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

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Noncompliant Code Example

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

void f() {
  char copypath[16PATH_MAX];
char /*temp = getenv("TEST_ENV");
if (temp != NULL) {Requires PATH_MAX to be defined */
  strcpy(copypath, tempgetenv("PATH"));
  /* Use path */
}

However, the string copied from temp may exceed the size of copy, leading to a buffer overflowEven 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 the following 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 *copypath = NULL;
  /* Avoid assuming $PATH is defined or has limited length */
  const char *temp = getenv("TEST_ENVPATH");
  if (temp != NULL) {
    copypath = (char *) malloc(strlen(temp) + 1);
    if (copypath !== NULL) {
      /* Handle error condition */
    } else {
      strcpy(copypath, 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  else= strdup(temp);
    if (path == NULL) {
      /* handleHandle 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

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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, "Communication with the environment"
\[[Open Group 04|AA. C References#Open Group 04]\] Chapter 8, "Environment Variables"
\[[Viega 03|AA. C References#Viega 03]\] Section 3.6, "Using Environment Variables Securely"

Related Guidelines

Bibliography

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Image Added Image Added Image Added ENV00-A. Do not store the pointer to the string returned by getenv()      10. Environment (ENV)       ENV02-A. Beware of multiple environment variables with the same effective name