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The fgets() function is and fgetws() functions are typically used to read a newnewline-line terminated line of input from a stream. The fgets() function takes a size parameter for the destination buffer and copies, at most, size-1 characters 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 the programmer assumes that the last character in the destination string is a newline.The fgetws() function is similarly affected 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 attempts to remove the trailing newline (\n) from an input linecopies the input string into a buffer, and assumes it captured all of the user's input.

#include <stdbool.h>
char buf[BUFSIZ + 1];

#include <stdio.h>
 
bool get_data(char *buffer, int size) {
  if (fgets(bufbuffer, sizeof(buf)size, stdin)) {
    return true;
  if (*buf}
  return false;
}
 
void func(void) {
 /* see FIO37-C */char buf[8];
  if  buf[strlen(get_data(buf, sizeof(buf) - 1] = '\0')) {
    printf("The user input %s\n", buf);
  }
}
 else {
    /* Handle error condition */
}
 printf("Error getting data from the user\n");
  }
}

However, if the last character in buf is not a new-line, this code overwrites an otherwise valid 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 uses strchr() to replace the newline character in the string (if it exists). The equivalent solution for fgetws() would use wcschr().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>
char buf[BUFSIZ + 1];
char *p;

if (fgets(buf, sizeof(buf), stdin)) {
  p = strchr(buf, '\n');
  if (p) {
    *p = '\0';
  }
}
else {
  /* Handle error condition */
}

An obvious alternative is to leave room in the buffer for one more character, and when no new-line is transferred, append a new-line followed by a null-termination character. This approach is unsafe because it quietly accepts an input that is not what was actually intended, with unknown consequences.

Risk Assessment

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

char *get_filled_buffer(void) {
  char temp[32];
  char *ret = NULL;
  size_t full_length = 0;
   
  while (fgets(temp, sizeof(temp), stdin)) {
    size_t len = strlen(temp);
    if (SIZE_MAX - len - 1 < full_length) {
      break;
    }
    char *r_temp = realloc(ret, full_length + len + 1);
    if (r_temp == NULL) {
      break;
    }
    ret = r_temp;
    strcpy(ret + full_length, temp); /* concatenate */
    full_length += len;
   
    if (feof(stdin) || temp[len-1] == '\n') {
      return ret;
    }
  }

  free(ret);
  return NULL;
}

Compliant Solution (POSIX getline())

The getline() function was originally a GNU extension, but is now standard in POSIX.1-2008. It also fills a string with characters from an input stream. In this case, the program passes it a NULL pointer for a string, indicating that getline() should allocate sufficient space for the string and the caller frees it later.

#include <stdio.h>

void func(void) {
  char* buf = NULL;
  size_t dummy = 0;
  if (getline(&buf, &dummy, stdin) == -1) {
	/* handle error */
  }
  printf("The user input %s\n", buf);
  free(buf);
}

Risk Assessment

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

Automated Detection

Related Vulnerabilities

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

Related Guidelines

CERT C++ Secure Coding Standard: FIO36-CPP. Do not assume a new-line character is read when using fgets()

ISO/IEC 9899:1999 Section 7.19.7.2, "The fgets function"

Bibliography

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