Latest Threat Research: LetMeowIn – Analysis of a Credential Dumper

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Outlook Zero-day Exploited Since April 2022

According to a recent report, a zero-day vulnerability in Microsoft Outlook, tracked as CVE-2023-23397, was being exploited by a Russian APT since April 2022. 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. The attacks appeared to be targeting a limited number of organizations within the government, transportation, energy, and military sectors in Europe.

Analyst Notes

Microsoft has previously released mitigation guidance and a CVE-2023-23397 script to assist with audit and cleanup of the attack. They have also provided the following guidelines to assist anyone looking for exploitation within their organization:

• 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

Organizations should follow hunting and mitigation steps to assure they have not been targeted by this vulnerability, especially those that fall under the targeted sectors. Binary Defense’s Managed Threat Hunting Services offering is an excellent solution to assist with these needs.