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Some implomentations implementations provide a nonportable environment pointer that is valid when main() is called but may be invalidated by operations that modify the environment.

The C Standard, J.5.1 2 [ISO/IEC 9899:20112024], states:

In a hosted environment, the main function receives a third argument, char *envp[], that points to a null-terminated array of pointers to char, each of which points to a string that provides information about the environment for this execution of the program (5.1.2.3.2).

Consequently, under a hosted environment supporting this common extension, it is possible to access the environment through a modified form of main():

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An environment pointer may also become invalidated by subsequent calls to getenv(). (see See ENV34-C. Do not store pointers returned by certain functions for more information.).

Noncompliant Code Example (POSIX)

After a call to the POSIX setenv() function or to another function that modifies the environment, the envp pointer may no longer reference the current environment. The Portable Operating System Interface (POSIX^®), Base Specifications, Issue 7 [IEEE Std 1003.1:2013], states:

Unanticipated results may occur if setenv() changes the external variable environ. In particular, if the optional envp argument to main() is present, it is not changed, and thus may point to an obsolete copy of the environment (as may any other copy of environ).

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Because envp may no longer point to the current environment, this program has undefined unanticipated behavior.

Compliant Solution (POSIX)

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According to the Visual C++ reference [MSDN],

The environment block passed to main and wmain is a "frozen" copy of the current environment. If you subsequently change the environment via a call to _putenv or _wputenv, the current environment (as returned by getenv / _wgetenv and the _environ / _wenviron variable) will change, but the block pointed to by envp will not change.

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Because envp no longer points to the current environment, this program has undefined unanticipated behavior.

Compliant Solution (Windows)

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Using the envp environment pointer after the environment has been modified can result in undefined behavior.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Key here (explains table format and definitions)

Bibliography

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