CVE-2026-27860: Dovecot LDAP Auth Bypass Vulnerability

CVE-2026-27860 Overview

CVE-2026-27860 is an LDAP Injection vulnerability affecting Dovecot's LDAP authentication mechanism. When the auth_username_chars configuration parameter is empty, attackers can inject arbitrary LDAP filter expressions into the authentication process. This vulnerability potentially allows attackers to bypass authentication restrictions and probe the underlying LDAP directory structure for sensitive information.

Critical Impact

Attackers can inject malicious LDAP filters to bypass authentication restrictions and enumerate LDAP directory information when auth_username_chars is misconfigured as empty.

Affected Products

• Dovecot LDAP Authentication (versions with empty auth_username_chars configuration)

Discovery Timeline

• 2026-03-27 - CVE CVE-2026-27860 published to NVD
• 2026-03-30 - Last updated in NVD database

Technical Details for CVE-2026-27860

Vulnerability Analysis

This vulnerability stems from improper LDAP distinguished name (DN) escaping in Dovecot's authentication module. The auth_username_chars configuration option defines which characters are allowed in usernames during authentication. When this parameter is left empty (cleared out), Dovecot fails to sanitize user-supplied input before incorporating it into LDAP filter queries.

The vulnerability is classified as CWE-90 (Improper Neutralization of Special Elements used in an LDAP Query), commonly known as LDAP Injection. This weakness occurs when an application constructs LDAP queries using externally-influenced input without properly sanitizing or validating that input.

An attacker who successfully exploits this vulnerability can modify the logic of LDAP queries, potentially bypassing authentication checks, extracting information about the LDAP directory structure, or enumerating valid usernames and attributes within the directory.

Root Cause

The root cause is insufficient input validation when the auth_username_chars parameter is empty. Under normal operation, this parameter restricts which characters are acceptable in usernames, effectively acting as an allowlist filter that prevents special LDAP metacharacters from being processed. When emptied, no character restrictions are enforced, allowing LDAP special characters such as *, (, ), \, and null bytes to pass through to the LDAP query construction.

Attack Vector

The attack is network-accessible and requires no prior authentication or user interaction. However, exploitation complexity is high as it requires the target Dovecot installation to have a specific misconfiguration (auth_username_chars set to empty). The attacker must craft specially formatted username values containing LDAP filter syntax to manipulate the authentication query logic.

Typical exploitation involves submitting malformed username strings during the authentication process. These strings contain LDAP metacharacters and filter expressions designed to alter the query semantics. For example, an attacker might inject filter components that always evaluate to true, or construct queries that return information about existing directory entries.

The Open-Xchange Security Advisory provides additional technical details about this vulnerability.

Detection Methods for CVE-2026-27860

Indicators of Compromise

• Authentication attempts containing LDAP metacharacters such as *, (, ), \, or null bytes in username fields
• Unusual patterns in authentication logs showing usernames with filter syntax like )(uid=* or *)(objectClass=*
• Increased failed authentication attempts followed by successful authentication with anomalous usernames
• LDAP server logs showing unexpected or malformed queries originating from Dovecot

Detection Strategies

• Monitor Dovecot authentication logs for usernames containing special LDAP characters that should normally be filtered
• Implement LDAP query logging on directory servers to identify injection patterns
• Deploy network intrusion detection signatures to flag IMAP/POP3 authentication attempts with suspicious username patterns
• Audit Dovecot configuration files for empty auth_username_chars settings

Monitoring Recommendations

• Enable verbose authentication logging in Dovecot to capture full username strings
• Configure LDAP server auditing to log all bind and search operations from the Dovecot service account
• Set up alerts for authentication anomalies such as unusual query response times or unexpected result counts
• Regularly review Dovecot configuration for security misconfigurations

How to Mitigate CVE-2026-27860

Immediate Actions Required

• Verify that auth_username_chars is not empty in your Dovecot LDAP configuration
• If auth_username_chars is empty, restore the default value or define an explicit character allowlist
• Review authentication logs for evidence of exploitation attempts
• Apply the vendor-provided security patch when available

Patch Information

Open-Xchange has released a security advisory addressing this vulnerability. Administrators should consult the Open-Xchange Security Advisory for specific patch information and updated package versions. Ensure you install the fixed version of Dovecot as recommended by the vendor.

Workarounds

• Ensure auth_username_chars contains an appropriate character set (the default setting provides adequate protection)
• Implement LDAP query parameterization or use dedicated LDAP escaping functions if custom integrations exist
• Consider implementing additional network-level restrictions on LDAP server access from Dovecot hosts
• Deploy a web application firewall (WAF) or authentication proxy that can filter malicious characters before they reach Dovecot

# Configuration example - Ensure auth_username_chars is not empty
# In dovecot-ldap.conf.ext or similar configuration file:

# BAD - leaves system vulnerable to LDAP injection:
# auth_username_chars =

# GOOD - use default or explicit safe character set:
auth_username_chars = abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ01234567890.-_@