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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 you call strtok() is called, you pass the string to be is parsed into tokens , the and a character delimiter, and the address of the variable to return the result in. 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 it's 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 string[]*path = "Hello secure coding wiki!";
char *tokengetenv("PATH");

token = strtok(stringpath, ' '":");
puts(token);

while (token = strtok(NULL0, ' '":")) {
  puts(token);
}

printf("PATH: %s\n", path);
/* stringPATH is hasnow been modifiedjust "/usr/bin" */

In this example, the {{strtok()}} function is used to parse the first argument into space-delimited tokens; it will output each word from the string on a new line. However, after the while loop ends, string\[\] will have been modified to look like this: {{"Hello\0secure\0coding\0wiki\0"}}. Any further manipulation of {{string}} operating on the assumption that it is still whole will see only "Hello" instead of the expected string value.

Compliant Solutions

After the loop ends, path is modified as follows: "/usr/bin\0/bin\0/usr/sbin\0/sbin\0". This is an issue because the local path variable becomes /usr/bin and because the environment variable PATH has been unintentionally changed, which 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 that is not referenced after the call One possible solution is to copy the string being tokenized into a temporary buffer which isn't referenced after the calls to strtok():

char *token;
const char string[]*path = "Hello secure coding wiki!";
char buff[256];
char *token;

strncpy(buff, stringgetenv("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);
token = strtok(buffcopy, ' '":");
puts(token);

while (token = strtok(NULL0, ' '":")) {
  puts(token);
}

free(copy);
copy = NULL;

printf("PATH: %s\n", path);
/* furtherPATH stringis manipulation on string[] succeedsstill "/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

The To quote 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.

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

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Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography

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References

C99 Section 7.21.5.8, "The strtok function"
Unix Man page strtok(3)