CVE-2023-23397 - Microsoft Outlook Elevation of Privilege Zero-Day Vulnerability

THE THREAT

On March 14th, as part of Microsoft’s monthly Patch Tuesday release, the company disclosed a critical, actively exploited vulnerability impacting Microsoft Office and Outlook. The vulnerability is tracked as CVE-2023-23397 (CVSS: 9.8) - Microsoft Outlook Elevation of Privilege vulnerability. Exploitation of this vulnerability would allow a threat actor to access an impacted user’s Net-NTLMv2 hash, enabling future NTLM Relay attacks to authenticate to other services as the user.

CVE-2023-23397 is relatively simple to exploit, and its success has already been proven in real world attacks; as such, widespread exploitation is expected in the immediate future. As exploitation has been observed in limited cases, immediate patching of CVE-2023-23397 is critical. eSentire Threat Intelligence assesses it is almost certain threat actors will adopt the technique for use in attacks in the immediate future. 

What we’re doing about it

• eSentire Managed Vulnerability Service (MVS) has plugins in place to identify CVE-2023-23397
• Threat hunts related to the exploitation of CVE-2023-23397 are ongoing
• Related malicious IP addresses have been added to the eSentire blocklist
• eSentire security teams are tracking this vulnerability for additional details and detection opportunities.

What you should do about it

• After performing a business impact review, apply the relevant security patches from Microsoft
• If patching is not immediately possible, Microsoft has provided temporary alternative mitigations
  • Add users to the Protected Users group in Active Directory
  • Block outbound SMB (TCP port 445) from the network by using a perimeter firewall, a local firewall, and via VPN settings

• Microsoft has released an impact assessment script that may be used to identify if organizations were targeted by threat actors attempting to exploit CVE-2023-23397

Additional information

CVE-2023-23397 is highly concerning as exploitation does not require previous access or any user interaction. In an attack scenario, threat actors would deliver a crafted email containing the PidLidReminderFileParameter extended Messaging Application Programming Interface (MAPI) property. This property is modified to reference an external universal naming convention (UNC) path to a Server Message Block (SMB) share on an attacker-controlled server. If the reminder triggers while Outlook is open, a connection to the SMB share is initiated containing the user’s hashed password in an NTLM negotiation message. These credentials can then be used in NTLM relay attacks against the victim. No user interaction is required.

Reports suggest the vulnerability is currently being leveraged by Russian state-sponsored APT groups. Microsoft acknowledges CERT-UA in the vulnerability report, implying that exploitation of the vulnerability was likely operationalized in Ukraine.

Exploitation of CVE-2023-23397 is not complex and technical reports on the vulnerability are already publicly available. While current exploitation is attributed to Russian actors, eSentire Threat Intelligence assesses it is almost certain other state actors and financially motivated threat actors will adopt the technique for use in attacks in the immediate future.

Impacted Products:

• Microsoft Outlook 2016 (64-bit edition)
• Microsoft Outlook 2013 Service Pack 1 (32-bit editions)
• Microsoft Outlook 2013 RT Service Pack 1
• Microsoft Outlook 2013 Service Pack 1 (64-bit editions)
• Microsoft Office 2019 for 32-bit editions
• Microsoft 365 Apps for Enterprise for 32-bit Systems
• Microsoft Office 2019 for 64-bit editions
• Microsoft 365 Apps for Enterprise for 64-bit Systems
• Microsoft Office LTSC 2021 for 64-bit editions
• Microsoft Outlook 2016 (32-bit edition)
• Microsoft Office LTSC 2021 for 32-bit editions

References:

[1] https://msrc.microsoft.com/update-guide/en-US/vulnerability/CVE-2023-23397
[2] https://attack.mitre.org/techniques/T1557/001/
[3] https://microsoft.github.io/CSS-Exchange/Security/CVE-2023-23397/
[4] https://www.mdsec.co.uk/2023/03/exploiting-cve-2023-23397-microsoft-outlook-elevation-of-privilege-vulnerability/