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Date: Mon, 17 Aug 2015 02:18:22 +0200
From: "DonVallejo ." <>
Subject: Poor security in SOHO routers, again. Changing configuration
 parameters with a click.

Hello all,

i would like to share with you a security issue that i found with some
Comtrend's routers and probably other manufacturer's routers.

The method would let us to configure some router models when a user
clicks a link created by us. I have not read about this method on the
internet, sorry if I am wrong and it’s not new. The method is quite
simple. It is usual to find routers with default passwords. And these
devices usually offers a HTTP based interface to configure them. And
some models accept configuration parameters through the URL.

For example, here it is the javascript source code of the VR-3025un’s
menu for DNS configuration:

function btnApply() {
  var loc = 'dnscfg.cgi?';
  with ( document.forms[0] ) {
    if ( isValidIpAddress(dnsPrimary.value) == false ) {
      alert('Primary DNS "' + dnsPrimary.value + '" has invalid IP address.');
    loc += 'dnsPrimary=' + dnsPrimary.value;
    if (dnsSecondary.value != '') {
      if ( isValidIpAddress(dnsSecondary.value) == false ) {
        alert('Secondary DNS "' + dnsSecondary.value + '" has invalid
IP address.');
      loc += '&dnsSecondary=' + dnsSecondary.value;
      loc += '&dnsSecondary=' + '';
    loc += '&dnsRefresh=1';
  loc += '&sessionKey=' + sessionKey;
  var code = 'location="' + loc + '"';

The default user:password for this router is admin:admin. The default
ip address for the network router is

If the user clicks this simple URL (for example if he receives it by
email), our malicious DNS server will be set to the device:


Other configurations could be modified too (proxies, DMZ, WAN
interface access,…).

(Note in the javascript code, a sessionKey is used. It seems to be
ignored by dnscfg.cgi and the new DNS configuration is set with no

Here you can find the complete article that i wrote in my web:

Models tested:

  Comtrend VR-3025un
  Comtrend AR-5387un
  Comtrend AR-5381un

These routers are currently being used by many jazztel ISP’s clients
in Spain (maybe other ISPs in other countries use these models too).

Almost sure, other Comtrend models are vulnerable. And probably,
models from other manufacturers are vulnerable to this method too
(adapting the URL to the specific device).


Internet Explorer doesn’t accept username and password in the URL (I
mean the syntax Currently Chrome and
Firefox are accepting username and password in the URL. I don’t know
about other browsers.

Javier Vicente Vallejo

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