Software vulnerability reports and reports of software exploitations continue to grow at an alarming rate, and a significant number of these reports result in technical security alerts. To address this growing threat to the government, corporations, educational institutions, and individuals, systems must be developed that are free of software vulnerabilities.
Coding errors cause the majority of software vulnerabilities. For example, 64 percent of the nearly 2,500 vulnerabilities in the National Vulnerability Database in 2004 were caused by programming errors [[Heffley 2004]].
Java is a relatively secure language: there is no explicit pointer manipulation; array and string bounds are automatically checked; attempts at referencing a null pointer are trapped; the arithmetic operations are well defined and platform independent, as are the type conversions. The built-in bytecode verifier ensures that these checks are always in place.
Moreover, there are comprehensive, fine-grained security mechanisms available in Java that can control access to individual files, sockets, and other sensitive resources. To take advantage of the security mechanisms, the Java Virtual Machine (JVM) must have a
security manager in place. This is an ordinary Java object of class java.lang.SecurityManager (or a subclass) that can be put in place programmatically but is more usually specified via a command line parameter.
There are, however, ways in which Java program safety can be compromised. The remainder of this chapter describes misuse cases under which Java programs might be exploited, and examples of guidelines which mitigate against these attacks. Not all of the rules apply to all Java language programs; frequently their applicability depend upon how the software is deployed and your assumptions concerning trust.
Input Validation and Data Sanitization
Leaking Sensitive Data
A system's security policy determines which information is sensitive. Sensitive data may include user information such as social security or credit card numbers, passwords, or private keys.
Java software components provide many opportunities to output sensitive information. Rules that address the mitigation of sensitive information disclosure include:
Type Safety
Java is believed to be a type-safe language [[LSOD 02]]. For that reason, it should not be
possible to compromise a Java program by misusing the type system. To see why type safety
is so important, consider the following types:
public class TowerOfLondon {
private Treasure theCrownJewels;
...
}
public class GarageSale {
public Treasure myCostumeJewerly;
...
}
If these two types could be confused, it would be possible to access the private field theCrownJewels as if it were the public field myCostumeJewerly. More generally, a type confusion attack could allow Java security to be compromised by making the internals of the security manager open to abuse. A team of researchers at Princeton University showed that any type confusion in Java could be used to completely overcome Javaâs security mechanisms (see Securing Java Ch. 5, Sec. 7 [[McGraw 1999]]).
Java’s type safety means that fields that are declared private or protected or that have default (package) protection should not be globally accessible. However, there are a number of vulnerabilities “built in†to Java that enable this protection to be overcome. These should come as no surprise to the Java expert, as they are well documented, but they may trap the unwary.
Public Fields
A field that is declared public may be directly accessed by any part of a Java program and may be modified from anywhere in a Java program (unless the field is declared final). Clearly, sensitive information must not be stored in a public field, as it could be
compromised by anyone who could access the JVM running the program.
Inner Classes
Inner classes have access to all the fields of their surrounding class. There is no bytecode support for inner classes, so they are compiled into ordinary classes with names like OuterClass$InnerClass. So that the inner class can access the private fields of the
outer class, the private access is changed to package access in the bytecode. For that reason, handcrafted bytecode can access these private fields (see âSecurity Aspects in Java Bytecode Engineeringâ [[Schoenefeld 04]] for an example).
Serialization
Serialization enables the state of a Java program to be captured and written out to a byte stream [[Sun 04b]]. This allows for the state to be preserved so that it can be reinstated (by deserialization). Serialization also allows for Java method calls to be transmitted over a network for Remote Method Invocation (RMI). An object (called someObject below) can be serialized as follows:
ObjectOutputStream oos = new ObjectOutputStream ( new FileOutputStream (âSerialOutputâ) ); oos.writeObject (someObject); oos.flush ( );
The object can be deserialized as follows:
ObjectInputStream ois = new ObjectInputStream ( new FileInputStream (âSerialOutputâ) ); someObject = (SomeClass)ois.readObject ( );
Serialization captures all the fields of a class, provided the class implements the Serializable interface, including the non-public fields that are not normally accessible (unless the field is declared transient). If the byte stream to which the serialized values are written is readable, then the values of the normally inaccessible fields may be read. Moreover, it may be possible to modify or forge the preserved values so that when the class is deserialized, the values become corrupted.
