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Propagating the content of exceptions without performing explicit filtering is often associated with information leakage. An attacker may craft input parameters such that underlying structures and mechanisms may get exposed inadvertently.

Non-Compliant Code Example

This example demonstrates code that accepts a file name as an input argument on which operations are to be performed. An attacker can gain insights on the underlying file system structure by repeatedly passing different paths to fictitious files. When a file is not found, the FileInputStream constructor throws a FileNotFoundException. Other risks such as revelation of the user's home directory and thus the user name also manifest themselves.

import java.io.FileInputStream;
import java.io.FileNotFoundException;

class exception {
  public static void main(String[] args) throws FileNotFoundException {
    FileInputStream dis = new FileInputStream("c:\\" + args[1]);
  }
}

Compliant Solution

Information leakage can result from both the exception message text and the type of exception. With FileNotFoundException, the message reveals the file system layout while the type conveys the absence of the file. The same exception must be caught while taking special care to sanitize the message before propagating it to the caller. In cases where the exception type itself can reveal too much, consider throwing a different exception (with a different message) altogether.

import java.io.FileInputStream;
import java.io.FileNotFoundException;

class exception {
  public static void main(String[] args) {
    try {
      FileInputStream dis = new FileInputStream("c:\\" + args[1]);
    }
    catch(FileNotFoundException fnf) { 
      System.out.println("Error: Operation could not be performed"); 
    } //sanitized message
  }
}

References

Secure coding in Java http://java.sun.com/security/seccodeguide.html^{Image Added}

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