One of the user sessions was terminated abnormally in the middle of a transaction. What does Oracle database do to recover it?

Undo data is used to roll back (undo) uncommitted changes, not to roll forward - roll forward operations use redo log data to reapply committed changes.

Flashback Logs support the Flashback Database feature to rewind an entire database to a prior point in time, not to recover individual aborted user transactions.

Online redo logs contain redo vectors used to roll forward committed changes during instance recovery, not to roll back changes from individual terminated sessions.

Oracle records 'before image' data in undo segments for every DML change. When a session ends abnormally, the PMON background process detects the dead process and uses this undo data to reverse all uncommitted changes, returning affected rows to their pre-transaction state. This automatic rollback ensures read consistency and data integrity without manual intervention.

RMAN is a backup and restore tool for recovering database files from physical failures, not an automatic mechanism for rolling back uncommitted user transactions.

SMON performs instance recovery after a full database crash by applying redo and rolling back uncommitted transactions from the undo, but for a single dead user process that role belongs to PMON.