Introducing a security manager does not prevent the normally inaccessible fields from being serialized and deserialized (although permission must be granted to write to and read from the file or network if the byte stream is being stored or transmitted). Network traffic (including RMI) can be protected, however, by using SSL.
Reflection
Reflection enables a Java program to analyze and modify itself. In particular, a program can find out the values of field variables and change them [[Forman 05], [Sun 02]. The Java reflection API includes a method call that enables fields that are not normally accessible to be accessed under reflection. The following code prints out the names and values of all fields of an object someObject of class SomeClass:
Field [ ] fields = SomeClass.getDeclaredFields( );
for (Field fieldsI : fields) {
if ( !Modifier.isPublic (fieldsI.getModifiers( )) ) {
fieldsI.setAccessible (true);
}
System.out.print (âField: â + fieldsI.getName( ));
System.out.println (â, value: â + fieldsI.get (someObject));
}
A field could be set to a new value as follows:
String newValue = reader.readLine ( ); fieldsI.set (someObject, returnValue (newValue, fieldsI.getType ( )) );
Introducing the default security manager does prevent the fields that would not normally be accessible from being accessed under reflection. The default security manager throws java.security.AccessControlException in these circumstances. However, it is
possible to grant a permission to override this default behavior: java.lang.reflect.ReflectPermission can be granted with action suppressAccessChecks.
The JVM Tool Interface
Java 5 introduced the JVM Tool Interface (JVMTI) [[Sun 04d]], replacing both the JVM Profiler Interface (JVMPI) and the JVM Debug Interface (JVMDI), which are now deprecated.
The JVMTI contains extensive facilities to find out about the internals of a running JVM, including facilities to monitor and modify a running Java program. These facilities are rather low level and require the use of the Java Native Interface (JNI) and C Language
programming. However, they provide the opportunity to access fields that would not normally be accessible. Also, there are facilities that can change the behavior of a running Java program (for example, threads can be suspended or stopped).
The JVMTI works by using agents that communicate with the running JVM. These agents must be loaded at JVM startup and are usually specified via one of the command line options {{–agentlib:}} or {{–agentpath:}}. However, agents can be specified in environment
variables, although this feature can be disabled where security is a concern. The JVMTI is always enabled, and JVMTI agents may run under the default security manager without requiring any permissions to be granted. More work needs to be done to determine under
exactly what circumstances the JVMTI can be misused.
Debugging
The Java Platform Debugger Architecture (JPDA) builds on the JVMTI and provides highlevel facilities for debugging running Java systems [[JPDA 2004]]. These include facilities similar to the reflection facilities described above for inspecting and modifying field values. In
particular, there are methods to get and set field and array values. Access control is not enforced so, for example, even the values of private fields can be set.
Introducing the default security manager means that various permissions must be granted in order for debugging to take place. The following policy file was used to run the JPDS Trace demonstration under the default security manager:
grant {
permission java.io.FilePermission "traceoutput.txt", "read,write";
permission java.io.FilePermission "C:/Program Files/Java/jdk1.5.0_04/lib/tools.jar", "read";
permission java.io.FilePermission "C:/Program", "read,execute";
permission java.lang.RuntimePermission "modifyThread";
permission java.lang.RuntimePermission "modifyThreadGroup";
permission java.lang.RuntimePermission "accessClassInPackage.sun.misc";
permission java.lang.RuntimePermission "loadLibrary.dt_shmem";
permission java.util.PropertyPermission "java.home", "read";
permission java.net.SocketPermission "<localhost>", "resolve";
permission com.sun.jdi.JDIPermission "virtualMachineManager";
};
Monitoring and Management
Java contains extensive facilities for monitoring and managing a JVM [[JMX 2006]]. In particular, the Java Management Extension (JMX) API enables the monitoring and control of class loading, thread state and stack traces, deadlock detection, memory usage, garbage
collection, operating system information, and other operations [[Sun 04a]]. There are also facilities for logging monitoring and management. A running JVM may be monitored and managed remotely.
For a JVM to be monitored and managed remotely, it must be started with various system properties set (either on the command line or in a configuration file). Also, there are provisions for the monitoring and management to be done securely (by passing the information using SSL, for example) and to require proper authentication of the remote server. However, users may start a JVM with remote monitoring and management enabled with no security for their own purposes, and this would leave the JVM open to compromise
from outsiders. Although a user could not easily turn on remote monitoring and management by accident, they might not realize that starting a JVM so enabled, without any security also switched on, could leave their JVM exposed to outside abuse.