CVE-2026-27575: Vikunja Authentication Bypass Vulnerability

CVE-2026-27575 Overview

CVE-2026-27575 is a critical weak password requirements and session persistence vulnerability in Vikunja, an open-source self-hosted task management platform. Prior to version 2.0.0, the application allows users to set weak passwords (e.g., 1234, password) without enforcing minimum strength requirements. Additionally, active sessions remain valid after a user changes their password, enabling attackers who compromise an account via brute-force or credential stuffing to maintain persistent access even after the victim resets their password.

Critical Impact

An attacker who gains unauthorized access through weak credentials can maintain persistent access to a victim's account indefinitely, even after password changes, potentially exposing sensitive task management data and enabling further malicious actions.

Affected Products

• Vikunja versions prior to 2.0.0

Discovery Timeline

• 2026-02-25 - CVE CVE-2026-27575 published to NVD
• 2026-02-25 - Last updated in NVD database

Technical Details for CVE-2026-27575

Vulnerability Analysis

This vulnerability combines two distinct security weaknesses that together create a significant risk for Vikunja deployments. The first issue (CWE-521: Weak Password Requirements) stems from the application's failure to enforce minimum password complexity requirements during account creation and password changes. Users can set trivially weak passwords such as 1234 or password, making accounts highly susceptible to brute-force and credential stuffing attacks.

The second, more concerning issue involves improper session management. When a user changes their password—typically done after discovering or suspecting unauthorized access—the application fails to invalidate existing authenticated sessions. This means an attacker who has already established a session through compromised credentials retains full access to the account, completely negating the protective action taken by the legitimate user.

Root Cause

The root cause lies in the absence of password policy enforcement within Vikunja's authentication module and inadequate session lifecycle management. The password validation logic does not check for minimum length, complexity requirements, or dictionary-based weakness before accepting user-provided passwords. Furthermore, the session management system lacks integration with password change events, failing to trigger session invalidation when credentials are updated.

Attack Vector

The attack requires network access to the Vikunja instance. An attacker can exploit this vulnerability through the following attack flow:

1. Initial Access: The attacker targets user accounts with weak passwords using automated brute-force or credential stuffing techniques against the Vikunja login endpoint
2. Session Establishment: Upon successful authentication with compromised credentials, the attacker establishes a valid session
3. Persistence: Even if the legitimate user detects the compromise and changes their password, the attacker's existing session remains valid
4. Continued Access: The attacker maintains unauthorized access to the victim's account, tasks, projects, and potentially sensitive data stored in the platform

The network-based attack vector combined with no required privileges or user interaction makes this vulnerability particularly dangerous for internet-exposed Vikunja instances. For more technical details, refer to the GitHub Security Advisory.

Detection Methods for CVE-2026-27575

Indicators of Compromise

• Multiple failed login attempts followed by successful authentication from unusual IP addresses or geographic locations
• Active sessions originating from IP addresses different from the user's typical access patterns
• Sessions that persist beyond expected timeframes or remain active after password change events
• Anomalous API activity or data access patterns from authenticated sessions

Detection Strategies

• Implement login attempt monitoring to detect brute-force and credential stuffing attacks against user accounts
• Deploy behavioral analytics to identify sessions exhibiting unusual access patterns or geographic anomalies
• Monitor authentication logs for successful logins following high volumes of failed attempts
• Cross-reference active session data with password change timestamps to identify potentially compromised persistent sessions

Monitoring Recommendations

• Enable detailed authentication logging including IP addresses, user agents, and timestamps for all login events
• Configure alerting for multiple failed login attempts targeting the same account within short time windows
• Implement session auditing to track all active sessions and their creation timestamps relative to password changes
• Review access logs regularly for signs of credential-based attacks against the Vikunja instance

How to Mitigate CVE-2026-27575

Immediate Actions Required

• Upgrade Vikunja to version 2.0.0 or later immediately to address both the weak password and session persistence issues
• Force password resets for all user accounts, requiring strong passwords that meet updated complexity requirements
• Invalidate all existing user sessions following the upgrade to ensure any potentially compromised sessions are terminated
• Audit user accounts for signs of compromise or unauthorized access prior to the upgrade

Patch Information

The vulnerability is fixed in Vikunja version 2.0.0. Organizations should upgrade to this version or later to remediate the vulnerability. For complete release information, see the Vikunja v2.0.0 Release Changelog. The security advisory is available at the GitHub Security Advisory.

Workarounds

• Implement network-level access controls to limit exposure of the Vikunja instance to trusted networks only
• Deploy a Web Application Firewall (WAF) with rate limiting to mitigate brute-force and credential stuffing attempts
• Enforce strong password policies at the organizational level and educate users on password security best practices
• Consider implementing additional authentication factors (MFA/2FA) if supported by your deployment configuration

# Example: Configure reverse proxy rate limiting for authentication endpoint
# nginx configuration snippet
location /api/v1/login {
    limit_req zone=login_limit burst=5 nodelay;
    proxy_pass http://vikunja_backend;
}