CVE-2026-40459: Pac4j LDAP Injection Vulnerability

CVE-2026-40459 Overview

PAC4J, a popular Java security engine for authentication and authorization, is vulnerable to LDAP Injection in multiple methods. A low-privileged remote attacker can inject crafted LDAP syntax into ID-based search parameters, potentially resulting in unauthorized LDAP queries and arbitrary directory operations. This vulnerability poses significant risk to applications using PAC4J for LDAP-based authentication mechanisms.

Critical Impact

Attackers with low-level access can manipulate LDAP queries to bypass authentication controls, enumerate directory information, and potentially execute arbitrary directory operations leading to unauthorized data access or privilege escalation.

Affected Products

• pac4j pac4j versions prior to 4.5.10
• pac4j pac4j versions 5.x prior to 5.7.10
• pac4j pac4j versions 6.x prior to 6.4.1

Discovery Timeline

• 2026-04-17 - CVE CVE-2026-40459 published to NVD
• 2026-04-20 - Last updated in NVD database

Technical Details for CVE-2026-40459

Vulnerability Analysis

This LDAP Injection vulnerability exists in multiple methods within the PAC4J security library. The flaw stems from insufficient validation and sanitization of user-supplied input that is subsequently incorporated into LDAP search queries. When applications pass user-controlled data to vulnerable PAC4J methods, attackers can inject malicious LDAP filter syntax to alter the intended query behavior.

The vulnerability is classified under CWE-90 (Improper Neutralization of Special Elements used in an LDAP Query), which describes scenarios where applications construct LDAP queries using externally-influenced input without proper neutralization of special LDAP characters or filter syntax.

Root Cause

The root cause of this vulnerability is improper input sanitization in ID-based search parameters within PAC4J's LDAP authentication modules. The library fails to adequately escape or validate special LDAP characters such as *, (, ), \, and NUL before incorporating user input into LDAP filter expressions. This allows attackers to break out of the intended query context and inject arbitrary LDAP filter clauses.

Attack Vector

The attack vector is network-based, requiring the attacker to have low-privileged access to the target application. The attacker crafts malicious input containing LDAP filter metacharacters and injects it through authentication or search parameters handled by vulnerable PAC4J methods. Upon processing, the injected syntax modifies the LDAP query structure, potentially allowing the attacker to:

• Bypass authentication by manipulating filter conditions
• Enumerate sensitive directory information
• Modify or delete directory entries if write permissions are present
• Extract user credentials or other sensitive attributes

The vulnerability can be exploited by submitting specially crafted payloads in username fields, search parameters, or other inputs that are processed by the affected PAC4J methods. Common LDAP injection payloads include filter manipulation using operators like )(, *)(objectClass=*, or attribute extraction techniques. For detailed technical analysis, refer to the CERT PL CVE-2026-40458 Analysis.

Detection Methods for CVE-2026-40459

Indicators of Compromise

• Unusual LDAP query patterns containing special characters like *, )(, or \00 in authentication logs
• Unexpected LDAP search operations returning more results than typical user lookups
• Authentication successes for accounts that should not have access
• LDAP server logs showing malformed or abnormally long filter expressions

Detection Strategies

• Monitor application logs for authentication attempts containing LDAP metacharacters in username or search fields
• Implement Web Application Firewall (WAF) rules to detect common LDAP injection patterns in request parameters
• Enable detailed LDAP server logging and alert on queries with unusual filter complexity or wildcard patterns
• Deploy runtime application self-protection (RASP) solutions to detect and block LDAP injection attempts

Monitoring Recommendations

• Configure SIEM rules to correlate authentication failures followed by successes with similar source IPs
• Monitor for enumeration behavior patterns such as rapid sequential queries against directory services
• Implement anomaly detection for LDAP query volume and response sizes per user session
• Review and audit PAC4J configuration and version across all deployed applications

How to Mitigate CVE-2026-40459

Immediate Actions Required

• Identify all applications using PAC4J for authentication and verify current versions
• Upgrade PAC4J to patched versions: 4.5.10, 5.7.10, or 6.4.1 depending on your major version
• Review application logs for potential prior exploitation attempts
• Implement input validation at the application layer as defense-in-depth

Patch Information

The PAC4J maintainers have released security patches addressing this vulnerability. Organizations should upgrade to the following fixed versions:

• PAC4J 4.x series: Upgrade to version 4.5.10 or later
• PAC4J 5.x series: Upgrade to version 5.7.10 or later
• PAC4J 6.x series: Upgrade to version 6.4.1 or later

For complete patch details and upgrade guidance, consult the Pac4j Security Advisory.

Workarounds

• Implement strict input validation to reject LDAP special characters in user-supplied parameters before they reach PAC4J methods
• Deploy a Web Application Firewall with LDAP injection detection rules as a temporary protective layer
• Restrict LDAP service account permissions to read-only access to limit potential impact
• Consider implementing additional authentication factors to reduce reliance on LDAP-only authentication

# Maven dependency update example for PAC4J 6.x
# Update your pom.xml to use the patched version:
# <dependency>
#     <groupId>org.pac4j</groupId>
#     <artifactId>pac4j-ldap</artifactId>
#     <version>6.4.1</version>
# </dependency>

# Verify current PAC4J version in your project
mvn dependency:tree | grep pac4j