Description

On March 24, 2023, Microsoft warned about a recently patched critical Outlook vulnerability that allows attackers to steal NT Lan Manager (NTLM) hashes and launch relay attacks without user interaction. Microsoft also shared guidance to find indicators of compromise linked to Outlook vulnerability. The critical Outlook vulnerability, tracked as CVE-2023-23397 and having a CVSS score of 9.8, is described as a privilege escalation flaw in Microsoft Outlook in Windows. Following an exploit, external attackers could send specially crafted emails to the victim that would force the victim to connect to one of the attackers' untrusted services. Later, the attacker could relay the victim's Net-NTLMv2 hash to another service and authenticate as the victim. Microsoft addressed the critical Outlook vulnerability in Patch Tuesday updates for March 2023, but not before, Russian-based threat actors weaponized the flaw against government, transportation, energy, and military sectors in Europe. Microsoft's incident response team says that they discovered the evidence of possible exploitation of the CVE-2023-23397 as early as April 2022. In one of the attacks, a threat actor was able to gain unauthorised access to an Exchange Server and modify mailbox folder permissions for persistent access due to a successful Net-NTLMv2 Relay attack. Further, the compromised email account was then utilized to extend the attacker's access within the infected network by sending additional malicious messages to target other members of the same organization. Furthermore, Microsoft claims that the exploit for CVE-2023-23397 is novel and stealthy in addition to leveraging NTLMv2 hashes to gain unauthorized access to resources. For identification of potential exploitation via CVE-2023-23397, organizations are requested to review SMBClient event logging, Process Creation events, and another available network telemetry. Also, an open-source incident response tool has been released by the Cybersecurity and Infrastructure Security Agency (CISA) to detect malicious activity in Microsoft cloud environments.