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Do not make any assumptions about the size of environment variables , as because an adversary could might have full control over the environment. Calculate If the environment variable needs to be stored, the length of the strings yourself, and dynamically allocate memory for your copies. There is nothing you can do to avoid the race conditions inherent here, but you can limit your exposure.

Non-Compliant Coding Example

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:This non-compliant code example copies into a buffer of fixed size. This can result in a buffer overflow.

char *temp;
char copy[16];

temp = getenv("TEST_ENV");

if(temp != NULL)
  strcpy(buff, temp);

Compliant Solution

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:Use strlen to calculate size and dynamically allocate space.

void f() {
  char *temp;
char *copy;

if ((path = 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 = strdup(temp);
    if (path == NULL)  else {
      /* handleHandle error condition */
    }
}
else {
  return -1;
}

Risk Assessment

    /* Use path */
    free(path);
  }
}

Risk Assessment

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

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", [strdup|http://www.opengroup.org/onlinepubs/009695399/functions/strdup.html] \[[Viega 03|AA. C References#Viega 03]\] Section 3.6, "Using Environment Variables Securely"