jail.c in jail in FreeBSD 8.0 and 8.1-PRERELEASE, when the "-l -U root" options are omitted, does not properly restrict access to the current working directory, which might allow local users to read, modify, or create arbitrary files via standard filesystem operations.
FreeBSD Security Advisory - The jail utility does not change the current working directory while imprisoning. The current working directory can be accessed by its descendants.