clock skew and preauth

Simo Sorce simo at
Mon Apr 16 07:41:54 EDT 2012

On Sun, 2012-04-15 at 23:52 -0400, Tom Yu wrote:
> Greg Hudson <ghudson at MIT.EDU> writes:
> > I have one concern about this approach, which is that an attacker could
> > create a false log entry for a successful preauthentication on the KDC
> > by forging the timestamp in a preauth-required error.  That is, you
> > attempt to kinit at noon; I forge a timestamp of 11pm in the
> > preauth-required error and capture your preauthenticated request; then
> > at 11pm I send that request to the KDC to make it look like you
> > authenticated at that time.
> >
> > This isn't necessarily a serious enough vulnerability to worry about
> > (when the alternative is for preauth to just fail with skewed clocks),
> > but I want to raise the issue before taking the patch.
> I think it's OK as long as we clearly communicate the auditing
> consequences in our documentation and elsewhere.  Does anyone see a
> security consequence besides auditing?

Being able to forge audit events may be annoying but caring for those is
not as important as having a working system. The tradeoff is more than
justified to me.


Simo Sorce * Red Hat, Inc * New York

More information about the krbdev mailing list