What factor is a security risk for VLAN trunking?

One significant security risk for VLAN trunking is the possibility of VLAN hopping attacks, where an attacker can send packets to a VLAN they do not belong to by manipulating the trunking protocol. This can happen through techniques like double tagging, where the attacker places a packet with two VLAN tags, allowing it to traverse trunk links and reach unauthorized VLANs. Proper configuration of VLANs, including using dynamic trunking protocols securely and implementing access control lists, is essential to mitigate these risks.