cloud-init has the ability to generate and set a randomized password for system users. This functionality is enabled at runtime by passing cloud-config data such as: 'chpasswd: list: | user1:RANDOM'
When instructing cloud-init to set a random password for a new user account, versions before 21.1.19 would write that password to the world-readable log file /var/log/cloud-init-output.log. This could allow a local user to log in as another user (CVE-2021-3429).