For those who haven't heard, there was a Bash exploit announced yesterday. Although a patch did come out (4.3.25), there are reports that it does not fully fix the problem.

Using variations of the test string that was posted to slashdot, it looks as if perl makes your system invulnerable:

sh-3.2$ env x='() { :;}; echo vulnerable' sh -c "echo this is a test"
vulnerable
this is a test
sh-3.2$ env x='() { :;}; echo vulnerable & echo' perl -e 'system "echo
+ test"'
test
sh-3.2$ env x='() { :;}; echo vulnerable' perl -e 'print `echo test`'
test

[download]

... but unfortunately, perl only protects you when you either pass system a list. In other cases, if it sees a shell meta character in your string, you're still vulnerable:

sh-3.2$ env x='() { :;}; echo vulnerable' perl -e 'print `echo test;`'
vulnerable
test
sh-3.2$ env x='() { :;}; echo vulnerable' perl -e 'system "echo test;"
+'
vulnerable
test
sh-3.2$ env x='() { :;}; echo vulnerable' perl -e 'system qw(echo test
+;)'
test;

[download]

Your main attack vector is CGIs -- anyone can set their user-agent, or pass in a query string, and the webserver will set environmental variables automatically. Should your scripts shell out, they're exploitable.

So, the moral of the story: always use the list form of system, and avoid backticks if you can. If you have to do strange things w/ redirecting output, look at IPC::Open2 and IPC::Open3 which can also take list inputs.

In reply to The importance of avoiding the shell by jhourcle

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