ENV33-C. Do not call system()

The C Standard system() function executes a specified command by invoking an implementation-defined command processor, such as a UNIX shell or CMD.EXE in Microsoft Windows. The POSIX popen() function also invokes a command processor but creates a pipe between the calling program and the executed command, returning a pointer to a stream that can be used to either read from or write to the pipe [IEEE Std 1003.1:2013]. 

Use of the system() function can result in exploitable vulnerabilities, in the worst case allowing execution of arbitrary system commands. Situations in which calls to system() have high risk include the following: 

• When passing an unsanitized or improperly sanitized command string originating from a tainted source
• If a command is specified without a path name and the command processor path name resolution mechanism is accessible to an attacker
• If a relative path to an executable is specified and control over the current working directory is accessible to an attacker
• If the specified executable program can be spoofed by an attacker

Noncompliant Code Example

In this noncompliant code example, the system() function is used to execute any_cmd in the host environment. Invocation of a command processor is not required.

#include <string.h>
#include <stdlib.h>

enum { BUFFERSIZE = 512 };

void func(const char *input) {
  char cmdbuf[BUFFERSIZE];
  int len_wanted = snprintf(cmdbuf, BUFFERSIZE,
                            "any_cmd '%s'", input);
  if (len_wanted >= BUFFERSIZE) {
    /* Handle error */
  } else if (len_wanted < 0) {
    /* Handle error */
  } else if (system(cmdbuf) == -1) {
    /* Handle error */
  }
}

If this code is compiled and run with elevated privileges on a Linux system, for example, an attacker can create an account by entering the following string:

happy'; useradd 'attacker

The shell would interpret this string as two separate commands,

any_cmd 'happy';
useradd 'attacker'

and create a new user account that the attacker can use to access the compromised system.

This noncompliant code example also violates STR02-C. Sanitize data passed to complex subsystems.

Compliant Solution (POSIX)

In this compliant solution, the call to system() is replaced with a call to execve(). The exec family of functions does not use a full shell interpreter, so it is not vulnerable to command-injection attacks, such as the one illustrated in the noncompliant code example.

The execlp(), execvp(), and (nonstandard) execvP() functions duplicate the actions of the shell in searching for an executable file if the specified file name does not contain a forward slash character (/). As a result, they should be used without a forward slash character (/) only if the PATH environment variable is set to a safe value, as described in ENV03-C. Sanitize the environment when invoking external programs.

The execl(), execle(), execv(), and execve() functions do not perform path name substitution.

Additionally, precautions should be taken to ensure the external executable cannot be modified by an untrusted user, for example, by ensuring the executable is not writable by the user.

#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
 
void func(char *input) {
  pid_t pid;
  int status;
  pid_t ret;
  char *const args[3] = {"any_exe", input, NULL};
  char **env;
  extern char **environ;

  /* ... Sanitize arguments ... */

  pid = fork();
  if (pid == -1) {
    /* Handle error */
  } else if (pid != 0) {
    while ((ret = waitpid(pid, &status, 0)) == -1) {
      if (errno != EINTR) {
        /* Handle error */
        break;
      }
    }
    if ((ret != -1) &&
      (!WIFEXITED(status) || !WEXITSTATUS(status)) ) {
      /* Report unexpected child status */
    }
  } else {
    /* ... Initialize env as a sanitized copy of environ ... */
    if (execve("/usr/bin/any_cmd", args, env) == -1) {
      /* Handle error */
      _Exit(127);
    }
  }
}

This compliant solution is significantly different from the preceding noncompliant code example. First, input is incorporated into the args array and passed as an argument to execve(), eliminating any concerns about buffer overflow or string truncation while forming the command string. Second, this compliant solution forks a new process before executing "/usr/bin/any_cmd" in the child process. Although this method is more complicated than calling system(), the added security is worth the additional effort.

The exit status of 127 is the value set by the shell when a command is not found, and POSIX recommends that applications should do the same. XCU, Section 2.8.2, of Standard for Information Technology—Portable Operating System Interface (POSIX®), Base Specifications, Issue 7 [IEEE Std 1003.1:2013], says:

If a command is not found, the exit status shall be 127. If the command name is found, but it is not an executable utility, the exit status shall be 126. Applications that invoke utilities without using the shell should use these exit status values to report similar errors.

