ENV01-C. Do not make assumptions about the size of an environment variable

Do not make any assumptions about the size of environment variables because an adversary might have full control over the environment. If the environment variable needs to be stored, the length of the associated string should be calculated and the storage dynamically allocated (see STR31-C. Guarantee that storage for strings has sufficient space for character data and the null terminator).

Noncompliant Code Example

This noncompliant code example copies the string returned by getenv() into a fixed-size buffer:

void f() {
  char path[PATH_MAX]; /* Requires PATH_MAX to be defined */
  strcpy(path, getenv("PATH"));
  /* Use path */
}

Even if your platform assumes that $PATH is defined, defines PATH_MAX, and enforces that paths not have more than PATH_MAX characters, the $PATH environment variable still is not required to have less than PATH_MAX chars. And if it has more than PATH_MAX chars, a buffer overflow will result. Also, if $PATH is not defined, then strcpy() will attempt to dereference a null pointer.

Compliant Solution

In this compliant solution, the strlen() function is used to calculate the size of the string, and the required space is dynamically allocated:

void f() {
  char *path = NULL;
  /* Avoid assuming $PATH is defined or has limited length */
  const char *temp = getenv("PATH");
  if (temp != NULL) {
    path = (char*) malloc(strlen(temp) + 1);
    if (path == NULL) {
      /* Handle error condition */
    } else {
      strcpy(path, temp);
    }
    /* Use path */
    free(path);
  }
}

Compliant Solution (POSIX or C2x)

In this compliant solution, the strdup() function is used to dynamically allocate a duplicate of the string:

void f() {
  char *path = NULL;
  /* Avoid assuming $PATH is defined or has limited length */
  const char *temp = getenv("PATH");
  if (temp != NULL) {
    path = strdup(temp);
    if (path == NULL) {
      /* Handle error condition */
    }
    /* Use path */
    free(path);
  }
}

Risk Assessment

Making assumptions about the size of an environmental variable can result in a buffer overflow.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography