Wednesday, July 22, 2009

DD-WRT (httpd service) Remote Command Execution Vulnerability

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")) {
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)............

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/%s905 server_dir != NULL ?
server_dir : "/www",file);
906 else
907 fprintf(exec, "/%s/%s\n",
908 server_dir != NULL ? server_dir : "/www",
909 fclose(exec);
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 }
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:


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:


Voila (pretty old-school, eheh). Here is some (poor) video demonstrating the

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:

Unlike the already documented CSRF vulnerability ( ) 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 for his support :)

Leka vecher :)

# [2009-07-20]

Berapa banyak hospot yang terdapat di indonesia ya ... metode cukup mudah ... :D apalagi hole ddwrt ini cukup menggairahkan ...

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