Do not modify the value returned by the {{getenv()}} function. Create a copy and make your changes locally, so that they are not overwritten.  According to C99 \[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\]:

The getenv function returns a pointer to a string associated with the matched list member. The string pointed to shall not be modified by the program, but may be overwritten by a subsequent call to the getenv function. If the specified name cannot be found, a null pointer is returned.

Non-Compliant Code Example

This non-compliant code example modifies the string returned by getenv().

char *env = getenv("TEST_ENV");
env[0] = 'a';

Compliant Solution

This is a compliant code solution. If it is necessary to modify the value of the string returned by the function getenv(), then the programmer should make a local copy of that string value, and then modify the local copy of that string.

char *env;
char *copy_of_env;

if ((env = getenv("TEST_ENV")) != NULL) {
   copy_of_env = malloc(strlen(env) + 1);

   if (copy_of_env != NULL) {
      strcpy(copy_of_env, env);
   }
   else {
      /* Error handling */
   }

   copy_of_env[0] = 'a';
}

Risk Assessment

The modified string may be overwritten by a subsequent call to the getenv function.

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 7.20.4.5, "The {{getenv}} function"
\[[Open Group 04|AA. C References#Open Group 04]\] [getenv|http://www.opengroup.org/onlinepubs/000095399/functions/getenv.html]