Yealink YMCS RPS before 2025-06-04 lacks SN verification attempt limits, enabling brute-force enumeration (last five digits).
https://www.yealink.com/en/trust-center/security-advisories/b8dc062eaa8d4f59
https://support.yealink.com/en/portal/knowledge/show?id=6476e7cd6a27da76bd06a9c9