CVE-2025-53940 is a high-severity vulnerability affecting the TryQuiet project's product "quiet," an alternative team chat application designed to operate without 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 in versions prior to 6.0.1 (specifically 6.1.0-alpha.4 and below). This insecure comparison allows an attacker to perform a timing attack by measuring subtle differences in response times when guessing token characters. Because incorrect characters cause the comparison to fail faster, an attacker can iteratively deduce the correct token one character at a time. This type of vulnerability is classified under CWE-208 (Observable Timing Discrepancy). The flaw compromises the confidentiality and integrity of authentication tokens, potentially allowing unauthorized access to communication channels or sensitive data within the Quiet application. The vulnerability has a CVSS 4.0 base score of 8.5, reflecting its high impact and complexity, with attack vector local, high attack complexity, no user interaction, and requiring low privileges. Although no known exploits are currently reported in the wild, the nature of the vulnerability makes it a significant risk if exploited. The issue was addressed and fixed in version 6.0.1 of Quiet by implementing a constant-time token comparison function to eliminate timing discrepancies. Organizations using affected versions should prioritize upgrading to the patched release to mitigate this risk.

For European organizations, the impact of CVE-2025-53940 can be substantial, especially for those relying on Quiet as a secure communication platform. Successful exploitation could lead to unauthorized access to internal chat communications, exposing sensitive business information, intellectual property, or personal data. This breach of confidentiality could also facilitate further lateral movement within networks, increasing the risk of broader compromise. The integrity of communication is also at risk, as attackers might impersonate users or manipulate messages if token verification is bypassed. Given the increasing regulatory scrutiny in Europe around data protection (e.g., GDPR), such breaches could result in significant legal and financial consequences. Additionally, organizations in sectors with high confidentiality requirements—such as finance, healthcare, government, and critical infrastructure—may face operational disruptions and reputational damage. The vulnerability's requirement for local access and low privileges means that insider threats or attackers who have gained limited footholds could leverage this flaw to escalate privileges or access sensitive communications.

Beyond the essential step of upgrading to Quiet version 6.0.1 or later, European organizations should implement several targeted mitigations: 1) Conduct a thorough inventory to identify all instances of Quiet in use, including alpha and pre-release versions, to ensure no vulnerable deployments remain. 2) Restrict local access to systems running Quiet to trusted users only, employing strict access controls and monitoring to detect suspicious activities indicative of timing attacks. 3) Implement network segmentation and application-layer firewalls to limit exposure of the Quiet API endpoints, reducing the attack surface. 4) Employ anomaly detection tools that can identify unusual timing patterns or repeated token verification attempts that may indicate an ongoing timing attack. 5) Educate developers and security teams about the risks of non-constant-time comparisons in authentication mechanisms to prevent similar vulnerabilities in custom or third-party software. 6) Regularly review and update cryptographic and authentication libraries to ensure they use constant-time operations where applicable. 7) In environments where upgrading is delayed, consider deploying compensating controls such as multi-factor authentication or additional token validation layers to reduce the risk of token compromise.