T1598.002Not expressible in Sigma

Spearphishing Attachment

ReconnaissancePRE

Where this stands

No Sigma rule can address this — the telemetry MITRE names has no Sigma logsource behind it.

This happens on infrastructure you do not own, so it never reaches your logs. MITRE's own guidance is internet scanning and threat intelligence, not log analysis.

What the technique is

Adversaries may send spearphishing messages with a malicious attachment to elicit sensitive information that can be used during targeting. Spearphishing for information is an attempt to trick targets into divulging information, frequently credentials or other actionable information. Spearphishing for information frequently involves social engineering techniques, such as posing as a source with a reason to collect information (ex: Establish Accounts or Compromise Accounts) and/or sending multiple, seemingly urgent messages. All forms of spearphishing are electronically delivered social engineering targeted at a specific individual, company, or industry. In this scenario, adversaries attach a file to the spearphishing email. In some cases, they may rely upon the recipient populating information, then returning the file. The text of the spearphishing email usually tries to give a plausible reason why the file should be filled-in, such as a request for information from a business associate. In other cases, adversaries may leverage techniques such as HTML Smuggling to harvest user credentials via fake login portals. Adversaries may also use information from previous reconnaissance efforts (ex: Search Open Websites/Domains or Search Victim-Owned Websites) to craft persuasive and believable lures.

Read it on attack.mitre.org

What MITRE says you would watch

AN1997
Monitor network data for uncommon data flows. Processes utilizing the network that do not normally have network communication or have never been seen before are suspicious. Monitor for suspicious email activity, such as numerous accounts receiving messages from a single unusual/unknown sender. Filtering based on DKIM+SPF or header analysis can help detect when the email sender is spoofed. Monitor and analyze traffic patterns and packet inspection associated to protocol(s) that do not follow the expected protocol standards and traffic flows (e.g extraneous packets that do not belong to established flows, gratuitous or anomalous traffic patterns, anomalous syntax, or structure). […]