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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, but typically it invokes 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 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 rule is a specific instance of rule IDS00-J. 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 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 permitting only a small group of whitelisted characters in the argument that will be passed to Runtime.exec(); all other characters are excluded.

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

Although this is a compliant solution, this sanitization approach rejects valid directories. Also, because the command interpreter invoked is system dependent, it is difficult to establish that this solution prevents command injections on every platform on which a Java program might run.

Compliant Solution (Restricted User Choice)

This compliant solution prevents command injection by passing only 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;
// only allow integer choices
int number = Integer.parseInt(System.getproperty("dir")); 
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 permitted directories from a properties file into a java.util.Properties object.

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.

Rule

Severity

Likelihood

Remediation Cost

Priority

Level

IDS07-J

high

probable

medium

P12

L1

Related Vulnerabilities

[CVE-2010-0886]

Sun Java Web Start plugin command line argument injection

[CVE-2010-1826]

Command injection in updateSharingD's handling of Mach RPC messages

[T-472]

Mac OS X Java command injection fl aw in updateSharingD lets local users gain elevated privileges

Related Guidelines

The CERT C Secure Coding Standard

ENV03-C. Sanitize the environment when invoking external programs

ENV04-C. Do not call system() if you do not need a command processor

The CERT C++ Secure Coding Standard

ENV03-CPP. Sanitize the environment when invoking external programs

ENV04-CPP. Do not call system() if you do not need a command processor

ISO/IEC TR 24772:2010

Injection [RST]

MITRE CWE

CWE-78. Improper neutralization of special elements used in an OS command ("OS command injection")

Bibliography

[Chess 2007]

Chapter 5, Handling Input, "Command Injection"

[OWASP 2005]

[Permissions 2008]

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