CVE-2025-53940 is a high-severity vulnerability affecting versions of the TryQuiet messaging platform prior to 6.0.1. Quiet is a decentralized team chat application designed as an alternative to centralized services like Slack, Discord, and Element, emphasizing privacy by avoiding reliance on central servers or self-hosting. The vulnerability arises from the use of a non-constant-time comparison function in the API responsible for backend/frontend token verification. Specifically, the token verification process compares user-supplied tokens against valid tokens using a timing-variant method, where incorrect characters cause the function to fail faster than correct ones. This timing discrepancy enables an attacker to perform a side-channel timing attack by measuring subtle differences in response times to iteratively guess the token one character at a time. Successfully extracting the token could allow unauthorized access to the communication backend, potentially compromising message confidentiality and integrity. The vulnerability is classified under CWE-208 (Observable Timing Discrepancy) and has a CVSS 4.0 base score of 8.5, indicating high severity. The attack vector is local (AV:L), requiring high attack complexity (AC:H) and low privileges (PR:L), but no user interaction (UI:N). The vulnerability impacts confidentiality, integrity, and availability with high scope and impact. Although no known exploits are currently reported in the wild, the vulnerability's nature makes it a significant risk, especially in environments where tokens protect sensitive communications. The issue was addressed in Quiet version 6.0.1 by implementing a constant-time token comparison function, mitigating timing side-channel leakage.

For European organizations using Quiet versions prior to 6.0.1, this vulnerability poses a significant risk to the confidentiality and integrity of internal communications. An attacker with network access and low privileges could leverage timing attacks to extract authentication tokens, potentially gaining unauthorized access to sensitive team chats and backend APIs. This could lead to data leakage, espionage, or disruption of collaboration workflows. Given Quiet's decentralized architecture, compromised tokens might allow lateral movement or impersonation within the communication network. The high impact on availability is also notable, as attackers might disrupt services by invalidating tokens or triggering denial-of-service conditions. European organizations in sectors with stringent data protection requirements (e.g., finance, healthcare, government) could face regulatory and reputational damage if such breaches occur. The lack of user interaction required for exploitation increases the threat's severity, especially in environments where network segmentation is limited. However, the high attack complexity and requirement for local network access somewhat limit the attack surface, though insider threats or compromised internal systems could exploit this vulnerability effectively.

European organizations should immediately verify their Quiet deployment versions and upgrade all instances to version 6.0.1 or later, where the constant-time token comparison fix is implemented. Network segmentation should be enforced to limit access to Quiet backend APIs, restricting token verification endpoints to trusted hosts and users only. Implement monitoring and anomaly detection for unusual token verification request patterns or timing irregularities that might indicate ongoing timing attacks. Employ additional layers of authentication or token rotation policies to reduce the window of exposure if tokens are compromised. Security teams should conduct penetration testing focusing on timing side-channel vulnerabilities in their communication platforms. Where possible, deploy application-layer firewalls or API gateways capable of detecting and mitigating timing-based attacks. Educate developers and system administrators on the importance of constant-time cryptographic operations to prevent similar vulnerabilities. Finally, maintain an incident response plan tailored to communication platform compromises to quickly contain and remediate any breaches.