Unrestricted deserializing from a privileged context allows an attacker to supply crafted input which, upon deserialization, can yield objects that the attacker does not have permissions to construct. Construction of a custom class loader is one example (See SEC07-J. Do not grant untrusted code access to classes existing in forbidden packages and SEC11-J. Do not allow unauthorized construction of classes in forbidden packages).
Noncompliant Code Example
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Consider the default security model of an applet that does not allow access to sun.util.calendar.ZoneInfo because all classes within the "sun" package are treated as untrusted. As a result, prior to JDK 1.6 u11, the acceptable method for an unsigned applet to deserialize a Zoneinfo object was to execute the call from a privileged context, such as a doPrivileged() block. This constitutes a vulnerability because there is no guaranteed method of knowing whether the serialized stream contains a Zoneinfo object and not a malicious serializable class. The vulnerable code casts the malicious object to the ZoneInfo type which typically causes a ClassCastException. This exception however, is of little consequence as it is possible to store a reference to the newly created object in some static context so that the garbage collector does not act upon it.
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