CVE-2025-30189: Authentication Bypass Vulnerability

CVE-2025-30189 Overview

CVE-2025-30189 is an authentication bypass vulnerability affecting Dovecot mail server when caching is enabled. When cache is enabled, some passdb/userdb drivers incorrectly cache all users with the same cache key, causing wrong cached information to be used for these users. After a cached login, all subsequent logins authenticate as the same user, leading to unauthorized access to other users' mailboxes and potential data exposure.

Critical Impact

After an initial cached login occurs, all subsequent authentication attempts may grant access to the wrong user account, allowing unauthorized access to email content, attachments, and sensitive communications.

Affected Products

• Dovecot mail server with caching enabled
• passdb drivers with caching functionality
• userdb drivers with caching functionality

Discovery Timeline

• 2025-10-31 - CVE-2025-30189 published to NVD
• 2026-03-27 - Last updated in NVD database

Technical Details for CVE-2025-30189

Vulnerability Analysis

This vulnerability is classified under CWE-1250 (Improper Preservation of Consistency Between Independent Representations of Shared State). The core issue lies in how Dovecot's authentication system generates and manages cache keys for user authentication data.

When caching is enabled for passdb (password database) or userdb (user database) drivers, the system is designed to store authentication results to improve performance by avoiding repeated lookups. However, certain driver configurations generate identical cache keys for different users, causing a cache collision. Once the first user authenticates and their data is cached, subsequent authentication attempts by other users incorrectly retrieve the cached credentials of the first user.

This cache key collision enables an attacker to gain unauthorized access to other users' mailboxes after any successful cached authentication occurs. The attack requires network access and exploits the flawed caching mechanism without requiring any prior privileges.

Root Cause

The root cause is improper cache key generation in certain passdb/userdb driver configurations. The drivers fail to include sufficient unique user identifiers when constructing cache keys, resulting in multiple distinct users sharing the same cache entry. This violates the fundamental assumption that each user's authentication data should be uniquely identifiable in the cache.

Attack Vector

The attack is network-based and does not require user interaction. An attacker must wait for or trigger a cached login to occur on the target system. Once any user has authenticated and their credentials are cached, subsequent authentication attempts—including those by the attacker—may be validated against the cached credentials of the original user rather than the attacker's actual credentials.

The exploitation scenario involves:

1. A legitimate user authenticates to the Dovecot server, populating the cache
2. Due to the cache key collision, subsequent authentication requests map to the same cache entry
3. An attacker attempting to authenticate may receive the cached authentication result of the first user
4. The attacker gains access to the first user's mailbox and associated data

No publicly available exploits are currently known for this vulnerability. For additional technical details, refer to the Open-Xchange Security Advisory.

Detection Methods for CVE-2025-30189

Indicators of Compromise

• Multiple users accessing the same mailbox from different source IP addresses in rapid succession
• Authentication logs showing successful logins for User A immediately followed by mailbox access patterns inconsistent with User A's normal behavior
• Unusual session patterns where a single cached identity is used across multiple connection sources
• User complaints about receiving emails intended for other users or seeing others' mailbox contents

Detection Strategies

• Monitor Dovecot authentication logs for anomalous patterns where different source IPs authenticate as the same user within short time windows
• Implement alerting on authentication events where the authenticated user's mailbox access patterns deviate significantly from their baseline
• Review passdb/userdb driver configurations to identify which caching configurations may be vulnerable
• Correlate authentication timestamps with cache refresh events to identify potential exploitation windows

Monitoring Recommendations

• Enable verbose logging for Dovecot authentication events including cache hit/miss indicators
• Deploy SIEM rules to detect multiple distinct source IPs authenticating to the same mailbox within configurable time thresholds
• Monitor for sudden changes in user mailbox access patterns that may indicate unauthorized access via cache collision
• Implement real-time alerting for authentication anomalies flagged by security monitoring tools

How to Mitigate CVE-2025-30189

Immediate Actions Required

• Disable caching globally in Dovecot configuration until patches can be applied
• Alternatively, disable caching selectively for the impacted passdb/userdb drivers
• Review authentication logs for signs of exploitation that may have already occurred
• Notify users if evidence of unauthorized mailbox access is discovered

Patch Information

Install the fixed version of Dovecot as recommended in the Open-Xchange Security Advisory. After upgrading, clear all existing authentication caches to ensure no corrupted cache entries remain. Additional discussion is available on the Full Disclosure Mailing List and Openwall OSS-Security Discussion.

Workarounds

• Set auth_cache_size = 0 in dovecot.conf to disable authentication caching entirely
• Configure driver-specific cache disabling for affected passdb/userdb backends
• Implement additional authentication controls such as IP-based restrictions to limit exposure
• Consider deploying network segmentation to restrict mail server access to trusted networks until patches are applied

# Configuration example - Disable authentication caching
# Add to /etc/dovecot/dovecot.conf or /etc/dovecot/conf.d/10-auth.conf

# Disable authentication caching globally
auth_cache_size = 0

# Alternatively, disable caching for specific passdb drivers
passdb {
  driver = ldap
  args = /etc/dovecot/dovecot-ldap.conf.ext
  # Disable caching for this specific driver
  cache_key = 
}