Filter classes, array lengths, and graph metrics during deserialization.

Warning: Deserialization of untrusted data is inherently dangerous and should be avoided. Untrusted data should be carefully validated according to the "Serialization and Deserialization" section of the Secure Coding Guidelines for Java SE. Serialization Filtering describes best practices for defensive use of serial filters.

To protect against deserialization vulnerabilities, application developers need a clear description of the objects that can be deserialized by each component or library. For each context and use case, developers should construct and apply an appropriate filter.

Deserialization Filtering Factory and Filters

The parts of deserialization filtering are the filters, composite filters, and filter factory. Each filter performs checks on classes and resource limits to determine the status as rejected, allowed, or undecided. Filters can be composed of other filters and merge or combine their results. The filter factory is responsible for establishing and updating the filter for each ObjectInputStream.

For simple cases, a static JVM-wide filter can be set for the entire application, without setting a filter factory. The JVM-wide filter can be set either with a system property on the command line or by calling Config.setSerialFilter. No custom filter factory needs to be specified, defaulting to the builtin filter factory. The builtin filter factory provides the static JVM-wide filter for each ObjectInputStream.

For example, a filter that allows example classes, allows classes in the java.base module, and rejects all other classes can be set:

As a command line property:
     % java -Djdk.serialFilter="example.*;java.base/*;!*" ...

Or programmatically:
     var filter = ObjectInputFilter.Config.createFilter("example.*;java.base/*;!*")
     ObjectInputFilter.Config.setSerialFilter(filter);

In an application with multiple execution contexts, the application can provide a filter factory to protect individual contexts by providing a custom filter for each. When the stream is constructed, the filter factory is called to identify the execution context from the available information, including the current thread-local state, hierarchy of callers, library, module, and class loader. At that point, the filter factory policy for creating or selecting filters can choose a specific filter or composition of filters based on the context. The JVM-wide deserialization filter factory ensures that a context-specific deserialization filter can be set on every ObjectInputStream and every object read from the stream can be checked.

Invoking the Filter Factory

The JVM-wide filter factory is a function invoked when each ObjectInputStream is constructed and when the stream-specific filter is set. The parameters are the current filter and a requested filter and it returns the filter to be used for the stream. When invoked from the ObjectInputStream constructors, the first parameter is null and the second parameter is the static JVM-wide filter. When invoked from ObjectInputStream.setObjectInputFilter, the first parameter is the filter currently set on the stream (which was set in the constructor), and the second parameter is the filter given to ObjectInputStream.setObjectInputFilter. The current and new filter may each be null and the factory may return null. Note that the filter factory implementation can also use any contextual information at its disposal, for example, extracted from the application thread context, or its call stack, to compose and combine a new filter. It is not restricted to only use its two parameters.

The active deserialization filter factory is either:

• The application specific filter factory set via ObjectInputFilter.Config.setSerialFilterFactory(BinaryOperator) or the system property jdk.serialFilterFactory or the security property jdk.serialFilterFactory.
• Otherwise, a builtin deserialization filter factory provides the static JVM-wide filter when invoked from the ObjectInputStream constructors and replaces the static filter when invoked from ObjectInputStream.setObjectInputFilter(ObjectInputFilter). See getSerialFilterFactory.

Filters

Filters can be created from a pattern string, or based on a predicate of a class to allow or reject classes.

The filter's checkInput(FilterInfo) method is invoked zero or more times while reading objects. The method is called to validate classes, the length of each array, the number of objects being read from the stream, the depth of the graph, and the total number of bytes read from the stream.

Composite filters combine or check the results of other filters. The merge(filter, anotherFilter) filter combines the status value of two filters. The rejectUndecidedClass(filter) checks the result of a filter for classes when the status is UNDECIDED. In many cases any class not ALLOWED by the filter should be REJECTED.

A deserialization filter determines whether the arguments are allowed or rejected and should return the appropriate status: ALLOWED or REJECTED. If the filter cannot determine the status it should return UNDECIDED. Filters should be designed for the specific use case and expected types. A filter designed for a particular use may be passed a class outside of the scope of the filter. If the purpose of the filter is to reject classes then it can reject a candidate class that matches and report UNDECIDED for others. A filter may be called with class equals null, arrayLength equal -1, the depth, number of references, and stream size and return a status that reflects only one or only some of the values. This allows a filter to be specific about the choice it is reporting and to use other filters without forcing either allowed or rejected status.

