Patch CVE

How is CVE-2023-23397 exploited?

The attacker sends a message to the victim with an extended Message Application Program Interface (MAPI) property with a Universal Naming Convention (UNC) path to a remote attacker-controlled Server Message Block (SMB, via TCP 445). Share-hosted on a server controlled by the attacker, the vulnerability is exploited whether the recipient has seen the message or not. The attacker remotely sends a malicious calendar invite represented by .msg — the message format that supports reminders in Outlook — to trigger the vulnerable API endpoint PlayReminderSound using “PidLidReminderFileParameter” (the custom alert sound option for reminders).

When the victim connects to the attacker’s SMB server, the connection to the remote server sends the user’s New Technology LAN Manager (NTLM) negotiation message automatically, which the attacker can use for authentication against other systems that support NTLM authentication.

NTLMv2 hashes are the latest protocol Windows uses for authentication, and it is used for a number of services with each response containing a hashed representation of users’ information, such as the username and password. As such, threat actors can attempt a NTLM relay attack to gain access to other services, or a full compromise of domains if the compromised users are admins. While online services such as Microsoft 365 are not susceptible to this attack because they do not support NTLM authentication, the Microsoft 365 Windows Outlook app is still vulnerable.

User interaction is not necessary to trigger (even before message preview) it, nor does it require high privileges. CVE-2023-23397 is a zero-touch vulnerability that is triggered when the victim client is prompted and notified (e.g., when an appointment or task prompts five minutes before the designated time). It is difficult to block outbound SMB traffic for remote users. The attacker could use the same credentials to gain access to other resources. We elaborate on this example in our webinar (at 04:23 of the video).

There have been reports of limited attacks abusing this gap. Microsoft has been coordinating with the affected victims to remediate this concern. All supported versions of Microsoft Outlook for Windows are affected. Other versions of Microsoft Outlook, such as Android, iOS, Mac, as well as Outlook on the web and other M365 services, are not affected.