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Observable response discrepancy in PostgreSQL SCRAM authentication allows an unauthenticated user to test the existence of a user via observing the SCRAM iteration count.  This requires the probed user to have a non-default scram_iterations count, because the authentication challenge for a nonexistent user reports the default scram_iterations.  Within major versions 16-18, minor versions before PostgreSQL 18.5, 17.11, and 16.15 are affected.  Versions before PostgreSQL 16 are unaffected.