FIO20-C. Avoid unintentional truncation when using fgets() or fgetws()

The fgets() and fgetws() functions are typically used to read a newline-terminated line of input from a stream. Both functions read at most one less than the number of narrow or wide characters specified by an argument n from a stream to a string. Truncation errors can occur if n - 1 is less than the number of characters appearing in the input string prior to the new-line narrow or wide character (which is retained) or after end-of-file.  This can result in the accidental truncation of user input.

Noncompliant Code Example

This noncompliant code example copies the input string into a buffer, and assumes it captured all of the user's input.

#include <stdbool.h>
#include <stdio.h>
 
bool get_data(char *buffer, int size) {
  if (fgets(buffer, size, stdin)) {
    return true;
  }
  return false;
}
 
void func(void) {
  char buf[8];
  if (get_data(buf, sizeof(buf))) {
    printf("The user input %s\n", buf);
  } else {
    printf("Error getting data from the user\n");
  }
}

However, if the last character in buf is not a newline and the stream is not at the end-of-file marker, the buffer was too small to contain all of the data from the user.  For example, because the buffer is only 8 characters in length, if the user input "Hello World\n", the buffer would contain "Hello W" terminated by a null character.

Compliant Solution (Fail on Truncation)

This compliant solution examines the end-of-file marker for the stream and the last character in the buffer to determine whether it is a newline or not.  If it is the end of file, or the last character is a newline, then the buffer contains all of the user's input.  However, if the last character is not at the end-of-file and not a newline then the user's input has been truncated.

#include <stdbool.h>
#include <stdio.h>
#include <string.h>
 
bool get_data(char *buffer, int size) {
  if (fgets(buffer, size, stdin)) {
    size_t len = strlen(buffer);
    return feof(stdin) || (len != 0 && buffer[len-1] == '\n');
  }
  return false;
}
 
void func(void) {
  char buf[8];
  if (get_data(buf, sizeof(buf))) {
    printf("The user input %s\n", buf);
  } else {
    printf("Error getting data from the user\n");
  }
}

Compliant Solution (Expanding Buffer)

This compliant solution solves the problem by expanding the buffer to read the entire contents from stdin instead of failing if the caller did not allocate enough space.  If the allocation fails, it will return NULL, but otherwise, it returns a buffer of the received data, which the caller must free.

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

char *get_filled_buffer(void) {
  char temp[32];
  char *ret = malloc(sizeof(temp));
  char *end = ret;
  if (!ret) {
    return NULL;
  }

  while (fgets(temp, sizeof(temp), stdin)) {
    size_t len = strlen(temp);
    const size_t full_size = end - ret + len;
    char *r_temp = realloc(ret, full_size + 1); /* NTBS */
    if (r_temp) {
      ret = r_temp;
      strcat(ret, temp);
      end = ret + full_size;
    } else {
      break;
    }

    if (feof(stdin) || temp[len] == '\n') {
      return ret;
    }
  }
  free(ret);
  return NULL;
}

Risk Assessment

Incorrectly assuming a newline character is read by fgets() or fgetws() can result in data truncation.

Related Vulnerabilities

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

Related Guidelines

Bibliography