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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 fails to sanitize untrusted input and allows uses it in the execution of arbitrary system commands (with carefully chosen arguments) or an external program.

Noncompliant Code Example

A weakness in a privileged program caused by relying on untrusted sources such as the environment (See ENV35-J. Provide a trusted environment and sanitize all inputs), can result in the execution of a command or a program that has more privileges than those possessed by a typical user. This noncompliant code example shows such a vulnerability; it is a variant of an 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 commandWhen 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 do not delimit the original command and an adversary is unable to proceed in 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 (programName"dir");
    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

A less likely, though more pernicious form of OS command injection is portrayed in this noncompliant code example. The program spawns a shell (*nix) or a command prompt (Windows) and allows passing arguments to external programs. Sometimes the shell or prompt is used to set an environment variable to a user defined value from within the program. The programName string is expected to hold the program's name, as well as the arguments.

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

Compliant Solution

"-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: System.out.println("Invalid option!");// Invalid
    break; 
}
if (dir == null) {

Compliant Solution

  // 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 An alternative is to read all the file names from a secure directory. The security policy file may grant permissions to the application to read files from a specific directory. The security manager must be used when running the application. (ENV30-J. Create a secure sandbox using a Security Manager)

Risk Assessment

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 attacksOS command injection can cause arbitrary programs to be executed.

Automated Detection

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TODO

Related Vulnerabilities

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

Other languages

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Tool	Version	Checker	Description
The Checker Framework	Include Page The Checker Framework_V The Checker Framework_V	Tainting Checker	Trust and security errors (see Chapter 8)
CodeSonar	Include Page CodeSonar_V CodeSonar_V	JAVA.IO.INJ.COMMAND	Command Injection (Java)
Coverity	7.5	OS_CMD_INJECTION	Implemented
Parasoft Jtest	Include Page Parasoft_V Parasoft_V	CERT.IDS07.EXEC	Do not use 'Runtime.exec()'
SonarQube	Include Page SonarQube_V SonarQube_V	S2076	OS commands should not be vulnerable to injection attacks

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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References

\[[OWASP 05|AA. Java References#OWASP 05]\] [Reviewing Code for OS Injection|http://www.owasp.org/index.php/Reviewing_Code_for_OS_Injection]
\[[Chess 07|AA. Java References#Chess 07]\] Chapter 5: Handling Input, "Command 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')"

MSC31-J. Never hardcode sensitive information      49. Miscellaneous (MSC)      MSC33-J. Prevent against SQL Injection