SEC00-J. Follow the principle of least privilege

According to the principle of least privilege, code should not be granted more privileges than those required for performing a task. This means that sections of code that require elevated privileges should be kept to a minimum. McGraw and Felten [[McGraw 00]] enlist various goals of the principle of least privilege in the context of the Java programming language:

• We want to grant each applet or application the minimum privileges it needs.
• Rather than assigning a given applet's entire collection of privileges to all of its classes, we want each class to get just what it needs.
• We want a class's privileges to be "turned off" except for brief periods of time.
• We even want to reduce the privileges of some of the built-in system classes.

The ways in which these goals can be achieved are discussed below.

Granting an applet the minimum privileges it needs is achieved by not signing it when it performs only unprivileged operations (ENV00-J. Do not sign code that performs only unprivileged operations). For applications, some additional steps may be required depending on the security policy. The default security policy file is restrictive. However, the security model allows the user to grant additional permissions to applications by using a custom security policy. Several guidelines deal with granting or suppressing permissions (for instance, ENV00-J. Do not sign code that performs only unprivileged operations, ENV31-J. Never grant AllPermission to untrusted code, ENV32-J. Do not grant ReflectPermission with target suppressAccessChecks and ENV33-J. Do not grant RuntimePermission with target createClassLoader).

There are several ways to practice privilege separation. Applets or applications that need to be signed can coexist with unsigned classes in the same package (or JAR file). Furthermore, it is possible to grant privileges to code on the basis of the code base or its signer.

The third goal is realized by using the AccessController mechanism. Only certain parts of code require elevated privileges and this mechanism allows just that. When a class needs to enable its own privileges, it calls the sensitive methods in a doPrivileged block. It is critical that these privileges be revoked as soon as the sensitive operations have concluded. Users most often do not understand complex security policies nor do they understand code signing. They cannot be relied upon to restrict permissions in non-default policy files. Consequently, privileged code must be written with utmost care assuming that the user is incapable of correctly configuring the security policy.

The fourth goal can be achieved, in part, by granting code specific permissions and protecting it from accessing classes in certain packages. This can be achieved by using the accessClassInPackage permission (SEC07-J. Do not grant untrusted code access to classes existing in forbidden packages). Doing so does not limit what system classes can do, however, it restricts the range of system packages that can be invoked from less-privileged code.

Noncompliant Code Example

This noncompliant example includes nonprivileged code following the privileged openPasswordFile statement in a doPrivileged block. This is a violation of the principle of least privilege.

AccessController.doPrivileged(new PrivilegedAction() {
  public Object run() {
    try {
      f[0] = openPasswordFile(password_file);  // call the privileged method here
      // other operations 
    }catch(FileNotFoundException cnf) { 
      System.err.println("Error: Operation could not be performed");
    }
    return null;
  }
});

Compliant Solution

It is critical to restrict the code that requires elevated privileges to a minimum. Privileged code must also be audited more carefully than non-privilege code to ensure than there are no other security vulnerabilities. This is essential to prevent the misuse of privileges.

AccessController.doPrivileged(new PrivilegedAction() {
  public Object run() {
    try {
      f[0] = openPasswordFile(password_file);  // call the privileged method here
    }catch(FileNotFoundException cnf) { 
      System.err.println("Error: Operation could not be performed");
    }
    return null;
  }
});

// other operations 

Noncompliant Code Example

Sometimes code needs to take into account the privileges of another context while performing a security critical action. If such code is granted the privileges of the currently executing thread, a vulnerability is exposed. This noncompliant code example shows a library method that allows all callers to perform a privileged operation (such as a read or write to a file) by using the wrapper method performActionOnFile(). The corresponding code is granted the permissions to both read and write the file. However, the caller wants only read access to the file to prevent any misuse. This code violates the principle of least privilege by also providing the caller with write access.

private void openFile() {
  final FileInputStream f[] = {null};
 
  AccessController.doPrivileged(new PrivilegedAction() {
    public Object run() {
      try {
        f[0] = new FileInputStream("file"); 
      } catch(FileNotFoundException cnf) { 
        System.err.println("Error: Operation could not be performed");
      }
      return null;
    }
  });
}

// wrapper method
public void performActionOnFile() {
  openFile();	
}

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 the permissions of the current execution context's domains and those of the context passed as the second argument. Consequently, a caller that requires only read permission to the file can pass a context that has only the file read permission.

private void openFile(AccessControlContext context) {
  final FileInputStream f[] = {null};
  AccessController.doPrivileged(new PrivilegedAction() {
    public Object run() {
      try {
	f[0] = new FileInputStream("file"); 
      } catch(FileNotFoundException cnf) { 
	System.err.println("Error: Operation could not be performed");
      }
      return null;
    }
  },context); // restrict the privileges by passing the context argument
}

// wrapper method
public void performActionOnFile(AccessControlContext acc) { 
  openFile(acc); // caller's AccessControlContext	
}

Risk Assessment

Failure to follow the principle of least privilege can lead to privilege escalation attacks when a vulnerability is exploited.

Automated Detection

TODO

Related Vulnerabilities

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

References

[[McGraw]]
[[MITRE 09]] CWE 272

02. Platform Security (SEC)      02. Platform Security (SEC)      SEC01-J. Provide sensitive mutable classes with unmodifiable wrappers