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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)

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].

#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

Version

Checker

Description

Compass/ROSE




LDRA tool suite
9.7.1

133 D

Fully implemented
Parasoft C/C++test

10.4.2

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

R2019b

CERT C: Rule ENV34-CChecks for misuse of return value from nonreentrant standard function (rule fully covered)
PRQA QA-C

9.7

2681, 2682, 2683
PRQA QA-C++

 4.4

2681, 2682, 2683

Related Guidelines

Key here (explains table format and definitions)

Taxonomy

Taxonomy item

Relationship

C Secure Coding StandardENV00-C. Do not store objects that can be overwritten by multiple calls to getenv() and similar functionsPrior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TR 24731-25.3.1.1, "The strdup Function"Prior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TS 17961:2013Using 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"



9 Comments

  1. Question: on which actual system does getenv() create copies of the environment variable?

  2. None that I know of.

  3. reading over this again, it seems like there is an inherent TOCTOU in getenv() between it returning a pointer, and a user reading it... why aren't we more worried about this?

    1. I think this whole rule is about the TOCTOU condition you cite, without ever mentioning it.

      I think the reason the race condition is not highlighted more is that we have agood handle on how the contents of the getenv return pointer can change...through another getenv call, (maybe through putenv/setenv too?)

      Unless you are talking about a multithreaded env...the rule does state that getenv() is not thread-safe.

  4. Would it be worthwhile to address the potential TOCTOU within the compliant code examples themselves, maybe in the form of a comment? I know the examples are already long enough but it might increase the chances of someone recognizing the implications if they're prone to cut-and-paste of these examples.

  5. Regarding the ROSE algorithm, do we also need to check for calls to putenv()/setenv() between a store from getenv() and an access?

    1. Good question, and one that applies to the rule as a whole, not just to ROSE. The POSIX standard says nothing about putenv() or setenv() invalidating pointers returned by getenv().

      I would guess that putenv() and setenv() do invalidate getenv() pointers, as they do modify the environment array, which is how env vars are usually stored these days. We have another rule that declares that they invalidate envp, so invalidating getenv() is likely.

  6. I think that the POSIX and C compliant solutions are hiding buffer overflow bugs.  There's no assurance that the string returned by getenv() or getenv_s() will be null terminated.  So, for instance, putenv() or its moral equivalent were called with an unterminated string, the call to getenv() would return an unterminated string that would be passed to strdup() or strlen()/ strcpy().

  7. I would like to extend this to "similar functions" meaning any function anyone writes that returns a pointer to a reusable buffer.  I know we considered this for TS 17961 but I don't recall why it didn't make it into the document.