CVE-2026-40263: Note Mark Auth Bypass Vulnerability

CVE-2026-40263 Overview

Note Mark is an open-source note-taking application. A timing attack vulnerability exists in versions 0.19.1 and prior where the login endpoint performs bcrypt password verification only when the supplied username exists, returning immediately for nonexistent usernames. This timing discrepancy allows unauthenticated attackers to enumerate valid usernames by measuring response times, enabling targeted credential attacks.

Critical Impact

Attackers can enumerate valid usernames through timing analysis of login responses, facilitating targeted credential stuffing and brute-force attacks against confirmed user accounts.

Affected Products

• Note Mark versions 0.19.1 and prior

Discovery Timeline

• 2026-04-17 - CVE CVE-2026-40263 published to NVD
• 2026-04-17 - Last updated in NVD database

Technical Details for CVE-2026-40263

Vulnerability Analysis

This vulnerability is classified as CWE-208 (Observable Timing Discrepancy), a type of side-channel attack that exploits measurable differences in system response times. The login endpoint in Note Mark exhibits different execution paths depending on whether a username exists in the system. When a valid username is submitted, the application performs computationally expensive bcrypt password hashing and comparison. However, when an invalid username is provided, the endpoint returns immediately without performing any password verification.

This behavioral difference creates a measurable timing gap that attackers can exploit. Since bcrypt is intentionally designed to be slow (to protect against brute-force attacks on password hashes), the time difference between verifying a password and skipping verification entirely is significant enough to be reliably detected over a network connection.

Root Cause

The root cause is improper handling of the authentication flow where password verification is conditionally executed based on username existence. The application fails to implement constant-time operations for the login process, allowing external observers to distinguish between valid and invalid usernames through response timing measurements.

Attack Vector

The attack is network-accessible and requires no authentication or user interaction. An attacker can systematically submit login requests with various usernames while measuring response times. Usernames that trigger bcrypt verification will exhibit noticeably longer response times compared to invalid usernames that return immediately.

The vulnerability enables reconnaissance for subsequent attacks. Once valid usernames are identified, attackers can focus credential stuffing, password spraying, or targeted phishing campaigns against confirmed accounts rather than wasting resources on non-existent users.

Detection Methods for CVE-2026-40263

Indicators of Compromise

• Unusual volume of failed login attempts from single IP addresses or ranges
• Sequential or rapid-fire login attempts with varying usernames but identical or no passwords
• Login attempt patterns indicating systematic username testing (alphabetical order, dictionary-based)
• Network traffic showing precise timing measurements between requests

Detection Strategies

• Monitor authentication logs for patterns of failed login attempts that span multiple usernames
• Implement rate limiting alerts that trigger on excessive login attempts per IP address
• Deploy anomaly detection for login request timing patterns that suggest automated enumeration
• Correlate authentication failures with subsequent targeted attacks against discovered accounts

Monitoring Recommendations

• Enable detailed logging on authentication endpoints including response timing metrics
• Set up alerting thresholds for repeated authentication failures from individual sources
• Monitor for unusual authentication traffic patterns during off-hours or from unexpected geographic locations

How to Mitigate CVE-2026-40263

Immediate Actions Required

• Upgrade Note Mark to version 0.19.2 or later immediately
• Review authentication logs for evidence of username enumeration attempts
• Consider implementing additional rate limiting on login endpoints
• Audit user accounts that may have been discovered through enumeration for signs of compromise

Patch Information

The vulnerability has been fixed in Note Mark version 0.19.2. The patch ensures constant-time behavior during authentication by performing password verification operations regardless of username validity. Technical details of the fix can be found in the GitHub Commit and the GitHub Security Advisory GHSA-w6m9-39cv-2fwp.

Workarounds

• Implement network-level rate limiting on authentication endpoints to slow enumeration attempts
• Deploy a Web Application Firewall (WAF) with rules to detect and block rapid sequential login attempts
• Consider adding CAPTCHA or similar challenges after a threshold of failed login attempts from the same source
• Use generic error messages that do not differentiate between invalid usernames and incorrect passwords

# Example rate limiting configuration for nginx
# Add to server block to limit login endpoint requests
limit_req_zone $binary_remote_addr zone=login:10m rate=5r/s;

location /api/login {
    limit_req zone=login burst=10 nodelay;
    limit_req_status 429;
    # ... existing proxy/backend configuration
}