Page History

...

The gets_s() function reads, at most, one less than the number of characters specified from the stream pointed to by stdin into an array.

The C Standard, Annex K 3.5.4.1 paragraph 4 [ISO/IEC 9899:20112024], states

No additional characters are read after a new-line character (which is discarded) or after end-of-file. The discarded new-line character does not count towards number of characters read. A null character is written immediately after the last character read into the array.

...

In this compliant solution, characters are no longer copied to buf once index == BUFFERSIZE - 1, leaving room to null-terminate the string. The loop continues to read characters until the end of the line, the end of the file, or an error is encountered. When chars_read > index truncated == true, the input string has been truncated.

#include <stdio.h>
 
enum { BUFFERSIZE = 32 };
 
void func(void) {
  char buf[BUFFERSIZE];
  int ch;
  size_t index = 0;
  size_t chars_readbool truncated = 0false;
 
  while ((ch = getchar()) != '\n' && ch != EOF) {
    if (index < sizeof(buf) - 1) {
      buf[index++] = (char)ch;
    } else {
     chars_read++ truncated = true;
    }
  }
  buf[index] = '\0';  /* Terminate string */
  if (ch == EOF) {
    /* Handle EOF or error */
  }
  if (chars_read > indextruncated) {
    /* Handle truncation */
  }
}

...

#include <stdio.h>
 
void func(const char *name) {
  char filename[128];
  sprintf(filename, "%.123s.txt", name);
}

Compliant Solution (snprintf())

A more general solution is to use the snprintf() functionYou can also use * to indicate that the precision should be provided as a variadic argument:

#include <stdio.h>
 
void func(const char *name) {
  char filename[128];
  snprintfsprintf(filename, "%.*s.txt", sizeof(filename), "%s.txt" - 5, name);
}

Risk Assessment

Compliant Solution (snprintf())

A more general solution is to use the snprintf() function, which also truncates name if it will not fit in the filename.

#include <stdio.h>
 
void func(const char *name) {
  char filename[128];
  int result = snprintf(filename, sizeof(filename), "%s.txt", name);
  if (result != strlen(filename) {
    /* truncation occurred */
  }
}

Risk Assessment

Copying string data to a buffer that is too small to hold that data results in a buffer overflow. Attackers can exploit this Copying string data to a buffer that is too small to hold that data results in a buffer overflow. Attackers can exploit this condition to execute arbitrary code with the permissions of the vulnerable process.

...

...

CVE-2009-0587 results from a violation of this rule. Before version 2.24.5, Evolution Data Server performed unchecked arithmetic operations on the length of a user-input string and used the value to allocate space for a new buffer. An attacker could thereby execute arbitrary code by inputting a long string, resulting in incorrect allocation and buffer overflow [xorl 2009].

CVE-2021-3156 results from a violation of this rule in versions of Sudo before 1.9.5p2. Due to inconsistencies on whether backslashes are escaped, vulnerable versions of Sudo enabled a user to create a heap-based buffer overflow and exploit it to execute arbitrary code. [BC].

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

...

...