IDS07-J. Do not pass untrusted, unsanitized data to the Runtime.exec() method

External programs are commonly invoked to perform a function required by the overall system. This is a form of reuse and might even be considered a crude form of component-based software engineering. Command and argument injection vulnerabilities occur when an application fails to sanitize untrusted input and uses it in the execution of external programs.

Every Java application has a single instance of class Runtime that allows the application to interface with the environment in which the application is running. The current runtime can be obtained from the Runtime.getRuntime() method. The semantics of Runtime.exec are poorly defined, so it's best not to rely on its behavior any more than necessary. It will invoke the command directly without a shell. If you want a shell, you can use /bin/sh, -c on POSIX or cmd.exe on Windows. The variants of exec() that take the command line as a single string split it using a StringTokenizer. On Windows, these tokens are concatenated back into a single argument string somewhere before being executed.

Consequently, command injection attacks cannot succeed unless a command interpreter is explicitly invoked. However, argument injection attacks can occur when arguments have spaces, double quotes, and so forth, or start with a - or / to indicate a switch.

This is a specific instance of the rule IDS00-J. Sanitize untrusted data passed across a trust boundary. Any string data that originates from outside the program's trust boundary must be sanitized before being executed as a command on the current platform.

Noncompliant Code Example (Windows)

This noncompliant code example provides a directory listing using the dir command. This is implemented using the Runtime.exec() to invoke the Windows dir command.

  
class DirList {
  public static void main(String[] args) throws Exception {
    String dir = System.getProperty("dir");
    Runtime rt = Runtime.getRuntime();
    Process proc = rt.exec("cmd.exe /C dir " + dir);
    int result = proc.waitFor();
    if (result != 0) {
      System.out.println("process error: " + result);
    }
    InputStream in = (result == 0) ? proc.getInputStream() : proc.getErrorStream();
    int c;
    while ((c = in.read()) != -1) {
      System.out.print((char) c);
    }
  }
}

Because Runtime.exec() receives unsanitized data originating from the environment, this code is susceptible to a command injection attack.

An attacker can exploit this program using the following command:

java -Ddir='dummy & echo bad' Java

the command executed is actually two commands:

cmd.exe /C dir dummy & echo bad

which first attempts to list a nonexistent dummy folder, and then prints bad to the console.

Noncompliant Code Example (POSIX)

This noncompliant code example provides the same functionality but uses the POSIX ls command. The only difference from the Windows version is the argument passed to Runtime.exec().

  
class DirList {
  public static void main(String[] args) throws Exception {
    String dir = System.getProperty("dir");
    Runtime rt = Runtime.getRuntime();
    Process proc = rt.exec(new String[] {"sh", "-c", "ls " + dir});
    int result = proc.waitFor();
    if (result != 0) {
      System.out.println("process error: " + result);
    }
    InputStream in = (result == 0) ? proc.getInputStream() : proc.getErrorStream();
    int c;
    while ((c = in.read()) != -1) {
      System.out.print((char) c);
    }
  }
}

The attacker can supply the same command shown in the previous noncompliant code example with similar effects. The command executed is actually:

sh -c 'ls dummy & echo bad'

Compliant Solution (Sanitization)

This compliant solution sanitizes the untrusted user input by only allowing a small number of whitelisted characters to be passed as part of the argument to the Runtime.exec() method.

// ...
if (!Pattern.matches("[0-9A-Za-z@.]+", dir)) {
  // Handle error
}
// ...

Although this is a compliant solution, the sanitization method rejects valid directories. Also, because the command interpreter invoked is system dependent, it is difficult to establish that this solution will not allow command injection on every possible platform in which a Java program might run.

Compliant Solution (Restricted User Choice)

This compliant solution prevents command injection by only passing trusted strings to Runtime.exec(). While the user has control over which string is used, the user cannot provide string data directly to Runtime.exec().

// ...
String dir = null;

int number = Integer.parseInt(System.getproperty("dir")); // only allow integer choices
switch (number) {
  case 1: 
    dir = "data1";
    break; // Option 1
  case 2: 
    dir = "data2";
    break; // Option 2
  default: // invalid
    break; 
}
if (dir == null) {
  // handle error
}

This compliant solution hard codes the directories that may be listed.

This solution can quickly become unmanageable if you have many available directories. A more scalable solution is to read all the available directories from a properties file into a java.util.Properties object. Alternately, the switch statement can operate on an enumerated type.

Compliant Solution (Avoid Runtime.exec())

When the task performed by executing a system command can be accomplished by some other means, it is almost always advisable to do so. This compliant solution uses the File.list() method to provide a directory listing, eliminating the possibility of command or argument injection attacks.

import java.io.File;

class DirList {
  public static void main(String[] args) throws Exception {
    File dir = new File(System.getProperty("dir"));
    if (!dir.isDirectory()) {
      System.out.println("Not a directory");
    } else {
      for (String file : dir.list()) {
        System.out.println(file);
      }
    }
  }
}

Risk Assessment

Passing untrusted, unsanitized data to the Runtime.exec() method can result in command and argument injection attacks.

Related Vulnerabilities

Related Guidelines

Bibliography

IDS06-J. Exclude user input from format strings            IDS08-J. Sanitize untrusted data passed to a regex