CVE-2025-53539 in fastapi-guard
Summary
by MITRE • 07/07/2025
FastAPI Guard is a security library for FastAPI that provides middleware to control IPs, log requests, and detect penetration attempts. fastapi-guard's penetration attempts detection uses regex 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 fixed in 3.0.1.
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Analysis
by VulDB Data Team • 12/31/2025
CVE-2025-53539 represents a critical performance vulnerability within the FastAPI Guard security library that affects applications leveraging this middleware for request monitoring and penetration attempt detection. The vulnerability stems from the library's implementation of regular expression patterns used to identify suspicious request patterns, where certain regex constructs exhibit pathological behavior under specific input conditions. This issue falls under the category of CWE-1333, which specifically addresses inefficient regular expressions that can lead to denial of service through excessive computational resources. The problematic regex patterns in fastapi-guard's penetration detection system are susceptible to catastrophic backtracking, a well-documented vulnerability pattern that occurs when regex engines perform exponential time complexity operations during pattern matching.
The technical flaw manifests when maliciously crafted inputs are processed through the inefficient regex patterns, causing the regular expression engine to explore an exponential number of possible matching paths. This behavior results in polynomial complexity backtracks that can consume excessive CPU resources and lead to significant performance degradation or complete system unresponsiveness. The vulnerability is particularly dangerous in high-traffic applications where the regex matching occurs on every incoming request, potentially allowing an attacker to craft inputs that trigger denial of service conditions. This type of vulnerability is classified under the ATT&CK technique T1499.004, which involves resource exhaustion through inefficient algorithms, and specifically aligns with the ATT&CK tactic TA0040, which covers resource exhaustion.
The operational impact of CVE-2025-53539 extends beyond simple performance degradation to encompass complete service availability issues that can compromise application integrity and user experience. When exploited, the vulnerability allows attackers to consume excessive computational resources through carefully crafted requests, potentially leading to application crashes, service interruptions, or complete system downtime. The vulnerability affects any FastAPI application that utilizes fastapi-guard middleware for request monitoring and penetration detection, making it particularly concerning for production environments where reliability and performance are paramount. Organizations relying on this security library may experience cascading failures as the system becomes unresponsive under attack conditions.
Mitigation strategies for CVE-2025-53539 require immediate action to upgrade to version 3.0.1 or later, which contains the necessary fixes for the inefficient regex patterns. Security teams should conduct thorough testing of their applications after applying the update to ensure no regression issues occur in the request handling logic. Additionally, organizations should implement monitoring and alerting mechanisms to detect unusual resource consumption patterns that may indicate exploitation attempts. The fix addresses the root cause by optimizing the regex patterns to prevent catastrophic backtracking while maintaining the intended security functionality. Security professionals should also consider implementing rate limiting and input validation controls as additional layers of defense, ensuring that even if the regex vulnerability were to be exploited, the impact would be limited through these complementary security measures.