MSC33-C. Do not pass invalid data to the asctime() function

Subclause 7.27.3.1 of the C Standard [ISO/IEC 9899:2011] provides the following sample implementation of the asctime() function:

char *asctime(const struct tm *timeptr) {
  static const char wday_name[7][3] = {
    "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"
  };
  static const char mon_name[12][3] = {
    "Jan", "Feb", "Mar", "Apr", "May", "Jun",
    "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"
  };
  static char result[26];
  sprintf(
    result, 
    "%.3s %.3s%3d %.2d:%.2d:%.2d %d\n",
    wday_name[timeptr->tm_wday],
    mon_name[timeptr->tm_mon],
    timeptr->tm_mday, timeptr->tm_hour,
    timeptr->tm_min, timeptr->tm_sec,
    1900 + timeptr->tm_year
  );
  return result;
}

This function is supposed to output a character string of 26 characters at most, including the terminating null character. If we count the length indicated by the format directives, we arrive at 25. Taking into account the terminating null character, the array size of the string appears sufficient.

However, this implementation assumes that the values of the struct tm data in timepiece are within normal ranges and does nothing to enforce the range limit. If any of the values print more characters than expected, the sprintf() function may overflow the result array. For example, if tm_year has the value 12345, then 27 characters (including the terminating null character) are printed, resulting in a buffer overflow.

Standard for Information Technology—Portable Operating System Interface (POSIX®), Base Specifications, Issue 7 [IEEE Std 1003.1:2013], says the following about the asctime() and asctime_r() functions:

These functions are included only for compatibility with older implementations. They have undefined behavior if the resulting string would be too long, so the use of these functions should be discouraged. On implementations that do not detect output string length overflow, it is possible to overflow the output buffers in such a way as to cause applications to fail, or possible system security violations. Also, these functions do not support localized date and time formats. To avoid these problems, applications should use strftime() to generate strings from broken-down times.

The C Standard, Annex K also defines asctime_s(), which can be used as a secure substitute for asctime().

The asctime() function appears in the list of obsolescent functions in MSC24-C. Do not use deprecated or obsolescent functions.

Noncompliant Code Example

Avoid using the asctime() function with unsanitized data:

#include <time.h>
 
void func(struct tm *time_tm) {
  char *time = asctime(time_tm);
  /* ... */
}

Noncompliant Code Example (asctime())

You can sanitize the data before invoking asctime():

#include <time.h>
 
int validate_tm(struct tm *time) {
  /* 
   * The range of valid values of the tm_sec member is [0, 60] 
   * inclusive (to allow for leap seconds).
   */
  if (time->tm_sec < 0 || time->tm_sec > 60) return 0;
  if (time->tm_min < 0 || time->tm_min >= 60) return 0;
  if (time->tm_hour < 0 || time->tm_hour >= 24) return 0;
  if (time->tm_mday <= 0 || time->tm_mday > 31) return 0;
  if (time->tm_mon < 0 || time->tm_mon >= 12) return 0;
  /* Other years are legit but may overflow asctime()'s buffer */
  if (time->tm_year < -999 || time->tm_year > 9999) return 0;
  if (time->tm_wday < 0 || time->tm_wday >= 7) return 0;
  if (time->tm_yday < 0 || time->tm_yday >= 366) return 0;
  return 1;
}
 
void func(struct tm *time_tm) {
  if (!validate_tm(time_tm)) {
    /* Handle error */
  }
  char *time = asctime(time_tm);
}

Note that although this example is safer due to sanitizing the data, it is still noncompliant because asctime() is obsolete. 

Compliant Solution (asctime_s())

The asctime_s() function from Annex K of the C Standard requires an additional argument that specifies the maximum size of the resulting time string:

#define __STDC_WANT_LIB_EXT1__ 1
#include <time.h>
 
void func(struct tm *time_tm) {
  const size_t maxsize = 26; 
  char buffer[maxsize];
 
  if (asctime_s(buffer, maxsize, &time_tm)) {
    /* Handle error */
  }
}

Compliant Solution (strftime())

The strftime() function allows you to specify a more rigorous format and also to specify the maximum size of the resulting time string:

#include <time.h>
 
void func(struct tm *time) {
  const size_t maxsize = 26;
  char s[maxsize];
  /* Current time representation for locale */
  const char *format = "%c";

  size_t size = strftime(s, maxsize, format, time);
}

This call has the same effects as asctime() but it also ensures that no more than maxsize chars are printed, preventing buffer overflow.

Risk Assessment

On implementations that do not detect output-string-length overflow, it is possible to overflow the output buffers, resulting in a vulnerability.

Related Vulnerabilities

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

Related Guidelines

Bibliography