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When a custom class loader needs to must override the getPermissions() method, the implementation should must consult the default system policy by explicitly invoking the superclass's getPermissions() method before assigning arbitrary permissions to the code source. A custom class loader that ignores the superclass's getPermissions() could load untrusted classes with elevated privileges. ClassLoader is abstract and must not be directly subclassed. 

Noncompliant Code Example

This noncompliant code example shows a fragment of a custom class loader that extends the class URLClassLoader. It overrides the getPermissions() method and but does not call the its superclass's more restrictive getPermissions() method. Note that URLClassLoader's getPermissions() method calls the Policy class's getPermissions() method which, by default, uses the global system-wide policy file to enforce access control. Consequently, a class defined using this custom class loader has permissions that are completely independent of those specified in the system-wide systemwide policy file; in . In effect, the class's permissions override them.

protected PermissionCollection getPermissions(CodeSource cs) {
  PermissionCollection pc = new Permissions();
  // Allow exit from the VM anytime
  pc.add(new RuntimePermission("exitVM"));   //allow exit from the VM anytime
  return pc;
}

Compliant Solution

In this compliant solution, the overridden getPermissions() method calls super.getPermissions(). ConsequentlyAs a result, the default system-wide systemwide security policy is applied , in addition to the custom policy.

protected PermissionCollection getPermissions(CodeSource cs) {
  PermissionCollection pc = super.getPermissions(cs);
  // Allow exit from the VM anytime
  pc.add(new RuntimePermission("exitVM"));
  return pc;
}

Risk Assessment

Failure to consult the default system policy while defining a custom classloader class loader violates the tenets of defensive programming and can result in classes defined with unintended permissions.

Automated Detection

This Violations of this rule can be addressed discovered with a heuristic checker in the style of FindBugs. As with all heuristic checks, achieving a low false-positive rate is essential.

Related Vulnerabilities

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

Bibliography

\[[API 2006|AA. Bibliography#API 06]\] [Class ClassLoader|http://java.sun.com/javase/6/docs/api/java/lang/ClassLoader.html]
\[[Oaks 2001|AA. Bibliography#Oaks 01]\]
\[[Security 2006|AA. Bibliography#Security 06]\]

Android Implementation Details

The java.security package exists on Android for compatibility purposes only, and it should not be used.

Bibliography

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Image Added Image Added Image AddedSEC10-J. Define custom security permissions for fine grained security      14. Platform Security (SEC)      SEC12-J. Do not grant untrusted code access to classes in inaccessible packages