CVE-2026-24031: Dovecot Auth Bypass Vulnerability

CVE-2026-24031 Overview

CVE-2026-24031 is a SQL Injection vulnerability affecting Dovecot's SQL-based authentication mechanism. When administrators clear the auth_username_chars configuration option, attackers can bypass authentication for any user and perform user enumeration attacks. This vulnerability exposes mail server infrastructure to unauthorized access, potentially compromising sensitive email communications and user credentials.

Critical Impact

Authentication bypass allowing unauthorized access to any user account and enabling user enumeration on affected Dovecot mail servers.

Affected Products

• Dovecot (versions with SQL-based authentication where auth_username_chars is cleared)

Discovery Timeline

• 2026-03-27 - CVE CVE-2026-24031 published to NVD
• 2026-03-30 - Last updated in NVD database

Technical Details for CVE-2026-24031

Vulnerability Analysis

This vulnerability stems from improper input validation in Dovecot's SQL-based authentication mechanism when the auth_username_chars configuration directive is cleared by an administrator. The auth_username_chars setting defines which characters are allowed in usernames, serving as a critical security control that prevents malicious input from reaching SQL queries.

When this safeguard is removed, attackers can inject SQL metacharacters through the username field during authentication attempts. This creates a classic SQL injection condition where user-supplied input is passed directly to database queries without proper sanitization.

The attack is network-accessible and requires no authentication or user interaction, though exploitation complexity is considered high due to the specific configuration requirement. Successful exploitation enables complete authentication bypass, allowing attackers to impersonate any user on the mail server. Additionally, the vulnerability facilitates user enumeration, enabling reconnaissance activities that could support further attacks.

Root Cause

The root cause is CWE-89: SQL Injection. When administrators clear the auth_username_chars configuration option, Dovecot no longer restricts which characters can appear in usernames. This removes a critical input validation layer that normally prevents SQL injection attacks. Without this character restriction, SQL metacharacters such as single quotes, semicolons, and comment delimiters can be included in username inputs and subsequently reach the underlying SQL authentication queries.

Attack Vector

The attack is conducted over the network against Dovecot's authentication interfaces (IMAP, POP3, or other mail protocols). An attacker submits specially crafted username values containing SQL injection payloads during the authentication process. Since auth_username_chars has been cleared, these malicious inputs bypass the normal character filtering and are passed to SQL queries responsible for user authentication verification.

The attacker can manipulate these queries to:

1. Bypass authentication entirely by altering query logic to always return true
2. Enumerate valid users by observing differences in server responses
3. Potentially extract sensitive data from the authentication database depending on the SQL backend configuration

The vulnerability requires no prior authentication and can be exploited remotely. However, the specific misconfiguration (clearing auth_username_chars) must be present on the target system.

Detection Methods for CVE-2026-24031

Indicators of Compromise

• Unusual authentication attempts with special characters, SQL keywords, or escape sequences in username fields
• Unexpected successful authentications for accounts that should have failed
• Database logs showing malformed or suspicious SQL queries originating from Dovecot authentication
• Multiple rapid authentication attempts from single IP addresses testing various username payloads

Detection Strategies

• Monitor Dovecot authentication logs for usernames containing SQL metacharacters (single quotes, semicolons, double dashes, OR, AND, UNION)
• Implement database query logging to detect anomalous authentication query patterns
• Deploy Web Application Firewall (WAF) or mail gateway rules to detect SQL injection patterns in authentication traffic
• Configure intrusion detection systems (IDS) to alert on SQL injection signatures in mail protocol traffic

Monitoring Recommendations

• Enable verbose logging on Dovecot authentication processes to capture detailed login attempt information
• Audit the auth_username_chars configuration setting regularly to ensure it has not been inadvertently cleared
• Monitor database authentication tables for unexpected entries or access patterns
• Implement alerting for authentication anomalies such as successful logins from unusual geographic locations or at unusual times

How to Mitigate CVE-2026-24031

Immediate Actions Required

• Verify that auth_username_chars is properly configured and not cleared in your Dovecot configuration
• If auth_username_chars must be cleared for operational reasons, prioritize installing the latest patched version of Dovecot
• Review authentication logs for signs of exploitation attempts or successful unauthorized access
• Consider temporarily restricting network access to mail authentication services while remediation is in progress

Patch Information

Install the latest fixed version of Dovecot that addresses this vulnerability. Refer to the Open-Xchange Security Advisory for specific version information and official guidance on patching affected installations.

Workarounds

• Ensure auth_username_chars is properly configured with a restrictive character set (do not clear this setting)
• If using passdb SQL, implement additional input validation at the database level using prepared statements or parameterized queries
• Apply network-level access controls to limit which IP addresses can connect to mail authentication services
• Consider implementing additional authentication layers such as multi-factor authentication to reduce the impact of credential compromise

# Configuration example
# Verify auth_username_chars is set in dovecot.conf or auth configuration
# Do NOT clear this setting - ensure it contains a restrictive character set
# Example of proper configuration:
auth_username_chars = abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789.-_@