An Account Was Taken While MFA Was Working. It Was Locked in Sixteen Minutes.
The Challenge
A staff member signed in during an ordinary working day, on their own machine, on the office network. The password was strong. Multi-factor authentication was enabled and it worked. The account was compromised anyway.
The attack was adversary-in-the-middle: the attacker sat between the user and the real Microsoft sign-in page, relayed every step to the genuine service, and kept a copy of the session token issued after MFA succeeded. Nothing about the sign-in looked unusual, because nothing about it was unusual.
What We Did
- Detection fired automatically and the account was locked while the attacker was still in the mailbox
- Found and removed a device the attacker had registered against the account — the access that survives a password reset
- Found a hidden inbox rule created to silently delete mail from one company, so a conversation could be conducted without the real owner seeing it
- Reconstructed the intrusion timeline independently from the tenant evidence rather than relying on the security vendor’s summary
- Revoked sessions, reset credentials, and reviewed every other account in the tenant for the same pattern
- Wrote the findings up so the same attack is recognisable next time
The Result
- The lesson we took from it is on the blog: resetting the password does not remove a registered device
- Conditional Access rules restricting device registration were tightened afterwards, here and across the wider estate