ENV30-C. Do not modify the string returned by getenv()

C99 [[ISO/IEC 9899:1999]] defines getenv as follows:

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.

Consequently, if the string returned by getenv() needs to be altered, a local copy should be created to ensure that the environment is not directly and unintentionally modified.

Noncompliant Code Example

This noncompliant code example modifies the string returned by getenv() by replacing all double quote (") characters with underscores.

void strtr(char *str, char orig, char rep) {
  while (*str != '\0') {
    if (*str == orig) {
      *str = rep;
    }
    str++;
  }
}

/* ... */

char *env = getenv("TEST_ENV");
if (env == NULL) {
  /* Handle Error */
}

strtr(env,'"', '_');


/* ... */

Compliant Solution (local copy)

For the case where the intent of the noncompliant code example is to use the modified value of the environment variable locally and not modify the environment, this compliant solution makes a local copy of that string value, and then modifies the local copy.

const char *env;
char *copy_of_env;

env = getenv("TEST_ENV");
if (env == NULL) {
  /* Handle Error */
}

copy_of_env = (char *)malloc(strlen(env) + 1);
if (copy_of_env == NULL) {
  /* Handle Error */
}

strcpy(copy_of_env, env);
strtr(copy_of_env,'\"', '_');

Compliant Solution (modifying the environment in POSIX)

For the case where the intent is to modify the environment, this compliant solution will save the altered string back into the environment by using the POSIX setenv() and strdup() functions.

const char *env;
char *copy_of_env;

env = getenv("TEST_ENV");
if (env == NULL) {
  /* Handle Error */
}

copy_of_env = strdup(env);
if (copy_of_env == NULL) {
  /* Handle Error */
}

strtr(copy_of_env,'\"', '_');

if (setenv("TEST_ENV", copy_of_env, 1) != 0) {
  /* Handle Error */
}

Risk Assessment

The modified string may be overwritten by a subsequent call to the getenv() function. Depending on the implementation, modifying the string returned by getenv() may or may not modify the environment.

Automated Detection

Compass/ROSE could detect violations of this rule by noting attempts to modify any variable that holds the result of getenv(). A stronger approach would be to report if the result of getenv() is assigned to a (non-const) char*.

Related Vulnerabilities

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

References

[[ISO/IEC 9899:1999]] Section 7.20.4.5, "The getenv function"
[[Open Group 04]] getenv

ENV04-C. Do not call system() if you do not need a command processor      10. Environment (ENV)