[vvanpo]

A pepper is just a salt that is "secret", i.e. not stored next to the hashes being salted. It indeed provides the desired defense-in-depth, in that a breach of the database does not allow spoofing requests to the application; a successful attack would require both an exploit giving database access as well as an exploit exposing the secret pepper. But this is just a result of splitting state across multiple data stores---you could theoretically split data across n databases which would require an attacker to find n exploits. As such I don't fully agree with their conclusion that "database state alone should not be able to grant authentication": just because you store application-wide secrets elsewhere doesn't mean it's not part of the application state, and isn't stored in a database.

For most monolithic applications I think the whole issue is be a bit moot; if the rest of the application state is in the primary database, then an attacker with database access could presumably accomplish anything without the need to spoof another user at the authentication layer.

Lastly, this scheme doesn't provide any mechanism for rotating the pepper.