CVE-2025-66489: Cal.com Authentication Bypass Vulnerability

CVE-2025-66489 Overview

CVE-2025-66489 is a critical authentication bypass vulnerability in Cal.com, an open-source scheduling software. The flaw exists in the login credentials provider and allows an attacker to bypass password verification when a Time-based One-Time Password (TOTP) code is provided. This vulnerability stems from problematic conditional logic in the authentication flow, potentially enabling unauthorized access to user accounts without valid credentials.

Critical Impact

Attackers can gain unauthorized access to Cal.com user accounts by exploiting flawed authentication logic, completely bypassing password verification when a TOTP code is submitted.

Affected Products

• Cal.com versions prior to 5.9.8
• Self-hosted Cal.com instances running vulnerable versions
• Cloud deployments using affected Cal.com authentication modules

Discovery Timeline

• 2025-12-03 - CVE-2025-66489 published to NVD
• 2026-02-13 - Last updated in NVD database

Technical Details for CVE-2025-66489

Vulnerability Analysis

This authentication bypass vulnerability (CWE-303: Incorrect Implementation of Authentication Algorithm) occurs due to flawed conditional logic in Cal.com's login credentials provider. When a user submits a TOTP code during the authentication process, the system fails to properly enforce password verification as a required step. The conditional checks in the authentication flow contain a logic error that allows the TOTP verification to serve as a standalone authentication mechanism, rather than as a second factor that supplements password verification.

The vulnerability is exploitable remotely without any prior authentication or user interaction. An attacker with knowledge of a target user's username/email and a valid TOTP code (or potentially any TOTP code depending on implementation specifics) can bypass the password requirement entirely. This represents a fundamental breakdown in the multi-factor authentication model, where the "something you know" (password) factor becomes optional rather than mandatory.

Root Cause

The root cause lies in the incorrect implementation of the authentication algorithm within the login credentials provider. The conditional logic evaluating authentication success improperly treats the presence of a TOTP code as sufficient for authentication, failing to verify that password validation has also been completed. This creates a logic shortcut that attackers can exploit to circumvent the expected authentication sequence.

Attack Vector

The attack can be executed remotely over the network with low complexity. An attacker targets the Cal.com login endpoint by submitting authentication requests that include TOTP codes while exploiting the flawed validation logic. The attack requires no privileges on the target system and no user interaction, making it highly exploitable. Due to the changed scope indicated in the vulnerability assessment, successful exploitation can impact resources beyond the vulnerable component itself, potentially affecting other connected systems and user data.

The exploitation involves crafting authentication requests to the login credentials provider that trigger the vulnerable code path. By manipulating the authentication parameters to include TOTP-related fields, attackers can cause the system to skip password verification checks. Technical details of the specific exploitation methodology are available in the GitHub Security Advisory.

Detection Methods for CVE-2025-66489

Indicators of Compromise

• Authentication logs showing successful logins with TOTP verification but abnormal or missing password validation events
• Unusual login patterns such as multiple successful authentications from disparate geographic locations or IP addresses
• Account access from previously unseen devices or user agents following TOTP-based authentication

Detection Strategies

• Monitor authentication logs for login sequences that include TOTP verification without corresponding password validation entries
• Implement anomaly detection for authentication patterns that deviate from normal user behavior
• Review access logs for accounts that may have been compromised through authentication bypass attempts

Monitoring Recommendations

• Enable detailed logging for all authentication events including password and TOTP verification steps
• Set up alerts for authentication anomalies such as impossible travel scenarios or rapid successive logins
• Audit user session activity for signs of unauthorized access following suspicious authentication events

How to Mitigate CVE-2025-66489

Immediate Actions Required

• Upgrade Cal.com to version 5.9.8 or later immediately to address this vulnerability
• Conduct a security audit of authentication logs to identify potential compromise during the exposure window
• Force password resets for accounts showing suspicious authentication activity
• Review and validate all recent account modifications and scheduled events for unauthorized changes

Patch Information

The vulnerability has been fixed in Cal.com version 5.9.8. Organizations running self-hosted Cal.com installations should update to this version or later. The patch corrects the flawed conditional logic in the login credentials provider to ensure password verification is properly enforced regardless of TOTP presence. Refer to the GitHub Security Advisory for detailed patch information.

Workarounds

• If immediate patching is not possible, consider temporarily disabling TOTP authentication until the upgrade can be completed
• Implement additional access controls at the network level to limit exposure of the authentication endpoint
• Enable additional logging and monitoring to detect potential exploitation attempts while awaiting the patch deployment

# Upgrade Cal.com to patched version
# For npm-based installations:
npm update @calcom/cal.com@5.9.8

# Verify the installed version
npm list @calcom/cal.com

# Review authentication logs for suspicious activity
grep -E "login|auth|totp" /var/log/calcom/auth.log | \
  grep -v "password_verified" | \
  tail -n 100