Filter Model Examples

For simple applications, a single predefined filter listing allowed or rejected classes may be sufficient to manage the risk of deserializing unexpected classes.

For an application composed from multiple modules or libraries, the structure of the application can be used to identify the classes to be allowed or rejected by each ObjectInputStream in each context of the application. The deserialization filter factory is invoked when each stream is constructed and can examine the thread or program to determine a context-specific filter to be applied. Some possible examples:

• Thread-local state can hold the filter to be applied or composed with a stream-specific filter. Filters could be pushed and popped from a virtual stack of filters maintained by the application or libraries.
• The filter factory can identify the caller of the deserialization method and use module or library context to select a filter or compose an appropriate context-specific filter. A mechanism could identify a callee with restricted or unrestricted access to serialized classes and choose a filter accordingly.

Example to filter every deserialization in a thread

This class shows how an application provided filter factory can combine filters to check every deserialization operation that takes place in a thread. It defines a thread-local variable to hold the thread-specific filter, and constructs a filter factory that composes that filter with the static JVM-wide filter and the stream-specific filter. The doWithSerialFilter method does the setup of the thread-specific filter and invokes the application provided Runnable.

 public static final class FilterInThread implements BinaryOperator<ObjectInputFilter> {

     private final ThreadLocal<ObjectInputFilter> filterThreadLocal = new ThreadLocal<>();

     // Construct a FilterInThread deserialization filter factory.
     public FilterInThread() {}

     // Returns a composite filter of the static JVM-wide filter, a thread-specific filter,
     // and the stream-specific filter.
     public ObjectInputFilter apply(ObjectInputFilter curr, ObjectInputFilter next) {
         if (curr == null) {
             // Called from the OIS constructor or perhaps OIS.setObjectInputFilter with no current filter
             var filter = filterThreadLocal.get();
             if (filter != null) {
                 // Wrap the filter to reject UNDECIDED results
                 filter = ObjectInputFilter.rejectUndecidedClass(filter);
             }
             if (next != null) {
                 // Merge the next filter with the thread filter, if any
                 // Initially this is the static JVM-wide filter passed from the OIS constructor
                 // Wrap the filter to reject UNDECIDED results
                 filter = ObjectInputFilter.merge(next, filter);
                 filter = ObjectInputFilter.rejectUndecidedClass(filter);
             }
             return filter;
         } else {
             // Called from OIS.setObjectInputFilter with a current filter and a stream-specific filter.
             // The curr filter already incorporates the thread filter and static JVM-wide filter
             // and rejection of undecided classes
             // If there is a stream-specific filter wrap it and a filter to recheck for undecided
             if (next != null) {
                 next = ObjectInputFilter.merge(next, curr);
                 next = ObjectInputFilter.rejectUndecidedClass(next);
                 return next;
             }
             return curr;
         }
     }

     // Applies the filter to the thread and invokes the runnable.
     public void doWithSerialFilter(ObjectInputFilter filter, Runnable runnable) {
         var prevFilter = filterThreadLocal.get();
         try {
             filterThreadLocal.set(filter);
             runnable.run();
         } finally {
             filterThreadLocal.set(prevFilter);
         }
     }
 }
 

Using the Filter Factory

To use FilterInThread utility create an instance and configure it as the JVM-wide filter factory. The doWithSerialFilter method is invoked with a filter allowing the example application and core classes:

        // Create a FilterInThread filter factory and set
        var filterInThread = new FilterInThread();
        ObjectInputFilter.Config.setSerialFilterFactory(filterInThread);

        // Create a filter to allow example.* classes and reject all others
        var filter = ObjectInputFilter.Config.createFilter("example.*;java.base/*;!*");
        filterInThread.doWithSerialFilter(filter, () -> {
              byte[] bytes = ...;
              var o = deserializeObject(bytes);
        });
 

Unless otherwise noted, passing a null argument to a method in this interface and its nested classes will cause a NullPointerException to be thrown.

     ObjectInputFilter f
         = allowFilter(cl -> cl.getClassLoader() == ClassLoader.getPlatformClassLoader() ||
                       cl.getClassLoader() == null, Status.UNDECIDED);
 

     ObjectInputFilter f = rejectFilter(cl ->
          cl.getClassLoader() == ClassLoader.ClassLoader.getSystemClassLoader(), Status.UNDECIDED);