CVE-2025-53548 is a high-severity vulnerability affecting the Clerk JavaScript library, specifically versions prior to 2.4.0. Clerk is a developer tool used for user management, including handling authentication and webhook events. The vulnerability arises from insufficient verification of data authenticity in the verifyWebhook() helper function. This function is intended to validate incoming webhook events to ensure they are legitimately signed by Clerk. However, due to improper verification logic, applications using vulnerable versions may accept webhook events that are improperly signed or forged. This can lead to attackers injecting malicious webhook payloads that the application trusts as authentic. The vulnerability is classified under CWE-345 (Insufficient Verification of Data Authenticity), indicating a failure to properly confirm the source and integrity of data. The CVSS 3.1 base score is 7.5, reflecting a high severity with network attack vector (AV:N), low attack complexity (AC:L), no privileges required (PR:N), no user interaction (UI:N), unchanged scope (S:U), no confidentiality impact (C:N), high integrity impact (I:H), and no availability impact (A:N). This means an unauthenticated attacker can remotely exploit this flaw without user interaction to alter data integrity by injecting or modifying webhook events. The issue was resolved in version 2.4.0 of @clerk/backend, so upgrading to this or later versions mitigates the risk. No known exploits are currently reported in the wild, but the vulnerability's nature makes it a significant risk for applications relying on Clerk for webhook verification.

For European organizations using Clerk's JavaScript library for user management and webhook processing, this vulnerability poses a serious risk to data integrity. Attackers could send forged webhook events that the application accepts as genuine, potentially leading to unauthorized actions such as user account manipulation, privilege escalation, or triggering unintended workflows. This could disrupt business processes, compromise user data integrity, and damage trust in affected services. Since the vulnerability does not impact confidentiality or availability directly, the primary concern is unauthorized data modification and the downstream effects on application logic and security. Organizations in sectors with strict regulatory requirements around data integrity and user authentication—such as finance, healthcare, and government—may face compliance risks if exploited. The lack of required authentication or user interaction for exploitation increases the threat level, making automated attacks feasible. Given the widespread adoption of JavaScript-based user management tools and the growing reliance on webhook integrations, the potential attack surface is significant within Europe.

European organizations should immediately verify if their applications use Clerk JavaScript versions prior to 2.4.0 and upgrade to version 2.4.0 or later to apply the official fix. Beyond upgrading, developers should implement additional webhook security measures such as: 1) Independently validating webhook signatures using cryptographic methods and secrets outside of the Clerk library to add defense in depth. 2) Employing strict input validation and sanitization on webhook payloads to prevent injection attacks. 3) Implementing logging and monitoring of webhook events to detect anomalies or unexpected patterns indicative of exploitation attempts. 4) Restricting webhook endpoints to accept requests only from Clerk's known IP ranges or using network-level controls like firewall rules. 5) Conducting regular security audits and penetration testing focused on webhook handling components. 6) Educating development teams on secure webhook processing best practices to avoid similar issues. These measures help reduce risk even if future vulnerabilities arise.