Ldap Injection with Hmac Signatures

LDAP injection in HMAC signature implementations typically occurs when user-supplied data is incorporated into LDAP queries without proper sanitization, allowing attackers to manipulate directory service operations. In HMAC-based authentication systems, this often manifests in user lookup or group membership verification processes.

A common vulnerability pattern appears in user authentication flows where the username from the HMAC payload is directly used in LDAP queries. Consider this vulnerable implementation:

public boolean verifyUser(String username, String password) throws NamingException {
    Hashtable env = new Hashtable<>();
    env.put(Context.INITIAL_CONTEXT_FACTORY, "com.sun.jndi.ldap.LdapCtxFactory");
    env.put(Context.PROVIDER_URL, "ldap://localhost:389/dc=example,dc=com");
    
    // Vulnerable: Direct concatenation of user input
    String searchFilter = "(uid=" + username + ")";
    
    DirContext ctx = new InitialDirContext(env);
    NamingEnumeration results = ctx.search("ou=users", searchFilter, new SearchControls());
    
    if (results.hasMore()) {
        SearchResult result = results.next();
        // Verify HMAC signature using retrieved user data
        return verifyHmac(result.getNameInNamespace(), password);
    }
    return false;
}

The critical vulnerability here is the searchFilter construction. An attacker can craft a username like:

admin*)(|(uid=*

This transforms the LDAP filter into:

(uid=admin*)(|(uid=*)

Which effectively becomes a wildcard search matching any user, potentially bypassing authentication entirely.

Another manifestation occurs in group membership checks for authorization decisions. HMAC signatures often include claims about user roles or permissions, but the verification of these claims might involve LDAP queries:

public boolean hasRole(String username, String role) throws NamingException {
    Hashtable env = new Hashtable<>();
    env.put(Context.INITIAL_CONTEXT_FACTORY, "com.sun.jndi.ldap.LdapCtxFactory");
    env.put(Context.PROVIDER_URL, "ldap://localhost:389/dc=example,dc=com");
    
    // Vulnerable: Role parameter not validated
    String searchFilter = String.format(
        "(uid=%s)(memberOf=cn=%s,ou=groups,dc=example,dc=com)", 
        username, role
    );
    
    DirContext ctx = new InitialDirContext(env);
    NamingEnumeration results = ctx.search("ou=users", searchFilter, new SearchControls());
    
    return results.hasMore();
}

An attacker could supply a role like:

admin)(|(objectClass=*

Creating a filter that matches any object with admin privileges, regardless of actual group membership.

HMAC signature implementations are particularly vulnerable when they use LDAP for token introspection or claim validation. The signature verification might succeed, but the subsequent LDAP query for user attributes or permissions can be manipulated through injection.

Detecting LDAP injection in HMAC signature systems requires examining both the authentication flow and the LDAP query construction patterns. MiddleBrick's black-box scanning approach can identify these vulnerabilities without requiring source code access.

During scanning, middleBrick tests for LDAP injection by submitting payloads that target common LDAP metacharacters and operators. For HMAC endpoints, the scanner first verifies the signature validity using a known-good key, then attempts injection through parameters that typically feed into LDAP queries.

Key detection patterns include:

MiddleBrick specifically tests HMAC endpoints by:

1. Validating the HMAC signature with a known key to ensure the request is properly formed
2. Modifying user identifier parameters in the payload to include LDAP injection patterns
3. Observing response differences that indicate successful query manipulation
4. Checking for timing differences that suggest different query execution paths

For API specifications, middleBrick analyzes the OpenAPI/Swagger definition to identify parameters that might be used in LDAP queries. The scanner looks for:

• Parameters named "username", "uid", "user", "dn", or similar LDAP identifiers
• Authorization endpoints that perform user lookups
• Group or role verification endpoints
• Endpoints with "search", "lookup", or "query" in their paths

The scanner then generates test cases that combine valid HMAC signatures with LDAP injection payloads, monitoring for authentication bypass or privilege escalation indicators.

Remediating LDAP injection in HMAC signature systems requires a defense-in-depth approach. The most effective strategy combines proper input validation, parameterized queries, and principle of least privilege.

The fundamental fix is using parameterized LDAP queries instead of string concatenation. In Java, this means using SearchControls with properly escaped input:

public boolean verifyUser(String username, String password) throws NamingException {
    Hashtable env = new Hashtable<>();
    env.put(Context.INITIAL_CONTEXT_FACTORY, "com.sun.jndi.ldap.LdapCtxFactory");
    env.put(Context.PROVIDER_URL, "ldap://localhost:389/dc=example,dc=com");
    
    // Secure: Use SearchControls with proper escaping
    SearchControls controls = new SearchControls();
    controls.setSearchScope(SearchControls.SUBTREE_SCOPE);
    
    // Use a safe filter pattern
    String searchFilter = "(uid={0})";
    
    DirContext ctx = new InitialDirContext(env);
    
    // The search method automatically escapes special characters
    NamingEnumeration results = ctx.search("ou=users", searchFilter, 
        new String[]{escapeLDAPFilter(username)}, controls);
    
    if (results.hasMore()) {
        SearchResult result = results.next();
        return verifyHmac(result.getNameInNamespace(), password);
    }
    return false;
}

private String escapeLDAPFilter(String input) {
    if (input == null) return "";
    
    StringBuilder escaped = new StringBuilder();
    for (char c : input.toCharArray()) {
        switch (c) {
            case '\u0000': escaped.append("\\00"); break;
            case '(': escaped.append("\\28"); break;
            case ')': escaped.append("\\29"); break;
            case '\': escaped.append("\\\\"); break;
            case '*': escaped.append("\\2a"); break;
            case '/': escaped.append("\\2f"); break;
            default: escaped.append(c); break;
        }
    }
    return escaped.toString();
}

For frameworks that support it, use the java.util.Base64 encoder for binary data and implement a whitelist approach for user identifiers:

public boolean verifyUser(String username, String password) throws NamingException {
    if (!isValidUsername(username)) {
        return false; // Reject invalid formats
    }
    
    // Continue with secure LDAP query
}

private boolean isValidUsername(String username) {
    // Allow only alphanumeric, underscore, and hyphen
    return username != null && username.matches("^[a-zA-Z0-9_-]{1,64}$");
}

In .NET implementations using System.DirectoryServices, use the DirectorySearcher with parameterized filters:

public bool VerifyUser(string username, string password) {
    using (var context = new PrincipalContext(ContextType.Domain)) {
        // Use principal-based authentication when possible
        UserPrincipal user = UserPrincipal.FindByIdentity(context, IdentityType.SamAccountName, username);
        if (user != null) {
            return VerifyHmac(user.DistinguishedName, password);
        }
    }
    return false;
}

For HMAC libraries that integrate with LDAP, ensure the library uses secure query patterns. When implementing custom solutions, always:

1. Use parameterized queries or prepared statements for LDAP operations
2. Implement strict input validation with whitelisting
3. Apply the principle of least privilege to LDAP service accounts
4. Log and monitor for unusual authentication patterns
5. Regularly update LDAP and HMAC libraries to patch known vulnerabilities

MiddleBrick's GitHub Action can be integrated into your CI/CD pipeline to automatically scan HMAC endpoints for LDAP injection vulnerabilities before deployment, ensuring these remediation measures are effective.