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The C99 C function strtok() is a string tokenization function which that takes three two arguments: an initial string to be parsed , and a const-qualified character delimiter, and . It returns a pointer to the first character of a token or to a pointer to modify to return the resultnull pointer if there is no token.

The first time strtok() is called, the string to be is parsed into tokens , and a character delimiter, and address of the variable in which to return the result are passed as arguments. The strtok() function parses the string up to the first instance of the delimiter character, replaces the character in place with a null byte ('\0'), and puts returns the address of the first character in the token to the passed-in variable. Subsequent calls to strtok() begin parsing immediately after the most recently - placed null character.

Because strtok() modifies its argumentthe initial string to be parsed, the string is subsequently unsafe and cannot be used in its original form. If you need to preserve the original string, copy it into a buffer and pass the address of the buffer to strtok() instead of the original string.

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Noncompliant Code Example

In this example, the strtok() function is used to parse the first argument into colon-delimited tokens; it outputs each word from the string on a new line. Assume that PATH is "/usr/bin:/usr/sbin:/sbin".

char *token;
char *path = getenv("PATH");
char *token;

token = strtok(path, ":");
puts(token);

while (token = strtok(0, ":")) {
  puts(token);
}

printf("PATH: %s\n", path);
/* PATH is now just "/usr/bin" */

However, after After the while loop ends, path will have been modified to look like this is modified as follows: "/usr/bin\0/bin\0/usr/sbin\0/sbin\0". This is an issue on several levels. If we check our because the local path variable , we will only see becomes /usr/bin now. Even worse, we have unintentionally changed and because the environment variable PATH has been unintentionally changed, which could cause unintended results.can have unintended consequences. (See ENV30-C. Do not modify the object referenced by the return value of certain functions.)

Compliant Solution

In this compliant solution, the string being tokenized is copied into a temporary buffer which buffer that is not referenced after the calls call to strtok():

char *token;
const char *path = getenv("PATH");
/* PATH is something like "/usr/bin:/bin:/usr/sbin:/sbin" */

char *copy = (char *)malloc(strlen(path) + 1);
if (copy == NULL) {
  /* Handle error */
}
strcpy(copy, path);
char *token;

token = strtok(copy, ":");
puts(token);

while (token = strtok(0, ":")) {
  puts(token);
}

free(copy);
copy = NULL;

printf("PATH: %s\n", path);
/* PATH is still "/usr/bin:/bin:/usr/sbin:/sbin" */

Another possibility is to provide your own implementation of strtok() which that does not modify the initial arguments.

Risk Assessment

To quote the The Linux Programmer's Manual (man) page on strtok(3) [Linux 2008] states:

Never use this function. This function modifies its first argument. The identity of the delimiting character is lost. This function cannot be used on constant strings.

However, The improper use of strtok() use will probably only is likely to result in truncated data, producing unexpected results later in program execution.

Automated Detection

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 7.21.5.8, "The strtok function"
\[Unix Man page\] strtok(3)

Related Guidelines

Bibliography

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Image Added Image Added Image AddedSTR05-A. Prefer making string literals const-qualified      07. Characters and Strings (STR)       STR07-A. Take care when calling realloc() on a null-terminated byte string