Varonis details TrustSink attack abusing Microsoft Entra external MFA providers

Varonis Threat Labs disclosed a technique called TrustSink in which an attacker with a highly privileged Entra account registers a rogue external MFA provider that inserts a fake password prompt into legitimate login flows, capturing users' plaintext passwords. The rogue provider still returns a valid signed token to Entra, so the sign-in completes normally with no visible error, and resetting a stolen password does not remove the malicious provider from the authentication flow. Varonis says the method could work with any authentication system using this external MFA model but demonstrated it specifically against Microsoft Entra.