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unsetenv.txt

unsetenv.txt
Posted Sep 28, 1999

The unsetenv function in glibc 2.1.1 suffers from a problem whereby when running through the environment variables, if the name of the variable being unset is present twice consecutively, the second is not destroyed.

tags | exploit
SHA-256 | e95f036725c67eeb8bbbae8a91e4c1024e21dd154ce64c42d89d5f8972a412d8

unsetenv.txt

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 o unsetenv() off-by-one error:
The unsetenv function in glibc 2.1.1 suffers from a problem whereby
when running through the environment variables, if the name of the
variable being unset is present twice consecutively, the second is
not destroyed.

unsetenv is sometimes used by programs that depend on it clearing out
variables for protection against evil environment variables.

It appears as though this was found by someone else before I stumbled
across it; glibc 2.1.2 should not be vulnerable.

To see if your libc has the problem, compile and run the following
program:

#include <stdlib.h>
#include <stdio.h>

extern char **environ;

int main()
{
char *env[] = {
"bob=trash",
"bob=uh-oh",
NULL
};

environ = env;

printf("bob = %s\n", env[0]);

unsetenv("bob");

printf("bob = %s\n", getenv("bob"));

return 0;
}

If the output isn't "bob = (null)", unsetenv() isn't doing its job.
(also note that not all libc's support unsetenv, or even the environ
variable, so this may not compile/link on many non-glibc systems).
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