FIO30-C. Exclude user input from format strings

Never call any formatted I/O function with a format string containing user input.

If the user can control a format string, they can write to arbitrary memory locations. The most common form of this error is in output operation. The rarely used and often forgotten %n format specification causes the number of characters written to be written to a pointer passed on the stack.

Non-Compliant Code Example 1

In this example, the input is outputted directly as a format string. By putting %n in the input, the user can write arbitrary values to whatever the stack happens to point to. This can frequently be leveraged to execute arbitrary code. In any case, by including other point operations (such as %s), fprintf() will interpret values on the stack as pointers. This can be used to learn secret information and almost certainly can be used to crash the program.

char input[1000];
if (fgets(input, sizeof(input)-1, stdin) == NULL) {
  /* handle error */
}
input[sizeof(input)-1] = '\0';
fprintf(stdout, input);

Non-Compliant Code Example 2

In this example, the library function syslog() interprets the string msg as a format string, resulting in the same security problem as before. This is a common idiom for displaying the same message in multiple locations or when the message is difficult to build.

void check_password(char *user, char *password) {
  if (strcmp(password(user), password) != 0) {
    char *msg = malloc(strlen(user) + 100);
    if (!msg) {
      /* handle error condition */
    }
    sprintf (msg, "%s password incorrect", user);
    fprintf(STDERR, msg);
    syslog(LOG_INFO, msg);
    free(msg);
  }
}

Compliant Solution 1

This example outputs the string directly instead of building it and then outputting it.

void check_password(char *user, char *password) {
  if (strcmp(password(user), password) != 0) {
    fprintf (stderr, "%s password incorrect", user);
  }
}

Compliant Solution 2

In this example, the message is built normally but is then outputted as a string instead of a format string.

void check_password(char *user, char *password) {
  if (strcmp(password(user), password) != 0) {
    char *msg = malloc(strlen(user) + 100);
    if (!msg) {
      /* handle error condition */
    }
    sprintf (msg, "%s password incorrect", user);
    fprintf (stderr, "%s", user);
    syslog(LOG_INFO, "%s", msg);
    free(msg);
  }
}

Risk Assessment

Failing to exclude user input from format specifiers may allow an attacker to execute arbitrary code.

Two recent examples of format string vulnerabilities resulting from a violation of this rule include Ettercap and Samba. In Ettercap v.NG-0.7.2, the ncurses user interface suffers from a format string defect. The curses_msg() function in ec_curses.c calls wdg_scroll_print(), which takes a format string and its parameters and passes it to vw_printw(). The curses_msg() function uses one of its parameters as the format string. This input can include user-data, allowing for a format string vulnerability [[VU#286468]]. The Samba AFS ACL mapping VFS plug-in fails to properly sanitize user-controlled filenames that are used in a format specifier supplied to snprintf(). This security flaw becomes exploitable when a user is able to write to a share that uses Samba's afsacl.so library for setting Windows NT access control lists on files residing on an AFS file system.

Related Vulnerabilities

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

References

[[ISO/IEC 9899-1999]] Section 7.19.6, "Formatted input/output functions"
[[MITRE 07]] CWE ID 134, "Uncontrolled Format String"
[[Seacord 05]] Chapter 6, "Formatted Output"
[[Viega 05]] Section 5.2.23, "Format string problem"
[[VU#286468]]
[[VU#649732]]