USN-8555-2: Ubuntu Advantage Tools (pro client) regression

USN-8555-1 fixed vulnerabilities in Ubuntu Advantage Tools. On Ubuntu
14.04 LTS only, it was discovered that some machines were unable to
enable esm-infra-legacy due to a preemptive apt-helper check. This
update fixes the problem.

We apologize for the inconvenience.

Original advisory details:

Bilal Teke discovered that Ubuntu Advantage Tools exposed the Pro bearer
token in command-line arguments when validating APT credentials. A local
attacker could possibly use this issue to obtain sensitive information
and gain unauthorized access to Ubuntu Pro repositories. (CVE-2026-9494)

Frederick Jerusha discovered that Ubuntu Advantage Tools did not properly
validate data received from the contract server when writing APT source
files. An attacker could possibly use this issue to inject arbitrary APT
configuration and execute arbitrary code. (CVE-2026-11386)

Mateusz Gierblinski discovered that Ubuntu Advantage Tools did not
properly handle symbolic links when collecting diagnostic logs. A local
attacker could possibly use this issue to obtain sensitive information
from files owned by the administrator. This issue only affected Ubuntu
16.04 LTS, Ubuntu 18.04 LTS, Ubuntu 20.04 LTS, Ubuntu 22.04 LTS,
Ubuntu 24.04 LTS and Ubuntu 26.04 LTS. (CVE-2026-12391)