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Code is usually signed when it requires more than the default set of permissions to perform some tasks. Although it is usually a bad idea to sign code, some actions necessitate this step. For example, if the application needs to establish an http 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 plugins plug-ins or extensions, a its vendor may provide signed code instead of having the user deal with complex security policiesrather 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 for packaging files to facilitate platform independent deployment. Be it desktop applications, Enterprise Java to package files for platform-independent deployment. JAR files are the preferred means of distribution for Enterprise Java-Beans (EJB), MIDlets (J2ME) or , and Weblogic Server J2EE applications, JAR files are the preferred means of distributionfor 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, however, this should not be interpreted to be the case that the code cannot be misused.. Signing certifies the authenticity of the code, but it cannot guarantee the security of the code.

According to the Java Tutorials [Java Tutorials], Wiki MarkupAccording to the Java Tutorials \[[Tutorials 2008|AA. Bibliography#Tutorials 08]\]

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.")

Depending on how the client code works, signatures may or may not be automatically checked programaticallyClient code may lack programmatic checks of code signatures. For example, any instances of URLClassLoader and its subclasses and java.util.jar automatically verify a signature whenever the JAR file is signed. If however, the developer implements a custom classloader that goes on to subclass java.lang.ClassLoader, this step is not performed automatically. Moreover, signatures of signed JAR files. Developer-implemented custom class loaders may lack this check. Moreover, even in the URLClassLoader case, the automatic verification just involves performs only an integrity check and does not ; it fails to authenticate the loaded class . This is because the check uses a the public key that is 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 and digest valuesalong 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 that , 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 2008]). It creates a JarClassLoader that loads an application update, plug-in, plugin 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://somewebsite.comwww.securecoding.cert.org/software-updates.jar^{Image Removed}) and ; any other arguments specify the arguments that are to be passed to the class to be that is loaded. Reflection is used 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,
    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 If the program expects the user to manually install the new JAR file, the user can explicitly check the signature from 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, an explicit signature verification check must be built within the invoking program . This can be achieved must verify the signature programmatically by obtaining the chain of certificates from the CodeSource of the class being loaded and checking if whether any one of the certificates belongs belong to the a trusted signer whose certificate has been securely obtained beforehand and stored in a local keystore. The invokeClass method can be modified to do this as shown in this compliant solution.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;  // return, do not execute if unsigned;  
  }  

  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");  
  // userCheck iswith the alias trusted public key, else throws exception
  certs[0].verify(pubCert.getPublicKey()); // check with the trusted public key, else throws exception
}

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

The It is not always the case that arbitrary code gets executed. By default, the URLClassLoader and all its subclasses are given by default only given enough permissions to interact with the URL that was specified when the URLClassLoader object was created. This , which means that the program loaded code can interact only interact with the specified host. However, this does not It fails to mitigate the risk completely as , however, because the loaded file code may have been granted appropriate privileges to perform that permit other sensitive operations such as updating an existing local JAR file.

Risk Assessment

Not verifying the Failure to verify a digital signature either , whether manually or programmatically, can lead to result in the execution of malicious code.

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this guideline on the CERT website.

Bibliography

\[[API 2006|AA. Bibliography#API 06]\] 
\[[Gong 2003|AA. Bibliography#Gong 03]\] 12.8.3 jarsigner
\[[Eclipse 2008|AA. Bibliography#Eclipse 08]\] [JAR Signing|http://wiki.eclipse.org/JAR_Signing] and [Signed bundles and protecting against malicious code|http://help.eclipse.org/stable/index.jsp?topic=/org.eclipse.platform.doc.isv/guide]
\[[Halloway 2001|AA. Bibliography#Halloway 01]\] 
\[[Flanagan 2005|AA. Bibliography#Flanagan 05]\] Chapter 24. The java.util.jar Package
\[[Oaks 2001|AA. Bibliography#Oaks 01]\] Chapter 12: Digital Signatures, Signed Classes
\[[Tutorials 2008|AA. Bibliography#Tutorials 08]\] [The JarRunner Class|http://java.sun.com/docs/books/tutorial/deployment/jar/jarrunner.html], [Lesson: API and Tools Use for Secure Code and File Exchanges|http://java.sun.com/docs/books/tutorial/security/sigcert/index.html] and [Verifying Signed JAR Files|http://java.sun.com/docs/books/tutorial/deployment/jar/verify.html]
\[[JarSpec 2008|AA. Bibliography#JarSpec 08]\] Signature Validation 
\[[Bea 2008|AA. Bibliography#Bea 08]\] 
\[[Muchow 2001|AA. Bibliography#Muchow 01]\] 
\[[MITRE 2009|AA. Bibliography#MITRE 09]\] [CWE ID 300|http://cwe.mitre.org/data/definitions/300.html] "Channel Accessible by Non-Endpoint (aka 'Man-in-the-Middle')", [CWE ID 319|http://cwe.mitre.org/data/definitions/319.html] "Cleartext Transmission of Sensitive Information", [CWE ID 494|http://cwe.mitre.org/data/definitions/494.html] "Download of Code Without Integrity Check", [CWE ID 347|http://cwe.mitre.org/data/definitions/347.html] "Improperly Verified Signature"

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

...

Image Added Image Added Image AddedSEC18-J. Define wrappers around native methods      02. Platform Security (SEC)      03. Declarations and Initialization (DCL)