In a significant security alert for users of Cal.com, the popular open-source scheduling platform, a critical has been unearthered. This flaw exposed user accounts to unauthorized access, allowing malicious actors to bypass crucial security measures with cleverly crafted fake TOTP (Time-based One-Time Password) codes. For anyone utilizing Cal.com for their scheduling needs, understanding the implications and implementing the patch is paramount.

Understanding the Cal.com Authentication Bypass Vulnerability

The core of this vulnerability lies in a critical flaw within Cal.com’s authentication logic. Tracked as CVE-2025-66489, this severe weakness allowed attackers to circumvent the intended multi-factor authentication (MFA) process. Specifically, the system failed to adequately validate TOTP codes submitted during the login sequence, effectively treating illegitimate codes as valid. This meant that even without knowing a user’s actual TOTP secret, an attacker could potentially generate and submit a fake code that the vulnerable system would accept, granting them unauthorized entry to the user’s account.

This type of vulnerability is particularly dangerous as it undermines a fundamental security layer designed to protect user identities. When MFA, especially TOTP, is compromised, the reliance on a secondary authentication factor becomes meaningless. The implications range from data breaches and identity theft to complete account takeover, leading to significant reputational and operational damage for individuals and organizations.

Affected Versions and Patch Information

According to reports, this critical authentication bypass vulnerability affected Cal.com versions up to 5.9.7. This broad range makes it imperative for all users and administrators running these older versions to prioritize an immediate update. The good news is that the Cal.com development team swiftly addressed the issue, releasing a patch in version 5.9.8.

Users are strongly advised to verify their current Cal.com installation and upgrade to the latest secure version without delay. Failure to do so leaves accounts exposed to the risk of unauthorized access via this easily exploitable flaw.

Remediation Actions and Best Practices

Addressing the Cal.com vulnerability requires immediate action. Here’s a clear breakdown of steps to secure your instance:

• Immediate Upgrade: The most crucial step is to upgrade your Cal.com instance to version 5.9.8 or newer. This version contains the necessary fixes for CVE-2025-66489. Check Cal.com’s official documentation or GitHub repository for detailed upgrade instructions relevant to your deployment.
• Review Logs for Suspicious Activity: After patching, review your Cal.com access logs for any unusual login attempts or account activities that occurred during the period your instance was vulnerable. Look for login attempts that might correspond to the timing of the vulnerability’s discovery or disclosure.
• Implement Strong Account Policies: While this vulnerability bypassed TOTP, it underscores the importance of a layered security approach. Ensure users have strong, unique passwords.
• Educate Users: If applicable, inform your users about the importance of promptly reporting any suspicious activity related to their Cal.com accounts.
• Regular Security Audits: Implement a routine schedule for security audits and penetration testing of your Cal.com deployment, especially after significant updates or configurations.

Tools for Detection and Mitigation

While the primary mitigation is updating Cal.com, here are some general cybersecurity tools that can aid in maintaining a secure environment and detecting potential anomalies.

Tool Name	Purpose	Link
Cal.com Official Updates	Primary source for patches and secure versions.	GitHub Repository
OWASP ZAP	Application security scanner for identifying vulnerabilities in web applications.	OWASP ZAP
Nessus	Vulnerability scanner to identify security issues across networks and applications.	Nessus
Splunk (or similar SIEM)	Security Information and Event Management for log aggregation, analysis, and alerting.	Splunk

Conclusion

The discovery of CVE-2025-66489 in Cal.com highlights the persistent challenges of securing authentication mechanisms. This critical vulnerability, which allowed attackers to bypass authentication using fake TOTP codes in versions up to 5.9.7, serves as a stark reminder of the need for rigorous security practices in any web application. Immediate action to upgrade to Cal.com version 5.9.8 or higher is essential to protect user accounts and maintain the integrity of your scheduling platform. Proactive patching, diligent monitoring, and adherence to security best practices remain the most effective defenses against such threats.