Accounts corrupted by the pre-fix SetPassword hold argon2(username + password)
tagged with an algorithm whose phrase rule omits the username. Nothing in the
hash distinguishes that from a forgotten password, so the only detection is to
retry with the other phrase on a failed verify -- which doubles the cost of
every failed login, and failed logins are the credential-stuffing surface.
Hence accountSecurity.repairMigratedPasswords, default false: an operator turns
it on for a migration window and off again. Repaired accounts are force-rehashed
because their stored parameters may already be current, in which case NeedsRehash
would decline and the account would verify once and stay broken.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
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