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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 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 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.

arbitrary system commands (with carefully chosen arguments) or of an external program. This is a specific instance of the guideline IDS01-J. Sanitize 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.

OS Command Injection Example

Suppose a Java program wants to send email using the mail program. It might ask the user for an email address. The command might take the form:

mail <ADDRESS>

However, if an attacker supplies the following value for <ADDRESS>:

noboday@nowhere.com ; useradd attacker

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 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 the command executed is actually two commands:

mail noboday@nowhere.com ;
useradd attacker

cmd.exe /C dir dummy & echo bad

which first attempts to list a nonexistent dummy folder and then prints bad to the consolewhich causes a new account to be created for the attacker.

Noncompliant Code Example

...

(POSIX)

This noncompliant code example attempts to send a message to an email address supplied by an untrusted user. Since no sanitization is done on the address, the attack outlined above would work as describedprovides 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 address[] args) throws Exception {
    String dir = System.getProperty("dir");
    Runtime rt = Runtime.getRuntime();
    Process proc = rt.exec("email");
if (address == nullnew String[] {"sh", "-c", "ls " + dir});
    int result = proc.waitFor();
    if (result != 0) {
   // handle error
}

Runtime runtime = Runtime.getRuntime();
Process proc = runtime.exec("mail " + address); 
}

Compliant Solution (Whitelisting)

   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 excludedThis compliant solution sanitizes the email address by permitting only a handful of correct characters to appear, thus preventing command injection.

// ...
  
String address = System.getProperty("email");
if (address == null) {
  // handle error
}
if (!Pattern.matches("[0-9A-Za-z@.]+", addressdir)) {
  // Handle error
}

Runtime runtime = Runtime.getRuntime();
Process proc = runtime.exec("mail " + address); 
}

...

// ...

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 addresses. This prevents untrusted data from being added to the commandpassing only trusted strings to Runtime.exec(). The user has control over which string is used but cannot provide string data directly to Runtime.exec().

// ...
String addressdir = null;

int filenamenumber = Integer.parseInt(System.getpropertygetProperty("addressdir")); // onlyOnly allow integer choices
switch (filenamenumber) {
  case 1: 
    addressdir = "root@localhostdata1";
    break; // Option 1
  case 2: 
    addressdir = "postmaster@localhostdata2";
    break; // Option 2
  default: // invalidInvalid
    break; 
}
if (addressdir == null) {
  // handleHandle error
}

Runtime runtime = Runtime.getRuntime();
Process proc = runtime.exec("mail " + address); 

Risk Assessment

OS command injection can cause arbitrary programs to be executed.

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.

Automated Detection

Related Vulnerabilities

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

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)

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 AddedIDS05-J. Library methods should validate their parameters      13. Input Validation and Data Sanitization (IDS)      IDS07-J. Prevent SQL Injection