Category Archives: Security

A Sobering Post About Code Review From Microsoft

It’s easy to get on the code review band-wagon, and tout it as the “silver bullet” for bugs, or the key to developing awesome, elegant software, etc.  It’s easy to get carried away, and forget that code review should probably be accompanied by rigorous testing, static analysis, and security integration from day one.

While the purpose of this blog post by Shawn Hernan from Microsoft may be to attack or question the merits of open source software, I see it as an interesting discussion on the role of code review in software engineering and how it relates to writing secure code.

Insert your own joke about Microsoft security here.  I, personally, think their IE team should read Shawn’s post.

Particularly interesting is one of the comments to the post by “danclarke_2000”:

I think another point is diminishing returns of code review..  Each extra code review brings less value than the preeding; review comments can already be known and awaiting action, not important enough to change etc

having extra eyes reviewing code means generating extra code review output.  Here is the true cost, all the code review comments of the many eyes have to pass through the bottleneck of the few people who have authority to make changes.  As each extra review has less value, processing the extra reviews has a higher and higher opportunity cost.

Sound kind of familiar?

Anyhow, Hernan’s post is an interesting read.  Click here to check it out.


Here’s a quote from Joshua Bloch of Google on a similar topic:

…We programmers need all the help we can get, and we should never assume otherwise. Careful design is great. Testing is great. Formal methods are great. Code reviews are great. Static analysis is great. But none of these things alone are sufficient to eliminate bugs: They will always be with us. A bug can exist for half a century despite our best efforts to exterminate it. We must program carefully, defensively, and remain ever vigilant.

Read the entire post here.

Fiddling Around with Skype

As I said last week, I’ve been working with a partner (Mohammad Jalali) on a project for our networks course.

The idea:  given an arbitrary IP and port number, we want to find a way of determining whether or not there is an FTP server, an HTTP server, or a Skype node on the other side.  FTP and HTTP are trivial – those protocols essentially announce themselves to the world.

Skype clients, on the other hand, act a little more strangely.  Skype goes out of its way to hide its traffic – from straight-up encryption, down to making their client executable really hard to reverse engineer.  Because of this, Skype has been an interesting challenge to the hacker community.

Anyhow, my partner and I have learned a few interesting things about Skype – and in particular, we’ve found a reliable way to determine whether or not Skype is running behind an arbitrary IP and port.  Cool.

Fact 1:  Skype pretends to be an HTTP server

I’m serious, it does.  Using Wireshark, we noticed that both UDP and TCP packets were being sent to one particular port.  Pretty funny behavior…so, we took a closer look.  And this is what we found.  Pop open your Skype client, connect to the network, then use nmap to find the ports that Skype is using:

$>nmap localhost -p10000-50000

Starting Nmap 5.00 ( ) at 2009-12-01 20:33 EST
Interesting ports on localhost (
Not shown: 39999 closed ports
48915/tcp open  unknown

Ok, cool – there’s something at 48915, and it looks like it accepts TCP connections.  Pop open Telnet, connect to it, and feed it an HTTP request:

$>telnet localhost 48915
Connected to localhost.
Escape character is '^]'.
GET / HTTP/1.1
HTTP/1.0 404 Not Found
Connection closed by foreign host.

Ok, we got an HTTP response – looks like there’s an HTTP server back there, right?

Wrong.  Reconnect, and send it some garbage:

$>telnet localhost 48915
Connected to localhost.
Escape character is '^]'.

See all of those funny characters down at the bottom?  That’s what I got back.  In the words of Obi-Wan Kenobi…that’s no HTTP server…it’s a space station (Skype node).

So we’ve learned something here – Skype opens a port, and “spoofs” an HTTP server.  We can easily check for this – just write a script that connects to a port, spews some garbage, and check to see if we got binary garbage back.

It’s so easy, that someone else has already done it.  Remember that nmap tool we used earlier?  Somebody over in that camp wrote a script for the Nmap Scripting Engine that runs this exact analysis on some ip/port.  Don’t believe me?  Read the script yourself! We stumbled upon that script while trying to figure out what Skype was doing with the spoofed HTTP server.
And sure enough:

$>nmap localhost -p48915 --script skype.nse
Starting Nmap 5.00 ( ) at 2009-12-01 20:45 EST
Interesting ports on localhost (
48915/tcp open  skype2

Nmap done: 1 IP address (1 host up) scanned in 0.14 seconds

Hmph.  So much for cutting edge, never-been-done research.  Go figure.

Fact 2:  Given some UDP packets, Skype echos back a predictable pattern

For this part, we’re pretty sure no one else has tried this.

While connected to Skype, we recorded some packets with tcpdump.  We wrote a script that loaded up those packets, and could “replay” the packet payloads to an arbitrary IP and port.

So, we played some packets against an IP/port with Skype behind it.  Most of the time, we got TCP packets with RST flags (which is TCP’s way of telling us to “shut yer trap”).  But wayyyy down in the middle, there was a section of UDP packets that actually got a response:

From Misc Blog Images was the computer we were playing the packets from, and was the computer with Skype running on it. See those UDP packets that are getting echoed back?  That’s the interesting part…instead of just shutting us down with RST’s, Skype appears to be saying something back.

