USN-6588-2: PAM vulnerability

USN-6588-1 fixed a vulnerability in PAM. This update
provides the corresponding updates for Ubuntu 14.04 LTS,
Ubuntu 16.04 LTS, and Ubuntu 18.04 LTS.

Original advisory details:

Matthias Gerstner discovered that the PAM pam_namespace module incorrectly
handled special files when performing directory checks. A local attacker
could possibly use this issue to cause PAM to stop responding, resulting in
a denial of service.

advisory attacker directory files issue local lts pam performing special ubuntu update updates usn vulnerability