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Upgrade to ProDetect Email Attachment Forces Outbound SMB Authentication to Harvest NTLM Hashes in Elastic Security
Rather than delivering an executable payload, actors such as TA577 email a ZIP attachment containing an HTML file (or, in earlier waves, a direct .url/.search-ms file) whose sole purpose is to reference a remote UNC path (e.g. \\attacker-server\share\file). When the victim opens the attachment, Windows automatically attempts SMB authentication to the attacker-controlled server to resolve the path — leaking the user's NetNTLMv2 challenge-response hash over the wire without ever executing malicious code on the endpoint. The captured hash is then cracked offline or relayed (NTLM relay) to gain credentials or a foothold. Because no payload runs and no persistence is established, this is reconnaissance/information-gathering via a phishing lure (T1598.002 — Phishing for Information: Spearphishing Attachment) rather than a delivery mechanism for malware; the objective is purely to elicit authentication material from the target. Detection focuses on the anomaly that outbound SMB (ports 445/139) is almost never legitimate when it originates from a user-facing application (browser, archive utility, Office app, or Explorer) and targets a public, non-RFC1918 IP address, especially when that connection follows shortly after a ZIP/HTML/ISO attachment was delivered to the same mailbox.
MITRE ATT&CK
- Tactic
- Reconnaissance
Elastic Detection Query
sequence by host.name with maxspan=1h
[
file where event.type == "creation" and
file.extension : ("zip", "html", "htm", "iso", "img")
]
[
network where destination.port in (445, 139) and
not cidrmatch(destination.ip, "10.0.0.0/8", "172.16.0.0/12", "192.168.0.0/16", "169.254.0.0/16", "127.0.0.0/8")
]Sequence-based Elastic EQL detection correlating creation of a ZIP/HTML/ISO/IMG file (the delivered lure attachment landing on disk after being opened from a mail client or browser) with a subsequent outbound SMB connection to a public IP from the same host within 1 hour — the file-drop-then-forced-auth pattern generated by TA577-style attachment lures.
Data Sources
Required Tables
False Positives & Tuning
- Legitimate download and extraction of ZIP archives from trusted vendors followed by unrelated SMB activity on the same host within the hour
- Software installers packaged as ISO/IMG that legitimately reach out to a licensing or update server that happens to be reachable over an SMB-adjacent port
- Authorized phishing-simulation exercises replicating the same file-then-network pattern
Other platforms for THREAT-Recon-AttachmentForcedSMBAuthNTLMHarvest
Testing Methodology
Validate this detection against 3 adversary techniques from Atomic Red Team. Each test below lists the behaviour to exercise and the telemetry you should expect to see. Executable commands and cleanup steps are available with Pro.
- Test 1Outbound SMB Connection Attempt to External IP from Browser Process
Expected signal: Sysmon Event ID 3 / DeviceNetworkEvents showing powershell.exe (or its child network stack) initiating a connection to 203.0.113.10 on port 445 from the test host.
- Test 2Explorer UNC Path Navigation to External Host
Expected signal: Sysmon Event ID 3 showing explorer.exe initiating an outbound connection to 203.0.113.10 on port 445; likely followed by a failed/timeout connection since no real share exists at the test IP.
- Test 3Simulated Attachment Delivery Followed by Outbound SMB Within Correlation Window
Expected signal: A synthetic JSON record simulating an EmailAttachmentInfo entry (FileType=zip) for [email protected], plus a genuine Sysmon Event ID 3 outbound connection to 203.0.113.10:445 within the same minute.
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