Julian Gilbey discovered that schroot, a tool allowing users to execute commands in a chroot environment, had too permissive rules on chroot or
session names, allowing a denial of service on the schroot service for
all users that may start a schroot session.
Note that existing chroots and sessions are checked during upgrade, and
an upgrade is aborted if any future invalid name is detected.
Problematic session and chroots can be checked before upgrading with the following command: