTTP traffic patterns
• Deploy Web Application Firewall (WAF) rules to detect and block requests with characteristic ReDoS payload patterns

Monitoring Recommendations

• Configure application performance monitoring (APM) to alert on regex processing time anomalies
• Set up resource utilization alerts for CPU spikes on servers running fastapi-guard protected applications
• Enable detailed logging for requests that exceed processing time thresholds
• Monitor for repeated requests from single sources containing long repetitive strings

How to Mitigate CVE-2025-53539

Immediate Actions Required

• Upgrade fastapi-guard to version 3.0.1 or later immediately
• Review and audit any custom regex patterns used in conjunction with fastapi-guard
• Implement request timeout limits at the web server or load balancer level as a defense-in-depth measure
• Consider temporarily disabling fastapi-guard penetration detection if immediate patching is not possible

Patch Information

The vulnerability is fixed in fastapi-guard version 3.0.1. The patch modifies the vulnerable regex patterns to use bounded quantifiers, limiting the maximum length of matched content and preventing catastrophic backtracking. The fix can be reviewed in the GitHub commit d9d50e8130b7b434cdc1b001b8cfd03a06729f7f. Additional details are available in the GitHub Security Advisory GHSA-j47q-rc62-w448.

Workarounds

• If immediate upgrade is not feasible, implement request body and header size limits at the reverse proxy or load balancer level
• Deploy a WAF with ReDoS protection capabilities to filter malicious requests before they reach the application
• Configure request timeout settings to automatically terminate long-running regex operations
• Consider temporarily bypassing fastapi-guard for non-critical endpoints while preparing for the upgrade

# Upgrade fastapi-guard to patched version
pip install --upgrade fastapi-guard>=3.0.1

# Verify installed version
pip show fastapi-guard | grep Version