The access switchport connecting to the OOBM access switch is not the only port with membership to the management VLAN.
The OOBM access switch will connect to the management interface of the managed network elements. The management interface can be a true OOBM interface or a standard interface functioning as the management interface. In either case, the management interface of the managed network element will be directly connected to the OOBM network. An OOBM interface does not forward transit traffic; thereby, providing complete separation of production and management traffic. Since all management traffic is immediately forwarded into the management network, it is not exposed to possible tampering. The separation also ensures that congestion or failures in the managed network do not affect the management of the device.
Ensure that the access switchport connecting to the OOBM access switch is the only port with membership to the management VLAN
Review the managed switch configuration and verify that the access port connected to the OOBM access switch is the only port has been assigned to the management VLAN.