A malicious, compromised, or man-in-the-middle PostgreSQL server can supply an arbitrarily large SCRAM-SHA-256 PBKDF2 iteration count during authentication. The client runs the key derivation inline with no upper bound, pinning a tokio worker thread for minutes per connection and possibly stalling the whole async runtime. Applications that connect only to trusted databases are not exposed. Fixed in 0.6.12, which caps the accepted iteration count at 100000 in postgres-protocol/src/authentication/sasl.rs.