November 27th, 2001, 10:57 AM
Malicious use of grc.com
I thought I'd share this info gained from a ntsecurity mailing list (also mentioned on the http://theregister.co.uk/content/6/23033.html)
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
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
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
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.