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Recipe 9.7 Testing Your Search Path
9.7.1 Problem
You want to avoid invoking the wrong
program of a given name.
9.7.2 Solution
Ensure that your search path contains no relative directories:
$ perl -e 'print "PATH contains insecure relative directory \"$_\"\n"
foreach grep ! m[^/], split /:/, $ENV{"PATH"}, -1;'
9.7.3 Discussion
Imagine you innocently type ls while your current
working directory is /tmp, and you discover to
your chagrin that you have just run a malicious program,
/tmp/ls, instead of the expected
/bin/ls. Worse, you might not notice at all, if
the rogue program behaves like the real version while performing
other nefarious activities silently. This can happen if your search
path contains a period ("."),
meaning the current working directory. The possibility of unexpected
behavior is higher if "." is early
in your search path, but even the last position is not safe: consider
the possibility of misspellings. A cracker could create a malicious
/tmp/hwo, a misspelling of the common
who command, and hope you type
"hwo" sometime while
you're in /tmp. As there is no
earlier "hwo" in your search path,
you'll unintentionally run the
cracker's ./hwo program. (Which
no doubt prints, `basename $SHELL`: hwo: command not
found to stderr while secretly demolishing your
filesystem.) Play it safe and keep
"." out of your search path.An empty search path elementtwo adjacent
colons, or a
leading or trailing colon also refers to the current working
directory. These are sometimes created inadvertently by scripts that
paste together the PATH environment variable with
":" separators, adding one too
many, or adding an extra separator at the beginning or end.In fact, any relative directories in your search path
are dangerous, as they implicitly refer to the current working
directory. Remove all of these relative directories: you can still
run programs (securely!) by explicitly typing their relative
directory, as in:
./myprogram
Our recipe uses a short Perl script to split the
PATH environment variable, complaining about any
directory that is not absolute (i.e., that
does not start with a "/"
character). The negative limit (-1) for split is important for
noticing troublesome empty directories at the end of the search path.
9.7.4 See Also
environ(5).