So, is there a pattern in all of this?  Actually yes.  We isolated 4 of those UDP packets, and repeatedly fired them at the same IP/Port on the computer running Skype, and we found a pattern.

The pattern:  the first two bytes that are sent in our UDP packets are echo’d back to us in the first two bytes of the UDP packets that come back.

So, for example,  one UDP payload we sent looked like this:

92 40 02 a1 66 65 ea 0d 8c 82 c3 0c 27 cd c5 e7
4e 78 fe a1 50 a6

And we got back:

92 40 17 c0 a8 00 13 74 a0 41 f0

See that common 92 40?  Bingo.  😉

And it’s pretty consistent – if we repeat the same UDP packet, we get (almost) the same response.

92 40 67 c0 a8 00 13 11 00 10 4f

And if we repeat again…

92 40 37 c0 a8 00 13 68 08 43 3a

92, 40, and c0, a8, 00, 13. Nice!  Looks like a fingerprint to me!


Except, remember, we already found a way of determining whether or not Skype was running behind a given IP/port.  This last finding was just bonus.  My partner and I aren’t sure if our instructor is going to let us stay with this topic, seeing as how it’s pretty much been solved by other people before.  We’ve only got 2 weeks before this project is due, so…if we get another project, let’s hope it’s relatively simple.  Push come to shove, we could always try to fingerprint a different protocol…maybe BitTorrent clients.

Either way, working on this stuff has been pretty cool…and it let me try out some pretty neat tools that are usually reserved for the people with coloured hats (and no, I didn’t mean Red Hat):

  • nmap: port scanner that can also do service/os fingerprinting
  • Scapy: sculpt, gut, spoof, manipulate, and send packets – the power of C, with the simplicity of Python!  We used Scapy as a library while writing our scripts.  Lots of potential with this tool.  Feel like poisoning an ARP cache?  Scapy is for you!
  • Wireshark: a network student’s best friend.

Click here to check out Mohammad’s blog post about this project.

Overriding Firefox’s Window.Alert – Part 4

So, I think I’m more or less done the extension.

Someday, when I’ve got more extension development experience under my belt, I’ll probably come back to this and fix it up.  Until then, this will have to do.

Click here to download.

If you’re interested in looking at the source, just change the file extension from “.xpi” to “.zip”, and decompress.  It’s all there.

There’s no license on this thing, no GPL, MIT, nothing.  Use it however you want.  If you find it useful though, I’d love to hear from you – send me email, post a comment, Facebook, Twitter, whichever.

Whew.  I think I’m going to reward myself with some orange sherbet.  Om nom nom…

Here’s a really annoying website to test the extension with.  I really don’t recommend that you visit it without my extension installed.

The window hops around a bit, so just double click on the location bar, and type in something like “”.  This will start up the flood of alerts, and (hopefully) you’ll be able to suppress them after the first one hits.

Here’s the site.  Visit at your own risk.


I’ve moved the extension to Mozilla Addons, and added Firefox 3.5 compatibility.

I’ve updated alertCheck.xpi so that it’ll play nice with Firefox 3.0b5, and hopefully Firefox 3.1.*.  Let me know if there are any behaviour foulups, and I’ll do my best to fix them.

Overriding Firefox’s Window.Alert – Part 3

Wow.  I think I got it.  I’ve got a Firefox plugin that can suppress all alert() dialogs on a page if the user checks a “suppress” box on the second alert() dialog.

The trick, was not to rely on the DOMContentLoaded event to fire to do the override.  Instead, I used the DOMWillOpenModalDialog to detect the first alert().  After detection, I overrode with an alertCheck which asked the user whether or not to “suppress more dialogs”.  If the user answers in the affirmative, alert() is simply overwritten with an empty function.

Piece of cake.

A couple of issues though…


In order to override the alert() function, I have to write to document.getElementById(‘content’).contentWindow.wrappedJSObject.alert.

Remember how I mentioned the distance between the Extension JavaScript, and the inline content JavaScript?  I said it felt like a security layer.

I was totally right.

Check this out. I’ll quote:

You should be aware of XPCNativeWrappers when working with untrusted content. With XPCNativeWrappers turned on (which is the default in Firefox 1.5+), your extension can safely access the DOM of the content document, but not the content JavaScript. Bypassing XPCNativeWrapper to work with content JavaScript directly can lead to security problems.

Hrmph.  So I seem to be violating some security rules here.  So maybe my approach isn’t the greatest idea.  “Mook” from #extdev suggested looking into commonDialog.xul…but I can’t seem to wrap my head around that just yet.


Not sure why yet, but while I can suppress dialog floods like this:

for (i = 0; i < 10; ++i) {

It seems to fail on this:

for (i = 0; i < 10; ++i) {

For some reason, regardless of whether or not I choose to suppress the dialogs, they just keep coming.  It works fine when I swap out the confirm() for a second alert().  Not exactly sure why.  Yet.

Ok, so I’m going to clean the code up, and post it soon.  I’ll also post a link to a real, brutally annoying website where you can test the alertCheck extension.  Just give me a bit.