This BattleEye exploit demonstrates a classic failure of trust boundary definition - they effectively created a system where client attestation was accepted without proper authentication or verification.
It means you trust something with lower trustworthiness without (re)validating, or even trusting it at all if the validation isn't all but guaranteed. The boundary is when you switch between levels of trust.
Trusting something outside of your control is a good example. When your trusted game server trusts the untrusted game client when it says "trust me, it was a headshot" without validating this.
