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The C Standard, subclause 7.22.4.6, paragraph 4 [ISO/IEC 9899:2011] states that getenv() has the following behavior:

The getenv function returns a pointer to a string associated with the matched list member. The string pointed to shall not be modified by the program but may be overwritten by a subsequent call to the getenv function.

Consequently, it is best do not to store this pointer because it may be overwritten by a subsequent call to the getenv() function or invalidated as a result of changes made to the environment list through calls to the POSIX functions putenv(), or  setenv(), or other implementation-specific means. Storing the pointer for later use can result in a dangling pointer or a pointer to incorrect data. This string should be referenced immediately and discarded; if later use is anticipated, the string should be copied so the copy can be safely referenced as needed.

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#include <stdlib.h>
#include <string.h>
#include <stdio.h>
 
void func(void) {
  char *tmpvar;
  char *tempvar;

  tmpvar = getenv("TMP");
  if (!tmpvar) {
    /* Handle return -1;
error */
  }
  tempvar = getenv("TEMP");
  if (!tempvar) {
    /* Handle return -1;
error */
  }
  if (strcmp(tmpvar, tempvar) == 0) {
   if (putsprintf("TMP and TEMP are the same.\n") == EOF) {;
    /* Handle error */
  }
}
} else {
   if (putsprintf("TMP and TEMP are NOT the same.\n") == EOF) {
    /* Handle error */;
  }
}

This code example is noncompliant because the string referenced by tmpvar may be overwritten as a result of the second call to the getenv() function. As a result, it is possible that both tmpvar and tempvar will compare equal even if the two environment variables have different values.

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This compliant solution uses only the C the malloc() and strcpy() functions to copy the string returned by getenv() into a dynamically allocated buffer:

#include <stdlib.h>
#include <string.h>
#include <stdio.h>
 
void func(void) {
  char *tmpvar;
  char *tempvar;

  const char *temp = getenv("TMP");
  if (temp != NULL) {
    tmpvar = (char *)malloc(strlen(temp)+1);
    if (tmpvar != NULL) {
      strcpy(tmpvar, temp);
    } else {
      /* Handle error */
    }
  } else {
    /* Handle error */
  }

  temp = getenv("TEMP");
  if (temp != NULL) {
    tempvar = (char *)malloc(strlen(temp)+1);
    if (tempvar != NULL) {
      strcpy(tempvar, temp);
    } else {
      /* Handle error */
    }
  } else {
    /* Handle error */
  }

  if (strcmp(tmpvar, tempvar) == 0) {
    printf("TMP and TEMP are the same.\n");
  } else {
    printf("TMP and TEMP are NOT the same.\n");
  }
  free(tmpvar);
  tmpvar = NULL;
  free(tempvar);
  tempvar = NULL;
}

Compliant Solution (

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Annex K)

C11 Annex K provides the getenv_s()function for getting a value from the current environment. However, note that according to the standard, getenv_s() can still have data races with other threads of execution that modify the environment list.

#define __STDC_WANT_LIB_EXT1__
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
 
void func(void) {
  char *tmpvar;
  char *tempvar;
  size_t requiredSize;
  errno_t err;
  err = getenv_s(&requiredSize, NULL, 0, "TMP");

  if (err) {
    /* Handle error */
  }
 
  tmpvar = (char *)malloc(requiredSize * sizeof(char));
  if (!tmpvar) {
    /* Handle error */
  }
  err = getenv_s(&requiredSize, tmpvar, requiredSize, "TMP" );

  if (err) {
    /* Handle error */
  }
  err = getenv_s(&requiredSize, NULL, 0, "TEMP");
  if (err) {
    /* Handle error */
  }
 
  tempvar = (char *)malloc(requiredSize * sizeof(char));
  if (!tempvar) {
    /* Handle error */
  }
  err = getenv_s(&requiredSize, tempvar, requiredSize, "TEMP" );

  if (err) {
    /* Handle error */
  }
  if (strcmp(tmpvar, tempvar) == 0) {
    printf("TMP and TEMP are the same.\n");
  } else {
    printf("TMP and TEMP are NOT the same.\n");
  }
  free(tmpvar);
  tmpvar = NULL;
  free(tempvar);
  tempvar = NULL;
}

Compliant Solution (Windows)

Microsoft Windows also provides the _dupenv_s() and wdupenv_s() functions for getting a value from the current environment [MSDN]. The _dupenv_s() function searches the list of environment variables for a specified name. If the name is found, a buffer is allocated; the variable's value is copied into the buffer, and the buffer's address and number of elements are returned. By allocating the buffer itself, _dupenv_s() and _wdupenv_s() provide a more convenient alternative to getenv_s() and _wgetenv_s().

The calling program is responsible for freeing any allocated buffers returned by these functions by calling free().

#include <stdlib.h>
#include <string.h>
#include <stdio.h>
 
void func(void) {
  char *tmpvar;
  char *tempvar;
  size_t len;

  errno_t err = _dupenv_s(&tmpvar, &len, "TMP");
  if (err) {
    /* Handle error */
  }
  err = _dupenv_s(&tempvar, &len, "TEMP");
  if (err) {
    /* Handle error */
  }

  if (strcmp(tmpvar, tempvar) == 0) {
    printf("TMP and TEMP are the same.\n");
  } else {
    printf("TMP and TEMP are NOT the same.\n");
  }
  free(tmpvar);
  tmpvar = NULL;
  free(tempvar);
  tempvar = NULL;
}

Compliant Solution (POSIX)

...

#include <stdlib.h>
#include <string.h>
#include <stdio.h>
 
void func(void) {
char *tmpvar;
  char *tempvar;

  const char *temp = getenv("TMP");
  if (temp != NULL) {
    tmpvar = strdup(temp);
    if (tmpvar == NULL) {
      /* Handle error */
    }
  } else {
    /* Handle error */
  }

  temp = getenv("TEMP");
  if (temp != NULL) {
    tempvar = strdup(temp);
    if (tempvar == NULL) {
      /* Handle error */
    }
  } else {
    /* Handle error */
  }

  if (strcmp(tmpvar, tempvar) == 0) {
    printf("TMP and TEMP are the same.\n");
  } else {
    printf("TMP and TEMP are NOT the same.\n");
  }
  free(tmpvar);
  tmpvar = NULL;
  free(tempvar);
  tempvar = NULL;
}

Risk Assessment

Storing the pointer to the string returned by getenv(), localeconv(), setlocale(), or strerror() can result in overwritten data.

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

Automated Detection

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