A logic error in the popular scheduling platform allowed attackers to bypass password and multi-factor authentication checks entirely, highlighting a growing trend of authentication bypass vulnerabilities.
NEW YORK - A critical security vulnerability has been discovered in Cal.com, the widely used open-source scheduling infrastructure, which allowed attackers to bypass authentication mechanisms completely. By leveraging a logic flaw in the platform's handling of Time-based One-Time Passwords (TOTP), malicious actors could gain unauthorized access to user accounts simply by submitting fake verification codes.
The disclosure comes amidst a surge of high-profile authentication bypass incidents affecting major enterprise software providers. This specific flaw, affecting versions prior to 5.9.8, strikes at the core of trust in Two-Factor Authentication (2FA) systems, demonstrating how implementation errors can render robust security protocols useless.
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The vulnerability resides within the application's authentication library, specifically in the file packages/features/auth/lib/next-auth-options.ts. According to reports from Cyber Security News, the flawed code failed to properly verify both passwords and TOTP codes during the login process.
Security researchers at GBHackers detailed that the vulnerability created specific attack scenarios where the system's logic broke down. In the primary scenario, attackers could bypass both password and TOTP verification simultaneously. By submitting a target's email address alongside any non-empty value in the totpCode field, the system effectively skipped the credential check.
"A flaw in the login credentials provider allows an attacker to bypass password verification when a TOTP code is provided... This issue exists due to problematic conditional logic in the authentication flow." - CVE Details
This "problematic conditional logic" meant that the presence of a TOTP code-regardless of its validity-was interpreted by the system as a signal to bypass the standard password validation, without adequately verifying the TOTP code itself. This effectively opened a backdoor for anyone who knew a user's email address.
The Cal.com incident is not an isolated event but part of a troubling trend in 2024 and 2025 where authentication mechanisms have been the primary point of failure. Just days prior, The Hacker News reported that vendors like Fortinet, Ivanti, and SAP issued urgent patches for similar authentication bypass flaws.
Furthermore, data from Palo Alto Networks highlights a recent CVE (CVE-2025-0108) involving an authentication bypass in their PAN-OS management interface. These repeated failures suggest that while encryption standards remain strong, the logic governing *who* gets access is becoming increasingly fragile in complex software stacks.
For businesses using Cal.com, the risks extend beyond simple unauthorized access. Scheduling platforms often house sensitive data, including client contact information, meeting agendas, and integration tokens for other services like Zoom or Google Calendar. An attacker gaining entry could not only scrape confidential data but also launch spear-phishing attacks from a legitimate internal account.
Experts argue that this vulnerability highlights the double-edged sword of open-source software. While transparency allows for rapid identification of bugs by the community, it also exposes code structures to potential attackers analyzing the repository for logical gaps. Teamwin reports emphasize that malicious actors can exploit these gaps to "bypass crucial security measures with cleverly crafted" inputs.
Cal.com has addressed the issue in version 5.9.8. Organizations running self-hosted instances are urged to update immediately. The incident serves as a stark reminder of the necessity for rigorous code reviews, particularly regarding authentication logic.
Looking ahead, reliance on traditional TOTP implementation is likely to be scrutinized more heavily. With vulnerabilities appearing in how these codes are processed-rather than the cryptographic strength of the codes themselves-the industry may accelerate the shift toward hardware keys and FIDO2 standards, which remove much of the conditional logic complexity that leads to these bypasses.
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