Oracle SCN Problem

In case you have not heard all the buzz about the Oracle SCN flaw, which was published by Infoworld after the release of January CPU, I summarize the essentials.

  • There is a risk that the SCN reaches it´s maximum value and this could lead to an outage of the database.
  • There is a bug in “ALTER DATABASE BEGIN BACKUP”, which increases the SCN dramatically. (Bug 12371955 – fixed in and others)
  • When you query a remote database via database link from a database with elevated SCN, the remote SCN gets increased to the higher value as well. This has the dramatic effect, that a database will infect the other database.
  • Imagine an unpatched environment with user-managed online backups (e.g. for SAN split mirror technology) and dozens of interconnected databases with database links.
  • Now imagine an evil employee with this little innocent database account with only “create session” privilege  … you get the picture.
  • The January 2012 CPU/PSU contains a patch that should restrict the ways to increase the SCN. However, I am not sure that the issue with db links is solved. I could bump up the SCN to 12562779343042 by a remote select even after patching with CPU January 2012.

Additionally, there are already quite a few articles showing how a potential attacker could use this flaw, e.g. this one.


  • Information on the System Change Number (SCN) and how it is used in the Oracle Database [ID 1376995.1]
  • Installing, Executing and Interpreting output from the “scnhealthcheck.sql” script [ID 1393363.1]
  • Bug 12371955 – Backup task can cause increased SCN growth rate leading to ORA-600 [2252] errors [ID 12371955.8]

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  1. Hopefully the SCN will not reach it’ s maximum limit till 21st December 2012 (Mayan Doomsday Date) 😉

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