MSC32-J. Prevent OS Command Injection

OS command injection vulnerabilities occur when an application does not sanitize externally obtained input and allows 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.

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 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.

  
// security manager check
String programName = System.getProperty("program.name");
if (programName != null){ 
  // 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.

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.

  
// programName can be 'ProgramName1 || ProgramName2'  
Process proc = runtime.exec("/bin/sh" + programName);  // "cmd.exe /C" on Windows

Compliant Solution

This compliant solution restricts the programs that a privileged application can invoke when using user controlled inputs.

Process proc;
int filename = Integer.parseInt(System.getproperty("program.name")); // only allow integer choices
Runtime runtime = Runtime.getRuntime();

switch(filename) {
  case 1: proc = runtime.exec("hardcoded\program1"); break; // option 1
  case 2: proc = runtime.exec("hardcoded\program2"); break; // option 2
  default: System.out.println("Invalid option!"); break; 
}

Compliant Solution

An alternative is to read 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

Automated Detection

TODO

Related Vulnerabilities

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

Other languages

This rule appears in the C Secure Coding Standard as ENV03-C. Sanitize the environment when invoking external programs.

This rule appears in the C++ Secure Coding Standard as ENV03-CPP. Sanitize the environment when invoking external programs.

References

[[OWASP 05]] Reviewing Code for OS Injection
[[Chess 07]] Chapter 5: Handling Input, "Command Injection"
[[MITRE 09]] CWE ID 78 "Failure to Preserve OS Command Structure (aka 'OS Command Injection')"

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