CVE-2025-53539 is a vulnerability identified in the fastapi-guard library, a security middleware used with the FastAPI framework to control IP addresses, log requests, and detect penetration attempts. The vulnerability arises from inefficient regular expression (regex) patterns used in the penetration attempt detection feature. Specifically, some regex patterns exhibit polynomial complexity backtracking when processing specially crafted inputs. This inefficiency can be exploited to cause excessive CPU consumption, leading to a denial-of-service (DoS) condition by slowing down or crashing the application. The vulnerability affects all versions of fastapi-guard prior to 3.0.1, where the issue has been fixed. The CVSS 4.0 base score is 6.9 (medium severity), reflecting that the vulnerability can be exploited remotely without authentication or user interaction, and it impacts availability by causing resource exhaustion. No known exploits are currently reported in the wild. The root cause is classified under CWE-1333, which relates to inefficient regular expression complexity, a common source of DoS vulnerabilities in web applications that rely on regex for input validation or detection mechanisms.

For European organizations, the impact of this vulnerability can be significant, especially for those relying on FastAPI and fastapi-guard for web application security. An attacker can exploit the inefficient regex patterns to trigger high CPU usage, potentially causing service outages or degraded performance. This can disrupt business operations, lead to loss of availability of critical web services, and damage organizational reputation. Industries with high web traffic or those providing critical online services (e.g., finance, healthcare, e-government) are particularly at risk. Additionally, the vulnerability could be leveraged as part of a larger attack chain, such as distracting security teams or creating a foothold for further exploitation. Since the vulnerability does not require authentication or user interaction, it can be exploited remotely by unauthenticated attackers, increasing the attack surface. However, the lack of known exploits in the wild suggests that immediate widespread exploitation is not yet observed, but proactive mitigation is recommended.

European organizations should promptly upgrade fastapi-guard to version 3.0.1 or later, where the inefficient regex patterns have been corrected. Until the upgrade is applied, organizations can implement the following specific mitigations: 1) Employ Web Application Firewalls (WAFs) with custom rules to detect and block requests containing suspicious payloads that could trigger regex backtracking; 2) Monitor application performance metrics closely to detect unusual CPU spikes indicative of exploitation attempts; 3) Limit request rates and implement IP-based throttling to reduce the impact of potential DoS attempts; 4) Review and harden input validation logic in the application to reduce reliance on complex regex patterns; 5) Conduct regular security testing including fuzzing inputs to identify similar inefficiencies; 6) Maintain an incident response plan that includes detection and mitigation of DoS attacks targeting regex vulnerabilities. These steps go beyond generic advice by focusing on immediate protective controls and monitoring tailored to this specific regex complexity issue.