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The getenv() function searches an environment list, provided by the host environment, for a string that matches a specified name. Do not rely on the pointer to the string returned by getenv() following a subsequent invocation. Wiki MarkupAccording to C99 \[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\]:C Standard, 7.24.4.6, paragraph 4 [ISO/IEC 9899:2024], 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. If the specified name cannot be found, a null pointer is returned.

This allows paragraph gives an implementation the latitude, for example, to copy the environmental variable to an internal static buffer and return a pointer to that a statically allocated buffer.

If you do not immediately make a copy of the value returned by getenv(), but instead store the pointer somewhere for later use, you could end up with a dangling pointer or a different value altogether.

Non-Compliant Coding Example

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Compliant Solution

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There is a race condition here even after you call getenv() and before you copy. Be careful to only manipulate the process environment from a single thread at a time.

Risk Assessment

Examples of vulnerabilities resulting from the violation of this recommendation can be found on the CERT website.

References

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.

Related Vulnerabilities

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

Automated Detection

Related Guidelines

Key here (explains table format and definitions)

Bibliography

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Image Added Image Added Image Added Wiki Markup\[[Dowd 06|AA. C References#Dowd 06]\] Chapter 10, "UNIX II: Processes" \[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] 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" \[[Wheeler 03|AA. C References#Wheeler 03]\] [Section 5.2, "Environment Variables"|http://www.dwheeler.com/secure-programs/Secure-Programs-HOWTO/environment-variables.html]