CVE-2025-26042 identifies a ReDoS vulnerability in Uptime Kuma, an open-source self-hosted monitoring tool, specifically in versions 1.23.0 and later. The vulnerability arises when an administrator creates a notification through the web interface and inputs a string that triggers catastrophic backtracking in the underlying regular expression engine. Catastrophic backtracking occurs when certain regex patterns cause the engine to consume exponentially increasing CPU resources when processing crafted input, leading to denial of service. This vulnerability is classified under CWE-1333, which relates to inefficient regular expressions causing performance degradation. The attack vector is network-based (AV:N), requiring high attack complexity (AC:H), administrator privileges (PR:H), and user interaction (UI:R). The scope is unchanged (S:U), with low confidentiality impact (C:L), but high integrity (I:H) and availability (A:H) impacts, resulting in a CVSS v3.1 base score of 6.0 (medium severity). Although no known exploits are currently reported in the wild, the vulnerability could be leveraged by attackers to disrupt monitoring services, potentially masking other malicious activities or causing operational downtime. The lack of available patches at the time of publication necessitates immediate mitigation strategies to reduce risk.

For European organizations, the impact of CVE-2025-26042 can be significant, particularly for those relying on Uptime Kuma for critical infrastructure monitoring and alerting. A successful ReDoS attack could exhaust server CPU resources, causing the monitoring service to become unresponsive or crash, leading to loss of visibility into system health and delayed incident response. This could indirectly affect confidentiality and integrity by allowing other attacks to go undetected. Organizations in sectors such as finance, healthcare, energy, and telecommunications, where uptime and monitoring are crucial, may face operational disruptions and compliance risks. The requirement for administrator privileges and user interaction somewhat limits the attack surface but does not eliminate risk, especially in environments with insufficient access controls or insider threats. The medium severity rating reflects these factors but underscores the need for proactive defense to maintain service reliability and security posture.

To mitigate CVE-2025-26042, European organizations should implement the following specific measures: 1) Restrict administrative access to Uptime Kuma to trusted personnel only and enforce strong authentication mechanisms such as multi-factor authentication (MFA). 2) Implement strict input validation and sanitization on notification creation forms to detect and reject potentially malicious regex patterns or excessively complex strings. 3) Monitor system resource usage closely for unusual CPU spikes that may indicate attempted ReDoS exploitation. 4) Isolate the Uptime Kuma service in a controlled network segment with limited exposure to reduce attack surface. 5) Stay informed about official patches or updates from Uptime Kuma developers and apply them promptly once available. 6) Conduct regular security audits and penetration testing focusing on web interface input handling. 7) Educate administrators about the risks of crafting complex regex patterns and encourage use of safe notification configurations. These targeted actions go beyond generic advice by focusing on access control, input handling, monitoring, and operational best practices specific to this vulnerability.