CVE-2025-68621: Trilium Notes Auth Bypass Vulnerability

CVE-2025-68621 Overview

CVE-2025-68621 is a critical timing attack vulnerability affecting Trilium Notes, an open-source, cross-platform hierarchical note-taking application designed for building large personal knowledge bases. Prior to version 0.101.0, the sync authentication endpoint is susceptible to a timing attack that allows unauthenticated remote attackers to recover HMAC authentication hashes byte-by-byte through statistical timing analysis. This vulnerability enables complete authentication bypass without requiring password knowledge, ultimately granting full read/write access to a victim's knowledge base.

Critical Impact

Attackers can bypass authentication entirely and gain complete read/write access to victim knowledge bases without any password knowledge through timing-based HMAC hash recovery.

Affected Products

• Trilium Notes versions prior to 0.101.0
• TriliumNext/Trilium (all versions before the security fix)
• Self-hosted and local Trilium Notes installations with sync functionality enabled

Discovery Timeline

• 2026-02-06 - CVE CVE-2025-68621 published to NVD
• 2026-02-09 - Last updated in NVD database

Technical Details for CVE-2025-68621

Vulnerability Analysis

This vulnerability falls under the category of Timing Attack (CWE-208: Observable Timing Discrepancy). The flaw exists in Trilium's sync authentication mechanism, which uses HMAC-based authentication to verify sync requests between clients and servers. The authentication comparison operation does not use constant-time comparison, creating an observable timing discrepancy that attackers can exploit.

When comparing HMAC hashes, the vulnerable implementation returns early upon encountering the first mismatched byte. This behavior creates measurable timing differences that vary based on how many bytes of the attacker's guessed hash match the actual authentication hash. By systematically testing each possible byte value and measuring response times with statistical precision, an attacker can determine the correct HMAC hash one byte at a time.

The network-accessible nature of this vulnerability means that any Trilium Notes instance with sync functionality exposed to untrusted networks is potentially at risk. Successful exploitation grants attackers complete access to the victim's entire knowledge base, including the ability to read sensitive notes and modify or delete content.

Root Cause

The root cause is the use of non-constant-time string comparison when validating HMAC authentication tokens in the sync endpoint. Standard string comparison functions in most programming languages perform byte-by-byte comparison and return immediately upon finding a mismatch. This optimization, while efficient for general string operations, creates exploitable timing side-channels in cryptographic contexts.

The HMAC comparison should use a constant-time comparison function that always takes the same amount of time regardless of how many bytes match, preventing attackers from inferring information about the secret hash through timing measurements.

Attack Vector

The attack is network-based and requires no authentication or user interaction. An attacker with network access to the Trilium Notes sync endpoint can perform the following attack sequence:

1. Initial Setup: The attacker identifies a Trilium Notes instance with sync functionality enabled and accessible over the network
2. Timing Measurement: The attacker sends crafted authentication requests with varying HMAC hash guesses, measuring the response time for each request with high precision
3. Statistical Analysis: By collecting multiple timing samples for each byte position, the attacker uses statistical analysis to identify which byte values produce slightly longer response times (indicating more matching prefix bytes)
4. Byte-by-Byte Recovery: Starting from the first byte, the attacker determines each correct byte value sequentially until the complete HMAC hash is recovered
5. Authentication Bypass: With the recovered HMAC hash, the attacker can authenticate to the sync endpoint and gain full read/write access to the victim's knowledge base

For detailed technical information about this vulnerability, refer to the GitHub Security Advisory GHSA-hxf6-58cx-qq3x.

Detection Methods for CVE-2025-68621

Indicators of Compromise

• Unusual patterns of rapid, repeated requests to the sync authentication endpoint from a single source
• High volume of failed authentication attempts with systematically varying hash values
• Network traffic analysis showing statistically unusual request timing patterns
• Unexpected successful sync connections from unrecognized IP addresses or clients

Detection Strategies

• Implement request rate limiting and monitoring on sync authentication endpoints to detect brute-force timing attacks
• Deploy network intrusion detection systems (NIDS) with rules to identify rapid sequential authentication attempts
• Enable detailed logging on Trilium Notes sync endpoints to capture authentication request metadata including source IP, timing, and failure reasons
• Monitor for successful authentication events from unexpected geographic locations or IP ranges

Monitoring Recommendations

• Establish baseline metrics for normal sync authentication traffic patterns and alert on statistical anomalies
• Configure real-time alerting for authentication failure rate spikes on sync endpoints
• Review authentication logs regularly for patterns consistent with byte-by-byte timing analysis attacks
• Implement application performance monitoring (APM) to detect unusual response time patterns in authentication flows

How to Mitigate CVE-2025-68621

Immediate Actions Required

• Upgrade all Trilium Notes installations to version 0.101.0 or later immediately
• Restrict network access to sync endpoints using firewall rules or VPN requirements until patching is complete
• Review sync authentication logs for any suspicious activity that may indicate prior exploitation attempts
• Consider rotating sync authentication credentials after upgrading as a precautionary measure

Patch Information

The vulnerability has been addressed in Trilium Notes version 0.101.0. The fix implements constant-time comparison for HMAC authentication validation, eliminating the timing side-channel. Details about the patch implementation can be found in GitHub Pull Request #8129. Users should upgrade to this version or later to remediate the vulnerability.

Workarounds

• Disable sync functionality entirely if not required until patching can be completed
• Place Trilium Notes instances behind a VPN or private network to restrict access to trusted clients only
• Implement a reverse proxy with aggressive rate limiting on the sync endpoint to make timing analysis infeasible
• Use network segmentation to isolate Trilium Notes instances from untrusted network segments

# Example: Restrict sync endpoint access using iptables
# Only allow sync connections from trusted IP ranges
iptables -A INPUT -p tcp --dport 8080 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 8080 -j DROP