SEC06-J. Do not rely on the default automatic signature verification provided by URLClassLoader and java.util.jar

Code should be signed only if it requires elevated privileges to perform one or more tasks (see ENV00-J. Do not sign code that performs only unprivileged operations for more information). For example, applets are denied the privilege of making HTTP connections to any hosts except the host from which they came. When an applet requires an HTTP connection with an external host to download plug-ins or extensions, its vendor may provide signed code rather than force the user to arbitrarily assign the permissions it requires. Because executing privilege-elevated signed code can be extremely dangerous, verifying the authenticity of its origin is of utmost importance.

Java-based technologies typically use the Java Archive (JAR) feature to package files for platform-independent deployment. JAR files are the preferred means of distribution for Enterprise Java-Beans (EJB), MIDlets (J2ME), and Weblogic Server J2EE applications, for example. The point-and-click installation provided by Java Web Start also relies on the JAR file format for packaging. Vendors sign their JAR files when required. Signing certifies the authenticity of the code, but it cannot guarantee the security of the code.

According to the Java Tutorials [Java Tutorials],

If you are creating applet code that you will sign, it needs to be placed in a JAR file. The same is true if you are creating application code that may be similarly restricted by running it with a security manager. The reason you need the JAR file is that when a policy file specifies that code signed by a particular entity is permitted one or more operations, such as specific file reads or writes, the code is expected to come from a signed JAR file. (The term "signed code" is an abbreviated way of saying "code in a class file that appears in a JAR file that was signed.")

Client code may lack programmatic checks of code signatures. For example, instances of URLClassLoader and its subclasses and java.util.jar automatically verify signatures of signed JAR files. Developer-implemented custom class loaders may lack this check. Moreover, even in the URLClassLoader case, the automatic verification performs only an integrity check; it fails to authenticate the loaded class because the check uses the public key contained within the JAR without validating the public key. The legitimate JAR file may be replaced with a malicious JAR file containing a different public key along with appropriately modified digest values.

The default automatic signature verification process may still be used but is not sufficient. Systems that use the default automatic signature verification process must perform additional checks to ensure that the signature is correct (such as comparing it against a known trusted signature).

Noncompliant Code Example

This noncompliant code example demonstrates the JarRunner application, which can be used to dynamically execute a particular class residing within a JAR file (abridged version of the class in The Java Tutorials [Java Tutorials]). It creates a JarClassLoader that loads an application update, plug-in, or patch over an untrusted network such as the Internet. The URL to fetch the code is specified as the first argument (for example, http://www.securecoding.cert.org/software-updates.jar); any other arguments specify the arguments that are to be passed to the class that is loaded. JarRunner uses reflection to invoke the main() method of the loaded class. Unfortunately, by default, JarClassLoader verifies the signature using the public key contained within the JAR file.

public class JarRunner {
  public static void main(String[] args)
       throws IOException, ClassNotFoundException,
              NoSuchMethodException, InvocationTargetException {
  
    URL url = new URL(args[0]);
    
    // Create the class loader for the application jar file
    JarClassLoader cl = new JarClassLoader(url);
    
    // Get the application's main class name
    String name = cl.getMainClassName();
    
    // Get arguments for the application
    String[] newArgs = new String[args.length - 1];
    System.arraycopy(args, 1, newArgs, 0, newArgs.length);
    
    // Invoke application's main class
    cl.invokeClass(name, newArgs);
  }
}

final class JarClassLoader extends URLClassLoader {
  private URL url;
  public JarClassLoader(URL url) {
    super(new URL[] { url });
    this.url = url;
  }

  public String getMainClassName() throws IOException {
    URL u = new URL("jar", "", url + "!/");
    JarURLConnection uc = (JarURLConnection) u.openConnection();
    Attributes attr = uc.getMainAttributes();
    return attr != null ? 
        attr.getValue(Attributes.Name.MAIN_CLASS) : null;
  }

  public void invokeClass(String name, String[] args)
      throws ClassNotFoundException, NoSuchMethodException,
             InvocationTargetException {
    Class c = loadClass(name);
    Method m = c.getMethod("main", new Class[] { args.getClass() });
    m.setAccessible(true);
    int mods = m.getModifiers();
    if (m.getReturnType() != void.class || !Modifier.isStatic(mods) ||
        !Modifier.isPublic(mods)) {
      throw new NoSuchMethodException("main");
    }
    try {
      m.invoke(null, new Object[] { args });
    } catch (IllegalAccessException e) {
      System.out.println("Access denied");
    }
  }
}

Compliant Solution (jarsigner)

Users can—but usually do not—explicitly check JAR file signatures at the command line. This solution may be adequate for programs that require manual installation of JAR files. Any malicious tampering results in a SecurityException when the jarsigner tool is invoked with the -verify option.

jarsigner -verify signed-updates-jar-file.jar

Compliant Solution (Certificate Chain)

When the local system cannot reliably verify the signature, the invoking program must verify the signature programmatically by obtaining the chain of certificates from the CodeSource of the class being loaded and checking whether any of the certificates belong to a trusted signer whose certificate has been securely obtained beforehand and stored in a local keystore. This compliant solution demonstrates the necessary modifications to the invokeClass() method:

public void invokeClass(String name, String[] args)
    throws ClassNotFoundException, NoSuchMethodException, 
           InvocationTargetException, GeneralSecurityException,
           IOException {
  Class c = loadClass(name);
  Certificate[] certs = 
      c.getProtectionDomain().getCodeSource().getCertificates();
  if (certs == null) {
    // Return, do not execute if unsigned
    System.out.println("No signature!");
    return;  
  }  

  KeyStore ks = KeyStore.getInstance("JKS");
  ks.load(new FileInputStream(System.getProperty(
      "user.home"+ File.separator + "keystore.jks")),
      "loadkeystorepassword".toCharArray());
  // User is the alias
  Certificate pubCert = ks.getCertificate("user");  
  // Check with the trusted public key, else throws exception
  certs[0].verify(pubCert.getPublicKey()); 
}

Because the invokeClass() method now has two additional exceptions in its throws clause, the catch block in the main() method must be altered accordingly.

The URLClassLoader and all its subclasses are given by default only enough permissions to interact with the URL that was specified when the URLClassLoader object was created, which means that the loaded code can interact only with the specified host. It fails to mitigate the risk completely, however, because the loaded code may have been granted privileges that permit other sensitive operations such as updating an existing local JAR file.

Risk Assessment

Failure to verify a digital signature, whether manually or programmatically, can result in the execution of malicious code.

Automated Detection

Automated detection is not feasible in the fully general case. However, an approach similar to Design Fragments [Fairbanks 2007] could assist both programmers and static analysis tools.

Related Guidelines

Bibliography