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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 does not sanitize externally obtained inputs and allows fails to sanitize untrusted input and uses it in the execution of arbitrary system commands (with carefully chosen arguments) or an external program.

Noncompliant Code Example

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 commandA weakness in a privileged program caused by relying on untrusted sources such as the environment (See MSC32-J. Treat the environment as untrusted and sanitize all inputs), can result in the execution of a command or a program which has more privileges than those possessed by a typical user. This code snippet shows such a vulnerability and can be best described as a variant of OS command injection. When the single argument version of the Runtime.exec() method is invoked, the arguments are parsed by a StringTokenizer into separate tokens. Consequently, any command separators maliciously inserted into the argument will not delimit the original command and an adversary will be unable to proceed with executing arbitrary system commands. This code is however, equally vulnerable as an attacker can easily invoke an external (privileged) program, despite the presence of a security manager.

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

Noncompliant Code Example

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()A less likely, though more pernicious form of OS command injection is portrayed in this noncompliant example. The program spawns a shell (*nix) or a command prompt (Windows) and allows passing arguments to external programs. The programName string is expected to hold the program's name, as well as the arguments. An adversary can terminate the command with a command separator (such as '&&' and '||') or cause the output of the program to be piped to a sensitive file for the purpose of causing a denial of service (privileged program), or even worse, redirect some sensitive output to a non sensitive 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

Compliant Solution

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 passing only trusted strings to Runtime.exec(). The user has control over which string is used but cannot provide string data directly to Runtime.exec()This compliant solution restricts the programs that a privileged application can invoke when using user controlled inputs.

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

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

switch(filename)switch (number) {
  case 1: 
    procdir = runtime.exec("hardcoded\program1"); "data1";
    break; // optionOption 1
  case 2: proc
    dir = runtime.exec("hardcoded\program2"); "data2";
    break; // optionOption 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("Invalid option!Not a directory"); break; 
}

...

    } 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

TODO

Related Vulnerabilities

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

References

\[[OWASP 05|AA. Java References#OWASP 05]\] [Reviewing Code for OS Injection|http://www.owasp.org/index.php/Reviewing_Code_for_OS_Injection]
\[[MITRE 09|AA. Java References#MITRE 09]\] [CWE ID 78|http://cwe.mitre.org/data/definitions/78.html] "Failure to Preserve OS Command Structure (aka 'OS Command Injection')"

Related Vulnerabilities

Related Guidelines

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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Image Added Image Added Image AddedMSC32-J. Treat the environment as untrusted and sanitize all inputs      49. Miscellaneous(MSC)      49.Miscellaneous (MSC)