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The {{getenv()}} function searches an environment list, provided by the host environment, for a string that matches a specified name.  The {{getenv()}} function returns a pointer to a string associated with the matched list member.  It is best not to store this pointer as it may be overwritten by a subsequent call to the {{getenv()}} function \[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] or invalidated as a result of changes made to the environment list through calls to {{putenv()}}, {{setenv()}}, or other means.  Storing the pointer for later use could result in a dangling pointer or a pointer to incorrect data.

Wiki MarkupAccording to C99 \[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\]:The 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 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.

immediately use and discard this string, make a copy of the referenced string returned by getenv() so that this copy may be safely referenced at a later time. The getenv() function is not thread-safe. Make sure to address any possible race conditions resulting from the use of this function.

Implementation Details

According to the Microsoft Visual Studio 2005/.NET Framework 2.0 help pages:

The getenv function searches the list of environment variables for varname. getenv is not case sensitive in the Windows operating system. getenv and _putenv use the copy of the environment pointed to by the global variable _environ to access the environment. getenv operates only on the data structures accessible to the run-time library and not on the environment "segment" created for the process by the operating system. Therefore, programs that use the envp argument to main or wmain may retrieve invalid information.

Non-Compliant Code Example

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 This non-compliant code example compares 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 non-compliant 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;

tmpvar  const char *temp = getenv("TMP");
  if (temp != NULL) {
    tmpvar = (char *)malloc(strlen(temp)+1) return -1;
tempvar;
    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) return -1;

{
      strcpy(tempvar, temp);
    } else {
      /* Handle error */
    }
  } else {
    /* Handle error */
  }

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

Compliant Solution (

...

Annex K)

Microsoft Visual Studio 2005 provides The C Standard, Annex K, provides the getenv_s() and _wgetenv_s() functions 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 * sizeof(char));
  if (!tmpvar) {
    /* handleHandle error condition */
  }
  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) {
    /* handleHandle error condition */
  }
  err = getenv_s(&requiredSize, tempvar, requiredSize, "TEMP" );

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

Compliant Solution (Windows)

...

Microsoft Visual Studio 2005 provides provides the {{\Windows provides the _dupenv_s()}} and {{\_wdupenv_s()}} functions for getting a value from the current environment . \[[Microsoft Visual Studio 2005/.NET Framework 2.0 help pages|http://msdn2.microsoft.com/en-us/library/ms175774(VS.80).aspx]\].[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, The _dupenv_s() provides a and _wdupenv_s() functions provide more convenient alternative alternatives to getenv_s(), and _wgetenv_s() because each function handles buffer allocation directly.

It is the calling program's responsibility to free the memory The caller is responsible for freeing any allocated buffers returned by these functions by calling free().

Code Block
bgColor #ccccff lang c
#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) return -1; errno_t{ /* Handle error */ } err = _dupenv_s(&tempvar, &len, "TEMP"); if (err) { free(tmpvar); return -1; /* Handle error */ } if (strcmp(tmpvar, tempvar) == 0) { puts printf("TMP and TEMP are the same.\n"); } else { putsprintf("TMP and TEMP are NOT the same.\n"); } free(tmpvar); tmpvar = NULL; free(tempvar); tempvar = NULL; }

Compliant Solution (POSIX or C2x)

The following compliant solution depends on the POSIX POSIX provides the strdup() function to , 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 = strdup(getenv("TMP"));
char *tempvar = strdup(getenv("TEMP"));
if (!tmpvar) return -1;
if (!tempvar) return -1;

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

If an environmental variable does not exist, the call to {{getenv()}} returns a null pointer.  In these cases, the call to {{strdup()}} should also return a null pointer, but it is important to verify this as this behavior is not guaranteed by POSIX \[[Open Group 04|AA. C References#Open Group 04]\]

Compliant Solution

This compliant solution is fully portable.

char *tmpvar;
char *tempvar;
char *temp;

if ( (temp = getenv("TMP")) char *tmpvar;
  char *tempvar;

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

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

if (strcmp(tmpvar, tempvar) == 0) {
  puts  printf("TMP and TEMP are the same.\n");
  }
 else {
    putsprintf("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() can , localeconv(), setlocale(), or strerror() can result in overwritten environmental data.

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"
\[[Open Group 04|AA. C References#Open Group 04]\] 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"

Automated Detection

Related Guidelines

Key here (explains table format and definitions)

Bibliography

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Image Added Image Added Image Added11. Environment (ENV)      11. Environment (ENV)       ENV01-A. Do not make assumptions about the size of an environment variable