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Date: 3/16/2023
Among the recent monthly Microsoft updates was a patch for CVE-2023-23397, an Elevation of Privilege Vulnerability in MS Outlook. Exploitation is very easy and occurs by sending a specially crafted calendar appointment which is processed by Outlook even before a user opens or previews it.
When the malicious appointment is processed by Outlook, a user’s NTLMv2 hash can be sent to a location specified by attackers. The hash could then be used in an NTLM Relay attack against another service to authenticate as the user. Evidence shows that ransomware actors have exploited this to establish a foothold within target networks. Exploitation attempts are likely to become very common quickly.
First and foremost: ensure that the available patches are applied throughout your network. Additionally, Internet-bound SMB/port 445 traffic should be blocked at the network perimeter to prevent hashes being sent during exploitation. Consider adding users to the Protected Users Security Group, which prevents the use of NTLM as an authentication mechanism.
Microsoft has also released scripts to check for mail items exploiting this vulnerability for on premises and Exchange Online environments.
Scripts are available here: https://microsoft.github.io/CSS-Exchange/Security/CVE-2023-23397/
According to the CVSS metrics, successful exploitation of this vulnerability could lead to major loss of confidentiality (C:H), integrity (I:H) and availability (A:H). What does that mean for this vulnerability?
An attacker who successfully exploited this vulnerability could access a user’s Net-NTLMv2 hash which could be used as a basis of an NTLM Relay attack against another service to authenticate as the user.
Is the Preview Pane an attack vector for this vulnerability?
The attacker could exploit this vulnerability by sending a specially crafted email that triggers automatically when it is retrieved and processed by the Outlook client. This could lead to exploitation BEFORE the email is viewed in the Preview Pane.
How could an attacker exploit this vulnerability?
External attackers could send specially crafted emails that will cause a connection from the victim to an external UNC location of attackers’ control. This will leak the Net-NTLMv2 hash of the victim to the attacker, who can then relay this to another service and authenticate as the victim.
Where can I find more information about NTLM relay attacks?
Download Mitigating Pass the Hash (PtH) Attacks and Other Credential Theft, Version 1 and 2. This document discusses Pass-the-Hash (PtH) attacks against the Windows operating systems and provides holistic planning strategies that, when combined with the Windows security features, will provide a more effective defense against pass-the-hash attacks.
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