Rekeying krbtgt and the behaviour of SSH and delegated credentials

[Greg Hudson]

On 08/05/2016 02:48 PM, Michael Howe wrote:
> When a client has an existing (forwardable) ticket, and the krbtgt is
> rekeyed with -keepold, most things keep working.  However, if that
> ticket is used with SSH using GSSAPIDelegateCredentials=yes it seems to
> make the forwarded ticket unusable - the KDC returns 'Bad encryption
> type' whenever it's used.  (I've not tested other applications that
> might forward credentials.)

I can reproduce this bug under 1.12 (the version of krb5 in Debian
Jessie) but not master.  I believe it to be a previously unknown
manifestation of
http://krbdev.mit.edu/rt/Ticket/Display.html?id=8139
which was previously believed to only affect S4U2Proxy operations.  That
bug was fixed in 1.14.

The fix for this bug is fairly invasive, so it wouldn't be easy to put
together a safe backport.  I think it would be possible to work around
the bug by ignoring errors from krb5_c_verify_checksum() in
kdc_authdata.c:verify_ad_signedpath_checksum().