SEC50-J. Avoid granting excess privileges

A Java security policy grants permissions to code to allow access to specific system resources. A code source (an object of type CodeSource), to which a permission is granted, consists of the code location (URL) and a reference to the certificate(s) containing the public key(s) corresponding to the private key(s) used to digitally sign the code. Reference to the certificate(s) is pertinent only if the code was digitally signed. A protection domain encompasses a CodeSource  and the permissions granted to code from that CodeSource, as determined by the security policy currently in effect. Consequently, classes signed by the same key and originating from the same URL are placed in the same protection domain. A class belongs to one and only one protection domain. Classes that have the same permissions but are from different code sources belong to different domains.

Each Java class runs in its appropriate domain, as determined by its code source. For any code running under a security manager to perform a secured action such as reading or writing a file, the code must be granted permission to perform that particular action. Privileged code can access privileged resources on behalf of an unprivileged caller by using the AccessController.doPrivileged() method. This is necessary, for example, when a system utility needs to open a font file on behalf of the user to display a document, but the application lacks permission to do so. To perform this action, the system utility uses its full privileges for obtaining the fonts, ignoring the privileges of the caller. Privileged code runs with all the privileges of the protection domain associated with the code source. These privileges often exceed those required to perform the privileged operation. Ideally, code should be granted only the minimum set of privileges required to complete its operation.  

SEC53-J. Define custom security permissions for fine-grained security describes another approach to eliminating excess privileges.

Noncompliant Code Example

This noncompliant code example shows a library method that allows callers to perform a privileged operation (reading a file) using the wrapper method performActionOnFile():

private FileInputStream openFile() {
  final FileInputStream f[] = { null };
 
  AccessController.doPrivileged(new PrivilegedAction() {
    public Object run() {
      try {
        f[0] = new FileInputStream("file"); 
      } catch(FileNotFoundException fnf) { 
        // Forward to handler
      }
      return null;
    }
  });
  return f[0];
}

// Wrapper method
public void performActionOnFile() {  
  try (FileInputStream f = openFile()){
    // Perform operation
  } catch (Throwable t) {
	// Handle exception		
  }	
}

In this example, the trusted code grants privileges beyond those required to read a file, even though read access to the file was the only permission needed by the  doPrivileged() block. Consequently, this code violates the principle of least privilege by providing the code block with superfluous privileges.

Compliant Solution

The two-argument form of doPrivileged() accepts an AccessControlContext object from the caller and restricts the privileges of the contained code to the intersection of privileges of the protection domain and those of the context passed as the second argument. Consequently, a caller that wishes to grant only permission to read the file can provide a context that has only the file-reading permissions. 

An AccessControlContext that grants the appropriate file-reading permissions can be created as an inner class:

private FileInputStream openFile(AccessControlContext context) {
  if (context == null) {
    throw new SecurityException("Missing AccessControlContext");
  }

  final FileInputStream f[] = { null };
  AccessController.doPrivileged(
    new PrivilegedAction() {
      public Object run() {
        try {
          f[0] = new FileInputStream("file");
        } catch (FileNotFoundException fnf) {
          // Forward to handler
        }
        return null;
      }
    },
    // Restrict the privileges by passing the context argument
    context);
  return f[0];
}

private static class FileAccessControlContext {
  public static final AccessControlContext INSTANCE;
  static {
    Permission perm = new java.io.FilePermission("file", "read");
    PermissionCollection perms = perm.newPermissionCollection();
    perms.add(perm);
    INSTANCE = new AccessControlContext(new ProtectionDomain[] {
      new ProtectionDomain(null, perms)});
  }
}

// Wrapper method
public void performActionOnFile() {
  try (final FileInputStream f = 
    // Grant only open-for-reading privileges
    openFile(FileAccessControlContext.INSTANCE)) { 
    // Perform action 
  } catch (Throwable t) {
    // Handle exception
  }
}

Callers that lack permission to create an appropriate AccessControlContext can request one using AccessController.getContext() to create the instance.

Applicability

Failure to follow the principle of least privilege can result in untrusted, unprivileged code performing unintended privileged operations. However, carefully restricting privileges adds complexity. This added complexity and the associated reduction of maintainability must be traded off against any security improvement.

Bibliography