- Functional Interface:
- This is a functional interface and can therefore be used as the assignment target for a lambda expression or method reference.
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 eachObjectInputStream.
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/*;!*" ...
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 propertyjdk.serialFilterFactoryor the security propertyjdk.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 construct a filter factory that composes that filter with the static JVM-wide filter and the stream-specific filter, rejecting any classes not handled by those two filters. If a stream specific filter is set and does not accept or reject a class, the combined JVM-wide filter and thread filter is applied. ThedoWithSerialFilter 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) {
// Merge to invoke the thread local filter and then the JVM-wide filter (if any)
filter = ObjectInputFilter.merge(filter, next);
return ObjectInputFilter.rejectUndecidedClass(filter);
}
return (next == null) ? null : ObjectInputFilter.rejectUndecidedClass(next);
} 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 merge to invoke it and then the current filter.
if (next != null) {
return ObjectInputFilter.merge(next, curr);
}
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 useFilterInThread 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.
- Since:
- 9
- See Also:
-
Nested Class Summary
Nested ClassesModifier and TypeInterfaceDescriptionstatic final classA utility class to set and get the JVM-wide deserialization filter factory, the static JVM-wide filter, or to create a filter from a pattern string.static interfaceFilterInfo provides access to information about the current object being deserialized and the status of theObjectInputStream.static enumThe status of a check on the class, array length, number of references, depth, and stream size. -
Method Summary
Modifier and TypeMethodDescriptionstatic ObjectInputFilterallowFilter(Predicate<Class<?>> predicate, ObjectInputFilter.Status otherStatus) Returns a filter that returnsStatus.ALLOWEDif the predicate on the class istrue.checkInput(ObjectInputFilter.FilterInfo filterInfo) Check the class, array length, number of object references, depth, stream size, and other available filtering information.static ObjectInputFiltermerge(ObjectInputFilter filter, ObjectInputFilter anotherFilter) Returns a filter that merges the status of a filter and another filter.static ObjectInputFilterrejectFilter(Predicate<Class<?>> predicate, ObjectInputFilter.Status otherStatus) Returns a filter that returnsStatus.REJECTEDif the predicate on the class istrue.static ObjectInputFilterReturns a filter that invokes a given filter and mapsUNDECIDEDtoREJECTEDfor classes, with some special cases, and otherwise returns the status.
-
Method Details
-
checkInput
Check the class, array length, number of object references, depth, stream size, and other available filtering information. Implementations of this method check the contents of the object graph being created during deserialization. The filter returnsStatus.ALLOWED,Status.REJECTED, orStatus.UNDECIDED.If
filterInfo.serialClass()isnon-null, there is a class to be checked. IfserialClass()isnull, there is no class and the info contains only metrics related to the depth of the graph being deserialized, the number of references, and the size of the stream read.- API Note:
- Each filter implementing
checkInputshould return one of the values ofObjectInputFilter.Status. Returningnullmay result in aNullPointerExceptionor other unpredictable behavior. - Parameters:
filterInfo- provides information about the current object being deserialized, if any, and the status of theObjectInputStream- Returns:
Status.ALLOWEDif accepted,Status.REJECTEDif rejected,Status.UNDECIDEDif undecided.
-
allowFilter
static ObjectInputFilter allowFilter(Predicate<Class<?>> predicate, ObjectInputFilter.Status otherStatus) Returns a filter that returnsStatus.ALLOWEDif the predicate on the class istrue. The filter returnsALLOWEDor theotherStatusbased on the predicate of thenon-nullclass andUNDECIDEDif the class isnull.When the filter's
checkInput(info)method is invoked, the predicate is applied to theinfo.serialClass(), the return Status is:UNDECIDED, if theserialClassisnull,ALLOWED, if the predicate on the class returnstrue,- Otherwise, return
otherStatus.
Example, to create a filter that will allow any class loaded from the platform or bootstrap classloaders.
ObjectInputFilter f = allowFilter(cl -> cl.getClassLoader() == ClassLoader.getPlatformClassLoader() || cl.getClassLoader() == null, Status.UNDECIDED);- Parameters:
predicate- a predicate to test a non-null ClassotherStatus- a Status to use if the predicate isfalse- Returns:
- a filter that returns
ALLOWEDif the predicate on the class istrue - Since:
- 17
-
rejectFilter
static ObjectInputFilter rejectFilter(Predicate<Class<?>> predicate, ObjectInputFilter.Status otherStatus) Returns a filter that returnsStatus.REJECTEDif the predicate on the class istrue. The filter returnsREJECTEDor theotherStatusbased on the predicate of thenon-nullclass andUNDECIDEDif the class isnull. When the filter'scheckInput(info)method is invoked, the predicate is applied to theserialClass(), the return Status is:UNDECIDED, if theserialClassisnull,REJECTED, if the predicate on the class returnstrue,- Otherwise, return
otherStatus.
Example, to create a filter that will reject any class loaded from the application classloader.
ObjectInputFilter f = rejectFilter(cl -> cl.getClassLoader() == ClassLoader.ClassLoader.getSystemClassLoader(), Status.UNDECIDED);- Parameters:
predicate- a predicate to test a non-null ClassotherStatus- a Status to use if the predicate isfalse- Returns:
- returns a filter that returns
REJECTEDif the predicate on the class istrue - Since:
- 17
-
merge
Returns a filter that merges the status of a filter and another filter. Ifanotherfilter isnull, thefilteris returned. Otherwise, afilteris returned to merge the pair ofnon-nullfilters. The filter returned implements thecheckInput(FilterInfo)method as follows:- Invoke
filteron theFilterInfoto get itsstatus; - Return
REJECTEDif thestatusisREJECTED; - Invoke
anotherFilterto get theotherStatus; - Return
REJECTEDif theotherStatusisREJECTED; - Return
ALLOWED, if eitherstatusorotherStatusisALLOWED, - Otherwise, return
UNDECIDED
- Parameters:
filter- a filteranotherFilter- a filter to be merged with the filter, may benull- Returns:
- an
ObjectInputFilterthat merges the status of the filter and another filter - Since:
- 17
- Invoke
-
rejectUndecidedClass
Returns a filter that invokes a given filter and mapsUNDECIDEDtoREJECTEDfor classes, with some special cases, and otherwise returns the status. If the class is not a primitive class and not an array, the status returned isREJECTED. If the class is a primitive class or an array class additional checks are performed; see the list below for details.Object deserialization accepts a class if the filter returns
UNDECIDED. Adding a filter to reject undecided results for classes that have not been either allowed or rejected can prevent classes from slipping through the filter.- Implementation Requirements:
- The filter returned implements the
checkInput(FilterInfo)method as follows:- Invoke the filter on the
FilterInfoto get itsstatus; - Return the
statusif the status isREJECTEDorALLOWED; - Return
UNDECIDEDif thefilterInfo.getSerialClass() serialClassisnull; - Return
REJECTEDif the class is not an array; - Determine the base component type if the
serialClassis an array; - Return
UNDECIDEDif the base component type is a primitive class; - Invoke the filter on the
base component typeto get itscomponent status; - Return
ALLOWEDif the component status isALLOWED; - Otherwise, return
REJECTED.
- Invoke the filter on the
- Parameters:
filter- a filter- Returns:
- an
ObjectInputFilterthat maps anObjectInputFilter.Status.UNDECIDEDstatus toObjectInputFilter.Status.REJECTEDfor classes, otherwise returns the filter status - Since:
- 17
-