CVE-2025-53544 is a high-severity vulnerability affecting Trilium Notes, an open-source, cross-platform hierarchical note-taking application designed for building large personal knowledge bases. Versions prior to 0.97.0 suffer from an improper restriction of excessive authentication attempts (CWE-307) specifically in the initial sync seed retrieval endpoint. This flaw allows unauthenticated attackers to perform brute-force password guessing without triggering any rate limiting or lockout mechanisms. Since Trilium is a single-user application that does not require usernames, the attack surface is simplified to guessing a single password, making exploitation more feasible. The vulnerability arises because the brute-force protection bypass enables unlimited password attempts, undermining the application's authentication security. Additionally, Trilium includes features such as multi-factor authentication (MFA), note sharing, and custom request handlers, which imply that instances of the application may be exposed to the internet, increasing the risk of remote exploitation. The vulnerability has a CVSS 3.1 base score of 7.5 (high), with an attack vector of network (AV:N), low attack complexity (AC:L), no privileges required (PR:N), no user interaction (UI:N), unchanged scope (S:U), high confidentiality impact (C:H), and no impact on integrity or availability (I:N/A:N). The vulnerability was published on August 5, 2025, and fixed in version 0.97.0. No known exploits are currently reported in the wild. The core technical issue is the lack of effective rate limiting or lockout on the password verification process during initial sync seed retrieval, which is critical for securing access to the user's encrypted notes and personal knowledge base.

For European organizations using Trilium Notes, especially those exposing the application to the internet or using it for sensitive personal or organizational knowledge management, this vulnerability poses a significant confidentiality risk. An attacker exploiting this flaw can gain unauthorized access to the user's encrypted notes by brute-forcing the password without detection, potentially leading to data leakage of sensitive or proprietary information. Since Trilium is often used for personal knowledge bases, the impact may be more pronounced for individual users or small teams relying on it for critical data. The lack of impact on integrity and availability reduces the risk of data tampering or denial of service, but the confidentiality breach alone can have serious consequences, including intellectual property theft, privacy violations, and compliance issues under GDPR if personal data is exposed. The ease of exploitation (no authentication or user interaction required) and network accessibility increase the threat level for European organizations that have not updated to the patched version. Organizations using Trilium in regulated sectors or handling sensitive data should consider this vulnerability a priority for remediation.

1. Immediate upgrade to Trilium version 0.97.0 or later, where the brute-force protection bypass is fixed, is the most effective mitigation. 2. If upgrading is not immediately possible, restrict network exposure of Trilium instances by limiting access to trusted IP addresses or VPNs to reduce the attack surface. 3. Implement external rate limiting or web application firewall (WAF) rules to detect and block excessive authentication attempts targeting the initial sync seed retrieval endpoint. 4. Monitor application logs for repeated failed password attempts and set up alerts for suspicious activity indicative of brute-force attacks. 5. Encourage users to use strong, complex passwords and enable multi-factor authentication (MFA) where supported to add an additional layer of security. 6. Conduct regular security audits and penetration testing on exposed Trilium deployments to identify and remediate any residual weaknesses. 7. Educate users about the risks of exposing personal knowledge bases to the internet and best practices for secure deployment.