Security Advisories can be found at: https://github.com/vmware/photon/wiki/Security-Advisories
In Photon OS version 4.0 onwards, the security advisories for the updated RPMs are available in the respective repositories. To see more details about all the published security advisories, use the updateinfo option in the tdnf command. To see the entire list of the security advisories, enable both the repositories.
Notes: The number of security advisories that you see in the message of the day is only for the enabled repository and might not refer to the total number of advisories.