CVE-2025-53539 is a vulnerability identified in the fastapi-guard library, a security middleware for the FastAPI framework. Fastapi-guard is designed to enhance security by controlling IP addresses, logging requests, and detecting penetration attempts. The vulnerability arises from the use of inefficient regular expression (regex) patterns within the penetration attempt detection component. 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. The vulnerability affects all versions of fastapi-guard prior to 3.0.1, where the issue has been addressed and fixed. The CVSS 4.0 base score is 6.9 (medium severity), reflecting a network attack vector with low attack complexity, no privileges or user interaction required, and a limited impact on availability. The vulnerability does not affect confidentiality or integrity but can degrade availability by exhausting server resources. No known exploits are currently reported in the wild. The root cause is classified under CWE-1333, which pertains to inefficient regular expression complexity causing performance issues. This vulnerability is particularly relevant for applications using fastapi-guard for security monitoring and request filtering, as attackers can craft inputs to trigger the regex inefficiencies and disrupt service availability.

For European organizations, this vulnerability poses a risk primarily to the availability of web applications and APIs built on the FastAPI framework that utilize fastapi-guard versions prior to 3.0.1. Exploitation could allow attackers to launch denial-of-service attacks by sending maliciously crafted requests that cause excessive CPU usage due to regex backtracking. This can degrade service performance or cause outages, impacting business continuity, customer trust, and regulatory compliance, especially under GDPR where service availability is a factor. Organizations relying on fastapi-guard for penetration detection and request filtering may find their defenses bypassed or overwhelmed. The impact is more pronounced for high-traffic services or critical infrastructure exposed to the internet. While the vulnerability does not compromise data confidentiality or integrity, the availability disruption can have cascading effects on dependent services and operational processes. Given the increasing adoption of FastAPI in Europe for modern web services, the threat is relevant across sectors including finance, healthcare, government, and technology.

European organizations should immediately verify if they are using fastapi-guard versions earlier than 3.0.1 in their FastAPI deployments. The primary mitigation is to upgrade fastapi-guard to version 3.0.1 or later, where the inefficient regex patterns have been optimized or replaced. Additionally, organizations should implement input validation and rate limiting at the API gateway or web application firewall (WAF) level to detect and block suspicious request patterns that could trigger regex backtracking. Monitoring CPU and memory usage on API servers can help detect anomalous spikes indicative of exploitation attempts. Employing anomaly detection tools to identify unusual request patterns may provide early warning. Developers should review custom regex patterns in their security middleware to ensure they do not introduce similar inefficiencies. Finally, maintaining an up-to-date inventory of dependencies and integrating automated vulnerability scanning in CI/CD pipelines will help prevent deployment of vulnerable versions.