The spanning-tree protocol is used to cut loops that redundant links create in bridge networks. These packets are not attested by the system, so an attacker could spoof the BPDU and compromise the network stability!
See below to understand BPDU attack:
In this example the Ciscozine1 switch is elected Root Bridge due to the lower MAC-address (suppose that [...]

The Spanning Tree Protocol is an OSI layer-2 protocol that ensures a loop-free topology for any bridged LAN. Spanning tree allows a network design to include spare (redundant) links to provide automatic backup paths if an active link fails, without the danger of bridge loops, or the need for manual enabling/disabling of these backup links. [...]