Serialized objects can be altered outside of any Java program unless they are protected using mechanisms such as sealing and signing. (See guideline ENV01-J. Place all security-sensitive code in a single JAR and sign and seal it.) If an object referring to a system resource becomes serialized, and an attacker can alter the serialized form of the object, it becomes possible to modify the system resource that the serialized handle refers to. For example, an attacker may modify a serialized file handle to refer to an arbitrary file on the system. In the absence of a security manager, any operations that use the file handle will be carried out using the attacker-supplied file path and file name.
Noncompliant Code Example
This noncompliant code example declares a serializable File object in the class BadSer. The serialized form of the object exposes the file path, which can be altered. When the object is deserialized, the operations will be performed using the altered path, which can cause the wrong file to be read or modified.
final class BadSer implements Serializable {
File f;
public BadSer() throws FileNotFoundException {
f = new File("c:\\filepath\\filename");
}
}
Compliant Solution (Not Implementing Serializable)
This compliant solution shows a final class Ser that does not implement java.io.Serializable. Consequently, the File object cannot be serialized.
final class Ser {
File f;
public Ser() throws FileNotFoundException {
f = new File("c:\\filepath\\filename");
}
}
Compliant Solution (Object Marked Transient)
This compliant solution declares the File object transient. Consequently, the file path is not exposed.
final class Ser implements Serializable {
transient File f;
public Ser() throws FileNotFoundException {
f = new File("c:\\filepath\\filename");
}
}
Applicability
Deserializing direct handles to system resources can allow the modification of the resources being referred to.
Bibliography
[Oracle 2012] Java Object Serialization