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Wiki MarkupDo not make any assumptions about the size of environment variables , as because an adversary could might have full control over the environment. Calculate the length of the strings yourself, and dynamically allocate memory for your copies \[[STR31-C|If the environment variable needs to be stored, the length of the associated string should be calculated and the storage dynamically allocated (see STR31-C. Guarantee that storage for strings has sufficient space for character data and the null terminator]\]).

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Noncompliant Code Example

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

void f() {
  char path[PATH_MAX]; /* Requires PATH_MAX to be defined */
char *temp;
char copy[16];

temp = getenv("TEST_ENV");
if (temp != NULL) {
  strcpy(buffpath, temp);
}

Compliant Solution

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, Use the strlen() function is used to calculate the size of the string, and dynamically allocate the required space .is dynamically allocated:

void f() {
  
char *temp;
char *copypath = NULL;

if ((  /* 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 = strdup(temp);
    else if (path == NULL) {
      /* handleHandle error condition */
    }
    /* Use path */
    free(path);
  }
}

Risk Assessment

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

References

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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Image Added Image Added Image Added Wiki Markup\[[ISO/IEC 9899-1999:TC2|AA. C References#ISO/IEC 9899-1999TC2]\] 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"