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Many programs and libraries, including the shared library loader on both UNIX and Windows systems, depend on environment variable settings. Because environment variables are inherited from the parent process when a program is executed, an attacker can easily sabotage variables, causing a program to behave in an unexpected and insecure manner \[[Viega 03|AA. C References#Viega 03]\]. |
All programs, in particularly those running with higher privileges than the caller (such as those with setuid/setgid flags), should treat their environment as untrusted user input. Because the environment is inherited by processes spawned by calls to the fork(), system(), or exec() functions, it is important to verify that the environment does not contain any values that can lead to unexpected behavior.
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C99 states that, "the set of environment names and the method for altering the environment list are implementation-defined." ThereforeConsequently, it is consequently important to understand what local functions are available for clearing, modifying, and looking up default values for environment variables. Because some programs may behave in unexpected ways when certain environment variables are not set, it is important to understand which variables are necessary on your system and what are safe values for them.
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POSIX also specifies the {{confstr()}} function, which, can then be used to look up default values for environment variables \[[Open Group 04|AA. C References#Open Group 04]\]. POSIX.1-2008 defines a new {{\_CS_V7_ENV}} argument to {{confstr()}} to retrieve a list of environment variable settings required for a default conforming environment \[[Austin Group 08|AA. C References#Austin Group 08]\]. A space-separated list of {{variable = value}} pairs is returned, with variable names guaranteed not to contain equal signs (=), and {{variable = value}} pairs guaranteed not to contain spaces. Used together with the {{\_CS_PATH}} request illustrated above, this completely describes the minimum environment variable settings required to obtain a clean, conforming environment. On systems conforming to the POSIX.1-2008 standard, this should be used to create a sanitized environment. |
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