Internal nodes are compromised + external RPC is DoSed, the attack and defense are asymmetric.

LayerZero incident report states that on April 18, its cross-chain protocol-based KelpDAO rsETH bridge was attacked, resulting in a loss of approximately 116,500 rsETH (about $292 million).
The attack began on March 6, with the attacker obtaining the developer session key through social engineering, infiltrating the RPC cloud environment, and contaminating internal nodes.
Subsequently, a DoS attack was launched against external RPC providers, forcing the DVN signing service to rely only on two compromised internal nodes, thereby providing an effective proof for forging cross-chain messages.
LayerZero indicated that the incident originated from the configuration of a single validator in the related OApp, and did not affect other OApps, channels, or transactions.