From the upstream advisory:
http://www.openssh.com/txt/legacy-cert.adv

        When generating legacy *-cert-v00 certificates,
        the nonce field was not being correctly filled with random
        data but was left uninitialised, containing the contents of
        the stack.

        The contents of the stack at this point in ssh-keygen's
        execution do not appear to leak the CA private key or other
        sensitive data, but this possibility cannot be excluded on
        all platforms and library versions.

        If certificates are generated using user-specified contents
        (as opposed to the CA specifying all fields) then they will
        be less resistant to hash collision attacks. Fortunately,
        such attacks are not currently considered practical for the
        SHA family of hashes used to sign these certificates.

This only affected OpenSSH versions 5.6 and 5.7, which Red Hat does not
currently ship.