According to Microsoft Defender for Identity documentation, when an alert such as "Suspected identity theft (Pass-the-Ticket)" appears, it indicates that a compromised Kerberos ticket might have been used by an attacker to impersonate a legitimate account. The goal in this scenario is to contain the spread of the attack while minimizing disruption to unaffected users and systems.
Microsoft recommends isolating or quarantining the compromised device first, as it is the likely source of the malicious activity. Quarantining Device1 blocks it from communicating with other networked systems, preventing further credential theft or lateral movement, while allowing SOC teams to investigate and remediate the issue.
Resetting all account passwords or disabling user accounts at this stage could cause significant operational disruption and is not advised unless there is evidence that multiple credentials were compromised. Defender for Identity's containment guidance explicitly notes:
* "Contain the affected device to stop further credential theft or movement."
* "Reset passwords only for accounts confirmed to be compromised."
Therefore, the least disruptive and most efficient containment step is to quarantine Device1 only . This action effectively isolates the threat while maintaining normal operations for other users and devices, satisfying the requirement to minimize administrative effort and user impact.
Hence, the verified answer is B. Quarantine Device1 only .