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Microsoft Pins Outlook Zero-Day Attacks on Russian Actor, Offers Detection Script

Microsoft blames a “Russian-based threat actor” for in-the-wild attacks hitting its flagship Microsoft Outlook and has released a detection script to help defenders.


Microsoft’s threat intelligence team is blaming a “Russian-based threat actor” for newly disclosed in-the-wild attacks targeting a critical vulnerability in its flagship Microsoft Outlook software.

One day after sounding an alarm for live exploitation of the Outlook security flaw, Microsoft said it traced the exploit to a Russian APT targeting a limited number of organizations in government, transportation, energy, and military sectors in Europe.

Redmond did not identify the actor or provide indicators of compromise (IOCs) to help defenders hunt for signs of compromise. 

However, in a nod to the severity of the issue, the Microsoft Security Response Center (MSRC) published mitigation guidance and offered a CVE-2023-23397 script to help with audit and cleanup.

“We strongly recommend all customers update Microsoft Outlook for Windows to remain secure,” Microsoft said.

From the MSRC’s new documentation:

“To determine if your organization was targeted by actors attempting to use this vulnerability, Microsoft is providing documentation and a script at

Organizations should review the output of this script to determine risk. Tasks, email messages and calendar items that are detected and point to an unrecognized share should be reviewed to determine if they are malicious. If objects are detected, they should be removed or clear the parameter. If no objects are detected, it is unlikely the organization was targeted via CVE-2023-23397.

The new documentation describes the CVE-2023-23397 bug as a critical privilege escalation issue in Microsoft Outlook that is triggered when an attacker sends a message with an extended MAPI property with a UNC path to an SMB (TCP 445) share on a threat actor-controlled server. 

“No user interaction is required,” Microsoft warned.  Because this flaw could lead to exploitation BEFORE the email is viewed in the Preview Pane, enterprise security teams are urged to prioritize the deployment of this update.

“The connection to the remote SMB server sends the user’s NTLM negotiation message, which the attacker can then relay for authentication against other systems that support NTLM authentication. Online services such as Microsoft 365 do not support NTLM authentication and are not vulnerable to being attacked by these messages,” Redmond added. 

The Microsoft Outlook zero-day headlined a busy Patch Tuesday that saw the rollout of patches for 80 vulnerabilities in a wide range of Windows operating system and software products.

Microsoft also flagged a second vulnerability — CVE-2023-24880  — for urgent attention and warned attackers are continuing to actively bypass its SmartScreen security feature.

The company has struggled to contain attackers bypassing the SmartScreen technology that has been fitted into Microsoft Edge and the Windows operating system to help protect users from phishing and social engineering malware downloads.

The notorious Magniber ransomware operation has been observed exploiting the SmartScreen bypass technique, prompting multiple attempts by Microsoft to mitigate the issue.

Related: Microsoft Warns of Outlook Zero-Day Exploitation, Patches 80 Security Vulns

Related: Microsoft Patches MotW Zero-Day Exploited for Malware Delivery

Related: Microsoft Plugs Windows Hole Exploited in Ransomware Attacks

Related: Adobe Warns of ‘Very Limited Attacks’ Exploiting ColdFusion Zero-Day

Written By

Ryan Naraine is Editor-at-Large at SecurityWeek and host of the popular Security Conversations podcast series. He is a security community engagement expert who has built programs at major global brands, including Intel Corp., Bishop Fox and GReAT. Ryan is a founding-director of the Security Tinkerers non-profit, an advisor to early-stage entrepreneurs, and a regular speaker at security conferences around the world.

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