Compliant Solution (Windows)

This compliant solution uses the Microsoft Windows CreateProcess() API:

#include <Windows.h>

void func(TCHAR *input) {
  STARTUPINFO si = { 0 };
  PROCESS_INFORMATION pi;
  si.cb = sizeof(si);
  if (!CreateProcess(TEXT("any_cmd.exe"), input, NULL, NULL, FALSE,
                     0, 0, 0, &si, &pi)) {
    /* Handle error */
  }
  CloseHandle(pi.hThread);
  CloseHandle(pi.hProcess);
}

This compliant solution relies on the input parameter being non-const. If it were const, the solution would need to create a copy of the parameter because the CreateProcess() function can modify the command-line arguments to be passed into the newly created process.

This solution creates the process such that the child process does not inherit any handles from the parent process, in compliance with WIN03-C. Understand HANDLE inheritance.

Noncompliant Code Example (POSIX)

This noncompliant code invokes the C system() function to remove the .config file in the user's home directory.

#include <stdlib.h>
 
void func(void) {
  system("rm ~/.config");
}

If the vulnerable program has elevated privileges, an attacker can manipulate the value of the HOME environment variable such that this program can remove any file named .config anywhere on the system.

Compliant Solution (POSIX)

An alternative to invoking the system() call to execute an external program to perform a required operation is to implement the functionality directly in the program using existing library calls. This compliant solution calls the POSIX unlink() function to remove a file without invoking the system() function [IEEE Std 1003.1:2013]:

#include <pwd.h>
#include <unistd.h>
#include <string.h>
 
void func(void) {
  const char *file_format = "%s/.config";
  size_t len;
  char *file;
  struct passwd *pwd;

  /* Get /etc/passwd entry for current user */
  pwd = getpwuid(getuid());
  if (pwd == NULL) {
    /* Handle error */
  }

  /* Build full path name home dir from pw entry */

  len = strlen(pwd->pw_dir) + strlen(file_format) + 1;
  pathname = (char *)malloc(len);
  if (NULL == file) {
    /* Handle error */
  }
  int r = snprintf(pathname, len, file_format, pwd->pw_dir);
  if (r < 0 || r >= len) {
    /* Handle error */
  }
  if (unlink(pathname) != 0) {
    /* Handle error */
  }

  free(pathname);
}

The unlink() function is not susceptible to a symlink attack where the final component of pathname (the file name) is a symbolic link because unlink() will remove the symbolic link and not affect any file or directory named by the contents of the symbolic link (see FIO01-C. Be careful using functions that use file names for identification) .  While this reduces the susceptibility of the unlink() function to symlink attacks, it does not eliminate it.  The unlink() function is still susceptible if one of the directory names included in the pathname is a symbolic link.  This could cause the unlink() function to delete a similarly named file in a different directory.

Compliant Solution (Windows)

This compliant solution uses the Microsoft Windows SHGetKnownFolderPath() API to get the current user's My Documents folder, which is then combined with the file name to create the path to the file to be deleted. The file is then removed using the DeleteFile() API.

#include <Windows.h>
#include <ShlObj.h>
#include <Shlwapi.h>
 
#if defined(_MSC_VER)
  #pragma comment(lib, "Shlwapi")
#endif

void func(void) {
  HRESULT hr;
  LPWSTR path = 0;
  WCHAR full_path[MAX_PATH];

  hr = SHGetKnownFolderPath(&FOLDERID_Documents, 0, NULL, &path);
  if (FAILED(hr)) {
    /* Handle error */
  }
  if (!PathCombineW(full_path, path, L".config")) {
    /* Handle error */
  }
  CoTaskMemFree(path);
  if (!DeleteFileW(full_path)) {
    /* Handle error */
  }
}

Risk Assessments

If the command string passed to system(), popen(), or other function that invokes a command processor is not fully sanitized, the risk of exploitation is high. In the worst case scenario, an attacker can execute arbitrary system commands on the compromised machine with the privileges of the vulnerable process.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography