The getenv() function searches an environment list for a string that matches a specified name, and returns a pointer to a string associated with the matched list member. Due to the manner in which environment variables are stored, multiple environment variables with the same name can cause unexpected results. You may check one value, but return another.

Implementation Details

Depending on the implementation, a program may not consistently choose the same value if there are multiple environment variables with the same name. The GNU glibc library addresses this issue in getenv() and setenv() by always using the first variable it comes across, and ignoring the rest. unsetenv() will remove all the entries matching the variable name. Other implementations are following this lead.

char *temp;
char *copy;

if ((temp = getenv("TEST_ENV")) != NULL) {
  copy= (char *)malloc(strlen(temp) + 1);
  if (copy != NULL) {
    strcpy(copy, temp);
  }
  else {
    /* handle error condition */
  }

  copy[0] = 'a';
  setenv("TEST_ENV", copy, 1);
}
else {
  return -1;
}

In addition, it is possible to search through environ checking for multiple entries of a variable. Any duplicate values are an indication of an attack; take appropriate action. It is unlikely that there would be a need for more than one variable of the same name.

Risk Assessment

An adversary could create several environment variables with the same name. If the program checks against one copy, but actually uses another, this could be a clear problem.

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"

ENV01-A. Do not make assumptions about the size of an environment variable      11. Environment (ENV)       ENV03-A. Sanitize the environment before invoking external programs