IDS06-J. Do not pass untrusted, unsanitized data to the Runtime.exec() method

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 getRuntime() method. The exec() method family executes the specified command by invoking an implementation-defined command processor, such as a UNIX shell or CMD.EXE in Windows NT and later.

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 interpreters, such as the POSIX command-language interpreter sh and the Windows CMD.EXE, however, provide functionality in addition to executing a simple command.

OS command injection vulnerabilities occur when an application fails to sanitize untrusted input but uses it in the execution of 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.

Noncompliant Code Example

This noncompliant code example attempts to send a message to an email address supplied by an untrusted user. Because untrusted data originating from the environment (see guideline [ENV06-J. Provide a trusted environment and sanitize all inputs]) without sanitization this code is susceptible to a command injection attack.

  
String address = System.getProperty("email");
if (address == null) {
  // handle error
}

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

If an attacker supplies the following value for the email system property:

noboday@nowhere.com ; useradd attacker

then two commands will actually be executed:

mail noboday@nowhere.com ;
useradd attacker

which causes a new account to be created for the attacker.

Compliant Solution (Whitelisting)

This compliant solution sanitizes the email address by permitting only a handful of correct characters to appear. This is an example of whitelisting, where characters that are not specifically listed are forbidden.

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

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

Although this is a compliant solution, the sanitization method is weak because:

• It will reject valid email addresses, with characters not on the whitelist.
• It doesn't require any syntax or regular expression pattern matching (for example, using the regular expression "\b[A-Z0-9._%+-]@[A-Z0-9.-]\.[A-Z]
  Unknown macro: {2,4}
  \b") to validate the input. The whitelist provides sanitization, not validation.
• Proper email validation is complicated. You can purchase or acquire an entire component / subsystem for email address validation, such as the Apache Class EmailValidator.
• The command interpreter invoked is system-dependent, as is the set of valid characters in the whitelist. It cannot be guaranteed that this solution will forbid command injection in every platform in which a Java program might run. Consequently this solution is non-portable.

Compliant Solution (Parametrization)

A further improvement to the previous compliant solution is to parametrize the call to the exec() method. There are six forms of the exec() method, most of which are convenience methods for the following method:

 public Process exec(String[] cmdarray,
                    String[] envp,
                    File dir)
             throws IOException

Using any form of the exec() method where the first argument is an array containing the command to call and its arguments is generally safer because the command itself does not contain untrusted data.

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

String[] command = {"mail", address};
Runtime runtime = Runtime.getRuntime();
Process proc = runtime.exec(command, null, null); 

In some cases, this can still result in an argument injection attack.

Compliant Solution (Not passing untrusted data to the exec() method)

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 command.

String address = null;

int filename = Integer.parseInt(System.getproperty("address")); // only allow integer choices
switch(filename) {
  case 1: 
    address = "root@localhost"
    break; // Option 1
  case 2: 
    address = "postmaster@localhost"
    break; // Option 2
  default: // invalid
    break; 
}
if (address == null) {
  // handle error
}

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

Risk Assessment

OS command injection can cause arbitrary programs to be executed.

Related Vulnerabilities

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

Bibliography

[[Chess 2007]] Chapter 5: Handling Input, "Command Injection"
[[MITRE 2009]] CWE ID 78 "Failure to Preserve OS Command Structure (aka 'OS Command Injection')"
[[OWASP 2005]] Reviewing Code for OS Injection
[[Permissions 2008]] Permissions in the Java™ SE 6 Development Kit (JDK), Sun Microsystems, Inc. (2008)

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