09. Prevent LDAP injection

The Lightweight Directory Access Protocol (LDAP) allows an application to remotely perform operations such as searching and modifying records in directories. LDAP injection results from inadequate input sanitization and validation and allows malicious users to glean restricted information using the directory service.

A whitelist can be used to restrict input to a list of valid characters. Characters and character sequences that must be excluded from whitelists—including Java Naming and Directory Interface (JNDI) metacharacters and LDAP special characters—are listed in the following table.

Characters and Sequences to Exclude from Whitelists

* This is a character sequence

LDAP Injection Example

Consider an LDAP Data Interchange Format (LDIF) file that contains records in the following format:

dn: dc=example,dc=com
objectclass: dcobject
objectClass: organization
o: Some Name
dc: example

dn: ou=People,dc=example,dc=com
ou: People
objectClass: dcobject
objectClass: organizationalUnit
dc: example

dn: cn=Manager,ou=People,dc=example,dc=com
cn: Manager
sn: John Watson
# Several objectClass definitions here (omitted)
userPassword: secret1
mail: john@holmesassociates.com

dn: cn=Senior Manager,ou=People,dc=example,dc=com
cn: Senior Manager
sn: Sherlock Holmes
# Several objectClass definitions here (omitted)
userPassword: secret2
mail: sherlock@holmesassociates.com

A search for a valid user name and password often takes the form

(&(sn=<USERSN>)(userPassword=<USERPASSWORD>))

However, an attacker could bypass authentication by using S* for the USERSN field and * for the USERPASSWORD field. Such input would yield every record whose USERSN field began with S.

An authentication routine that permitted LDAP injection would allow unauthorized users to log in. Likewise, a search routine would allow an attacker to discover part or all of the data in the directory.

Noncompliant Code Example

This noncompliant code example allows a caller of the method searchRecord() to search for a record in the directory using the LDAP protocol. The string filter is used to filter the result set for those entries that match a user name and password supplied by the caller.

// String userSN = "S*"; // Invalid
// String userPassword = "*"; // Invalid
public class LDAPInjection {        
  private void searchRecord(String userSN, String userPassword) throws NamingException {        
    Hashtable<String, String>  env = new Hashtable<String, String>();
    env.put(Context.INITIAL_CONTEXT_FACTORY, "com.sun.jndi.ldap.LdapCtxFactory");
    try {
      DirContext dctx = new InitialDirContext(env);
             
      SearchControls sc = new SearchControls();
      String[] attributeFilter = {"cn", "mail"};
      sc.setReturningAttributes(attributeFilter);
      sc.setSearchScope(SearchControls.SUBTREE_SCOPE);
      String base = "dc=example,dc=com";
 
      // The following resolves to (&(sn=S*)(userPassword=*))      
      String filter = "(&(sn=" + userSN + ")(userPassword=" + userPassword + "))"; 
 
      NamingEnumeration<?> results = dctx.search(base, filter, sc);
      while (results.hasMore()) {
        SearchResult sr = (SearchResult) results.next();
        Attributes attrs = (Attributes) sr.getAttributes();
        Attribute attr = (Attribute) attrs.get("cn");
        System.out.println(attr);
        attr = (Attribute) attrs.get("mail");
        System.out.println(attr);
      }    
      dctx.close();
    } catch (NamingException e) {
      // Forward to handler
    }
  }  
}

When a malicious user enters specially crafted input, as outlined previously, this elementary authentication scheme fails to confine the output of the search query to the information for which the user has access privileges.

Compliant Solution

This compliant solution uses a whitelist to sanitize user input so that the filter string contains only valid characters. In this code, userSN may contain only letters and spaces, whereas a password may contain only alphanumeric characters.

// String userSN = "Sherlock Holmes"; // Valid
// String userPassword = "secret2";   // Valid

// ... beginning of LDAPInjection.searchRecord()...
sc.setSearchScope(SearchControls.SUBTREE_SCOPE);
String base = "dc=example,dc=com";
           
if (!userSN.matches("[\\w\\s]*") || !userPassword.matches("[\\w]*")) {
  throw new IllegalArgumentException("Invalid input");
} 
            	
String filter = "(&(sn = " + userSN + ")(userPassword=" + userPassword + "))";
 
// ... remainder of LDAPInjection.searchRecord()...     

When a database field such as a password must include special characters, it is critical to ensure that the authentic data is stored in sanitized form in the database and also that any user input is normalized before the validation or comparison takes place. Using characters that have special meanings in JNDI and LDAP in the absence of a comprehensive normalization and whitelisting-based routine is discouraged. Special characters must be transformed to sanitized, safe values before they are added to the whitelist expression against which input will be validated. Likewise, normalization of user input should occur before the validation step.

Applicability

Failure to sanitize untrusted input can result in information disclosure and privilege escalation.

Bibliography