Shields Down?

November 26, 2001 7:54 PM UTC
Hackers Banquet @ GRC.

From: Magni@HammerofGod.com
To: BUGTRAQ@SECURITYFOCUS.COM
Subject: Malicious use of grc.com
Date: Mon, 26 Nov 2001 14:53:16 -0500

Greetings:

ShieldsUp(tm) is an application developed by Steve Gibson of Gibson Research Corporation that allows a web user to request a remote port scan of their local system via the GRC.Com web site (https://grc.com/x/ne.dll?bh0bkyd2). The 'Probe my Ports' option performs a scan of many common tcp ports and reports the status of each port back to the user's browser.

The development of the application and its method of identifying the client IP address is quite insecure. As a result, ShieldsUp! allows the web user to perform a port scan against any other machine on the Internet and return the results to the web user. The remote system will log the scan as having originated from one of Steve Gibson's machines.

Gibson has chosen to use a simple hidden tag in the client-side HTML code to identify the IP address that is passed to the scanning engine. Though the client's IP address is hashed, it is trivial to alter the value of the hidden tag in order to request that a different IP address be scanned. The true IP address is never checked in the HTTP header during the scan - ShieldsUp happily scans the other box while returning the result set into the browser of the box that requested the scan.

Fenris, The Wolf, a member of Hammer of God, quickly reviewed the hash algorithm used to represent the IP address and found it weak; therefore, one can easily submit requests, via the Shields Up web page, for specific IP addresses to be scanned. These findings are not my own, and I have not included the details of the hash here as it is used to display a copyrighted page. The Wolf may post his findings if he chooses to do so, but I will not make that choice for him.

Instead, we can easily bypass the need to crack the hash by simply using the 'IP Agent' supplied by Gibson. Over a year ago, a hacked version of IP Agent was published that allowed one to supply an address to scan - Gibson discounted this as a non-issue, but reportedly fixed IP Agent to perform a check to prevent this from happening.

However, IP Agent now supports multiple client IP addresses. One simply needs to bind the targeted IP addresses to a local interface and perform a scan request. In this case, ShieldsUp presents friendly command buttons listing the IP addresses bound to the local interfaces and allows you to select any one that you want scanned. Again, no other checking is done, and ShieldsUp will scan whatever IP address you ask it to and display the results in your own browser.

According to the scanning page, 'Information gained will NOT be retained, viewed, or used by us in any way for any purpose whatsoever' which basically invites anyone to use Gibson's site to do port scans of other people's boxes without fear of detection.

Additionally, multiple post requests can be easily scripted to perform scans against a site in attempts to perform a denial of service attack against a host. In these cases, with sufficient requests generated, one could ask grc.com to attack another site and it will comply.

One would have hoped that instead of Mr. Gibson spending so much time expounding on the theoretical DoS capabilities of Raw Sockets, that he instead had used that time to properly develop his own application in order to prevent the same. Those concerned with malicious attacks from grc.com should block Gibson's netblock at the border.

Cheers,
Magni