The ISO/IEC 9796:1991 standard has been withdrawn in 2000 by ISO/IEC,
but can still be purchased as an INCITS standard.
A free online source is section 11.3.5 in the excellent (though slightly dated) Handbook of Applied Cryptography.
I announced the attack on August 30, 1999 in an sci.crypt.research post.
Here is the mentioned file with messages allowing the attack. Same gzipped, for hash verification purposes.
This page was revised on 2022-05-28