Microsoft says it has evidence that Russian APT actors were exploiting a nasty Outlook zero-day as far back as April 2022, a disclosure that ups the stakes on organizations to start hunting for signs of compromise.
The vulnerability, tracked as CVE-2023-23397, was flagged in the ‘already exploited’ category when Redmond shipped a fix earlier this month and Microsoft’s incident responders have pinned the attacks on Russian government-level hackers targeting organizations in Europe.
“Microsoft has traced evidence of potential exploitation of this vulnerability as early as April 2022,” the company said in fresh documentation that provides guidance for investigating attacks linked to the Outlook flaw.
Microsoft warned that exploitation of the critical-severity bug “leaves very few forensic artifacts to discover in traditional endpoint forensic analysis” and urged defenders in the targeted sectors to use “an in-depth and comprehensive threat hunting strategy” to ferret out the nation-state hacking teams.
Microsoft has pinned the blame for the attacks on an unidentified “Russian-based threat actor” seen hitting a limited number of organizations in government, transportation, energy, and military sectors in Europe.
In a nod to the severity of the issue, the Microsoft Security Response Center (MSRC) previously published mitigation guidance and provided a CVE-2023-23397 script to help with audit and cleanup and now the company is going further with threat-hunting tips and guidance for defenders.
Microsoft noted that the vulnerability triggers a Net-NTLMv2 hash leak that has been abused for initial access, credential access and lateral movement and even persistence in compromised mailboxes.
The company is recommending that defenders be strategic about hunting for infections linked to the Outlook for Windows vulnerability. Microsoft is recommending that defenders:
- Review suspicious messages, calendar items, or tasks with reminders that were reported by users
- Examine network logging and endpoint logging for evidence of known atomic indicators
- Scan Exchange for delivered messages with the PidLidReminderFileParameter set
- Hunt for anomalous behaviors based on:
- NTLM authentication involving untrusted or external resources. This can be observed in Exchange Server logging, Microsoft Defender for Identity, and Microsoft Defender for Endpoint telemetry.
- WebDAV connection attempts through process execution events.
- SMBClient event log entries.
- Firewall logs for suspicious outbound SMB connection
Microsoft also shipped a CVE-2023-23397 detection script and urged organizations to review the output of this script to determine whether an exploit was successful.
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.
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