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Errors can occur when incorrect assumptions are made about the type of data being read. These assumptions may be violated, for example, when binary data has been read from a file instead of text from a user's terminal or the output of a process is piped to stdin. (See FIO14-C. Understand the difference between text mode and binary mode with file streams.) On some systems, it may also be possible to input a null byte (as well as other binary codes) from the keyboard.

Subclause 7.23.7.2 of the C Standard paragraph 3 [ISO/IEC 9899:2024] says,

The fgets function returns s if successful. If end-of-file is encountered and no characters have been read into the array, the contents of the array remain unchanged and a null pointer is returned.

...

If a read error occurs during the operation, the members of the array have unspecified values and a null pointer is returned.

The wide-character function fgetws() has the same behavior. Therefore, if fgets() or fgetws() returns a non-null pointer, it is safe to assume that the array contains data. However, it is erroneous to assume that the array contains a nonempty string because the data may contain null characters.

Noncompliant Code Example

This non-compliant noncompliant code example attempts to remove the trailing newline (\n) from an input line. The fgets() function is typically used to read a newline-terminated line of input from a stream. It takes a size parameter for the destination buffer and copies, at most, size - 1 characters from a stream to a stringcharacter array.

#include <stdio.h>
#include <string.h>
 
enum { BUFFER_SIZE = 1024 };

void func(void) {
  char buf[BUFSIZ + 1BUFFER_SIZE];

  if (fgets(buf, sizeof(buf), fpstdin) == NULL) {
    /* handleHandle error */
  }
  buf[strlen(buf) - 1] = '\0';
}

Wiki MarkupThe {{strlen()}} function computes the length of a string by determining the number of characters that precede the terminating null character. A problem occurs if the first character read from the input by {{fgets()}} is a null happens to be a null character. This may occur, for example, if a binary data file is read by the {{fgets()}} call \[ [Lai 06|AA. C References#Lai 06]\]. If the first character in {{buf}} is a null character, {{strlen(buf)}} will return 0 and a 2006]. If the first character in buf is a null character, strlen(buf) returns 0, the expression strlen(buf) - 1 wraps around to a large positive value, and a write-outside-array-bounds error will occur. occurs.

Compliant Solution

This compliant solution uses strchr() to replace the newline character in the string , if it exists (see FIO36-C. Do not assume a newline character is read when using fgets()).:

#include <stdio.h>
#include <string.h>
 
enum { BUFFER_SIZE = 1024 };

void func(void) {
  char buf[BUFSIZ + 1BUFFER_SIZE];
  char *p;

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

Risk Assessment

Assuming Incorrectly assuming that character data has been read can result in an out-of-bounds memory writeswrite or other flawed logic.

Automated Detection

Fortify SCA Version 5.0 is able to detect violations of this rule.

Related Vulnerabilities

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

References

\[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] Section 7.19.7.2, "The fgets function"
\[[Lai 06|AA. C References#Lai 06]\]
\[[Seacord 05|AA. C References#Seacord 05]\] Chapter 2, "Strings"

Related Guidelines

Key here (explains table format and definitions)

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-241 and FIO37-C

CWE-241 = Union( FIO37-C, list) where list =

• Improper handling of unexpected data type that does not come from the fgets() function.

Bibliography

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Image Added Image Added Image AddedFIO36-C. Do not assume a newline character is read when using fgets()      09. Input Output (FIO)       FIO38-C. Do not use a copy of a FILE object for input and output