ENV01-J. Place all privileged code in a single package and seal the package

In Java SE 6, privileged code either uses the AccessController mechanism or must be signed by an owner (or provider) who is trusted by the user. Attackers could link privileged code with malicious code if the privileged code directly or indirectly invokes code from another package. This is called a mix and match attack. A mix and match attack is not possible if the code is signed because, by default, the jarsigner tool signs the finished manifest that contains the names of the included classes along with their digests.

Execution of untrusted code causes loss of privileges. If trusted code calls some untrusted code that attempts to perform some action requiring permissions not granted by the security policy, the action is not allowed. However, privileged code may use a class that exists in an untrusted container, performing only unprivileged operations. If the attacker replaces this class with a malicious implementation, the trusted code will retrieve incorrect results.

According to the Java API [[JarSpec 2008]], JAR file specification

A package sealed within a JAR specifies that all classes defined in that package must originate from the same JAR. Otherwise, a SecurityException is thrown.

Sealing a JAR file automatically enforces the requirement of keeping privileged code together. In addition, it is important to adhere to guideline SEC01-J. Minimize the accessibility of classes and their members.

Noncompliant Code Example

This noncompliant code example uses a doPrivileged block and calls a method defined in a class that exists in a different, untrusted package.

package trusted;
import untrusted.RetValue;

public class MixMatch {
  private void privilegedMethod() throws IOException {
    try {
      final FileInputStream fis = AccessController.doPrivileged(
	       new PrivilegedExceptionAction<FileInputStream>() {
                 public FileInputStream run() throws FileNotFoundException {
	           return new FileInputStream("file.txt");
	         }
               }
	     );
      try {
        RetValue rt = new RetValue();

        if (rt.getValue() == 1) {
  	// do something with sensitive file
        }
      } finally {
        fis.close();
      }
    } catch (PrivilegedActionException e) {
      // forward to handler and log
    }

  }

  public static void main(String[] args) throws IOException {
    MixMatch mm = new MixMatch();
    mm.privilegedMethod();
  }
}

package untrusted;

class RetValue {
  public int getValue() {
    return 1;
  }
}

An attacker can provide an implementation of class RetValue so that the privileged code uses the wrong return value. Even if class MixMatch trusted only signed code, an attacker can still cause this behavior by maliciously deploying a legally signed class in the class path of the privileged code.

Compliant Solution

This compliant solution combines all privileged code into the same package and reduces the accessibility of the getValue() method to package-private. Sealing the package is necessary to prevent attackers from inserting any rogue classes.

package trusted;

public class MixMatch {
  // ...
}

package trusted;

class RetValue {
  int getValue() {
    return 1;
  }
}

To seal a package, use the sealed attribute in the manifest file header. This is shown below.

Name: trusted/ // package name
Sealed: true  // sealed attribute

Exception

ENV01-EX1: Independent groups of privileged code may be placed in separate sealed packages. The enabling condition is that the code in any one of these packages lacks any dynamic or static dependency on any of the other packages. This means that code from one such package must not invoke code from any of the others, whether directly or transitively.

Risk Assessment

Failure to place all privileged code together in one package and seal the package can lead to mix and match attacks.

Automated Detection

Detecting code that should be considered privileged or sensitive requires programmer assistance. Given identified privileged code as a starting point, automated tools could compute the closure of all code that can be invoked from that point. Such a tool could plausibly determine whether all code in that closure exists within a single package. A further check of whether the package is sealed appears feasible.

Related Vulnerabilities

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

Related Guidelines

MITRE CWE: CWE-349: Acceptance of Extraneous Untrusted Data With Trusted Data

Bibliography

[[API 2006]]
[[McGraw 1999]] Rule 7: If You Must Sign Your Code, Put It All in One Archive File (sic)
[[Ware 2008]]

      15. Runtime Environment (ENV)      ENV02-J. Create a secure sandbox using a Security Manager