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The C Standard, 7.22.4.6, paragraph 4 [ISO/IEC 9899:2011], states

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.

This paragraph gives an implementation the latitude, for example, to return a pointer to a statically allocated buffer. Consequently, do not store this pointer because the string data it points to may be overwritten by a subsequent call to the getenv() function or invalidated by modifications to the environment. 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.

The getenv() function is not thread-safe. Make sure to address any possible race conditions resulting from the use of this function.

The asctime(), localeconv(), setlocale(), and strerror() functions have similar restrictions. Do not access the objects returned by any of these functions after a subsequent call.

Noncompliant Code Example

This noncompliant code example attempts to compare the value of the TMP and TEMP environment variables to determine if they are the same:

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

  tmpvar = getenv("TMP");
  if (!tmpvar) {
    /* Handle error */
  }
  tempvar = getenv("TEMP");
  if (!tempvar) {
    /* 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");
  }
}

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.

Compliant Solution

This compliant solution uses 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);
  free(tempvar);
}

Compliant Solution (Annex K)

The C Standard, Annex K, provides the getenv_s() function for getting a value from the current environment. However, getenv_s() can still have data races with other threads of execution that modify the environment list.

#define __STDC_WANT_LIB_EXT1__ 1
#include <errno.h>
#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);
  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);
  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 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. The _dupenv_s() and _wdupenv_s() functions provide more convenient alternatives to getenv_s() and _wgetenv_s() because each function handles buffer allocation directly.

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

#include <stdlib.h>
#include <errno.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 or C2x)

POSIX provides the strdup() function, which can make a copy of the environment variable string [IEEE Std 1003.1:2013]. The strdup() function is also included in Extensions to the C Library—Part II [ISO/IEC TR 24731-2:2010]. Further, it is expected to be present in the C2x standard.

#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.

Rule	Severity	Likelihood	Remediation Cost	Priority	Level
ENV34-C	Low	Probable	Medium	P4	L3

Related Vulnerabilities

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

Automated Detection

Tool	Checker	Description
Compass/ROSE
Helix QAC	DF2681, DF2682, DF2683
Klocwork	MISRA.STDLIB.ILLEGAL_REUSE.2012_AMD1
LDRA tool suite	133 D	Fully implemented
Parasoft C/C++test	CERT_C-ENV34-a	Pointers returned by certain Standard Library functions should not be used following a subsequent call to the same or related function
Polyspace Bug Finder	CERT C: Rule ENV34-C	Checks for misuse of return value from nonreentrant standard function (rule fully covered)

Related Guidelines

Key here (explains table format and definitions)

Taxonomy	Taxonomy item	Relationship
C Secure Coding Standard	ENV00-C. Do not store objects that can be overwritten by multiple calls to getenv() and similar functions	Prior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TR 24731-2	5.3.1.1, "The `strdup` Function"	Prior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TS 17961:2013	Using an object overwritten by `getenv`, `localeconv`, `setlocale`, and `strerror` [libuse]	Prior to 2018-01-12: CERT: Unspecified Relationship

Bibliography

[IEEE Std 1003.1:2013]	Chapter 8, "Environment Variables" XSH, System Interfaces, `strdup`
[ISO/IEC 9899:2011]	Subclause 7.22.4, "Communication with the Environment" Subclause 7.22.4.6, "The `getenv` Function" Subclause K.3.6.2.1, "The `getenv_s` Function"
[MSDN]	`_dupenv_s()`, `_wdupenv_s()`
[Viega 2003]	Section 3.6, "Using Environment Variables Securely"