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OS command External programs are commonly invoked to perform a function required by the overall system. This practice 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 externally obtained untrusted input and allows uses it in the execution of arbitrary system commands (with carefully chosen arguments) or of an external program.

Noncompliant Code Example

A weakness in a privileged program caused by relying on untrusted sources such as the environment (guideline ENV06-J. Provide a trusted environment and sanitize all inputs) can result in the execution of a command or of a program that has privileges beyond those possessed by a typical user. This noncompliant code example shows a variant of the OS command injection vulnerability.

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 is 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 when they start with a - or / to indicate a switch.

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. It is implemented using Runtime.exec() to invoke the Windows dir commandThe single argument version of the Runtime.exec() method uses a StringTokenizer to parse the argument into separate tokens. Consequently, command separators maliciously inserted into the argument fail to delimit the original command, so an adversary is unable to execute arbitrary system commands. Nevertheless, this noncompliant code example remains vulnerable, because a lax security policy could permit an attacker to invoke an external (privileged) program.

class DirList {
  public static void main(String[] args) throws Exception {
    String programNamedir = System.getProperty("program.name");
if (programName"dir");
    Runtime rt = Runtime.getRuntime();
    Process proc = rt.exec("cmd.exe /C dir " + dir);
    int result = proc.waitFor();
    if (result != null0) {
 
  // Runs user controlled program 
  Runtime runtime = Runtime.getRuntime();
  Process proc = runtime.exec(programName); 
}

Noncompliant Code Example

This noncompliant code example demonstrates a less likely, but more pernicious, form of OS command injection. The program spawns a shell (on POSIX based platforms) or a command prompt (on Windows), and permits passing arguments to external programs. The shell or prompt is often used to set an environment variable to a user-defined value from within the Java program. In this example, the programName string is expected to hold both the program's name and its arguments.

     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()An adversary can execute arbitrary commands by terminating the command with a command separator, such as '&&' or '||'. The attacker can use this technique to cause a denial of service by piping the output of the program to a sensitive file; even worse, he can expose sensitive data by redirecting some sensitive output to an insecure location.

class DirList 
// programName can be 'ProgramName1 || ProgramName2'  
{
  public static void main(String[] args) throws Exception {
    String dir = System.getProperty("dir");
    Runtime rt = Runtime.getRuntime();
    Process proc = runtimert.exec("/bin/sh" + programName);  // "cmd.exe /C" on Windows

...

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 it 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 requiring the user to select one of a predefined group of programs or commands. Further, both the programs and their arguments are hard-coded to prevent modification by the userpassing only trusted strings to Runtime.exec(). The user has control over which string is used but cannot provide string data directly to Runtime.exec().

Process proc// ...
String dir = null;

int filenamenumber = Integer.parseInt(System.getpropertygetProperty("program.namedir")); // onlyOnly allow integer choices
Runtime runtime = Runtime.getRuntime();

switch(filenameswitch (number) {
  case 1: 
    procdir = runtime.exec("hardcoded\program1"); "data1";
    break; // Option 1
  case 2: 
    procdir = runtime.exec("hardcoded\program2"); "data2";
    break; // Option 2
  default: // Invalid
    System.out.println("Invalid option!");break; 
}
if (dir == null) {
  // Handle break; error
}

This approach also prevents exposure of the file system structure.

Compliant Solution

An alternative compliant solution is to:

1. Store command names and arguments in a secure directory that is inaccessible to an attacker.
2. Use a security manager to regulate both access permissions for that directory and also execute permissions for the commands to be invoked.
3. Use the security manager's checkExec(String cmd) method to check whether the program is permitted to create the subprocess and to execute the external program.

This approach requires that the security manager must be used when running the application, and that the security policy file cannot be modified by an attacker. Use the security policy file to grant permissions to the application to execute files from a specific directory. See guideline ENV02-J. Create a secure sandbox using a Security Manager for additional information.

The security policy file must grant the {{java.io.FilePermission}} as follows \[[Permissions 2008|AA. Bibliography#Permissions 08]\]: 

• When cmd is an absolute path, java.io.FilePermission "{cmd}", "execute"
• otherwise, java.io.FilePermission "-", "execute";.

Note that the second alternative grants permission to execute any program. Consequently, we strongly recommend that permissions should be restricted per file whenever possible. Do this by specifying absolute paths in the security policy file (the first alternative above).

Risk Assessment

OS command injection can cause arbitrary programs to be executed.

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.

Automated Detection

Related Vulnerabilities

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

Other languages

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Related Vulnerabilities

Related Guidelines

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Android Implementation Details

Runtime.exec() can be called from Android apps to execute operating system commands.

Bibliography

[Chess 2007]	Chapter 5, "Handling Input," section "Command Injection"
[OWASP 2005]	A Guide to Building Secure Web Applications and Web Services
[Permissions 2008]	Permissions in the Java™ SE 6 Development Kit (JDK)
[Seacord 2015]	Image Added IDS07-J. Do not pass untrusted, unsanitized data to the Runtime.exec() method LiveLesson

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Bibliography

\[[Chess 2007|AA. Bibliography#Chess 07]\] Chapter 5: Handling Input, "Command Injection"
\[[MITRE 2009|AA. Bibliography#MITRE 09]\] [CWE ID 78|http://cwe.mitre.org/data/definitions/78.html] "Failure to Preserve OS Command Structure (aka 'OS Command Injection')"
\[[OWASP 2005|AA. Bibliography#OWASP 05]\] [Reviewing Code for OS Injection|http://www.owasp.org/index.php/Reviewing_Code_for_OS_Injection]
\[[Permissions 2008|AA. Bibliography#Permissions 08]\] [Permissions in the Java™ SE 6 Development Kit (JDK)|http://java.sun.com/javase/6/docs/technotes/guides/security/permissions.html], Sun Microsystems, Inc. (2008)

IDS05-J. Library methods should validate their parameters      13. Input Validation and Data Sanitization (IDS)      IDS07-J. Prevent SQL Injection