The Vendor's App Is the Attacker — A Multi-Tenant Application Identity Opens the Cloud Tenant — severe-but-plausible scenario — ITOL

Vector: Third-Party & Concentration · Severity: severe

The bank's backup, security or management vendor holds an application identity with standing consent in the bank's Microsoft 365 tenant. The vendor's credential is stolen. Overnight an agentic framework attempts sign-ins to thousands of tenants with it and succeeds in a few hundred. It assigns the vendor's own service principals (the logins a vendor's software uses in the bank's cloud) to administrator roles, adds new secrets to other applications, and reads key vaults at scale. The bank's users see nothing. The vendor's advisory names the application IDs and says to disable them. Disabling the backup app pauses the backups.