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Re: md5 for bgp tcp sessions

  • From: Todd Underwood
  • Date: Thu Jun 23 05:58:12 2005

ras, all,

On Thu, Jun 23, 2005 at 12:14:12AM -0400, Richard A Steenbergen wrote:
> On Wed, Jun 22, 2005 at 10:04:09PM -0400, Todd Underwood wrote:

> > 	a) many (all?) implementations of md5 protection of tcp expose
> > new, easy-to-exploit vulnerabilities in host OSes.  md5 verification
> > is slow and done on a main processor of most routers.  md5
> > verification typically takes places *before* the sequence number,
> > ports, and ip are checked to see whether they apply to a valid
> > session.  as a result, you've exposed a trivial processor DOS to your
> > box.  
> Well, I think they've finally fixed this one by now, at least everyone 
> that I'm aware of has done so. Immediately following the whining to start 
> deploying MD5 is was certainly the case that many implementations did 
> stupid stuff like process MD5 before running other validity checks like 
> sequence numbers which are far less computationally intensive, and there 
> were a few MSS bugs that popped up, but they should have all been worked 
> out by now. I don't think that anyone running modern code is suffering any 
> more attack potential because of this.

my understanding is that md5 is still checked before the ttl-hack
check takes place on cisco (and perhaps most router platforms).  new
attack vector for less security than you had before.  oh well.  ras:
can you confirm that it is possible to implement ttl-hack and have it
check *before* md5 signature checks?

the chaos (and crappy quality of the implementations) during the panic
demonstrates two other things:  rolling out magic code because your
vendor tells you to is a bad idea;  slapping together a hack on top of 
a well-designed protocol without careful thought and testing is a
terrible idea.  


todd underwood
director of operations & security
renesys - interdomain intelligence
[email protected]