Network Connection via Compiled HTML File

## Triage and analysis

### Investigating Network Connection via Compiled HTML File

CHM (Compiled HTML) files are a format for delivering online help files on Windows. CHM files are compressed compilations of various content, such as HTML documents, images, and scripting/web-related programming languages such as VBA, JScript, Java, and ActiveX.

When users double-click CHM files, the HTML Help executable program (`hh.exe`) will execute them. `hh.exe` also can be used to execute code embedded in those files, PowerShell scripts, and executables. This makes it useful for attackers not only to proxy the execution of malicious payloads via a signed binary that could bypass security controls, but also to gain initial access to environments via social engineering methods.

This rule identifies network connections done by `hh.exe`, which can potentially indicate abuse to download malicious files or tooling, or masquerading.

> **Note**:
> This investigation guide uses the {security-guide}/invest-guide-run-osquery.html[Osquery Markdown Plugin] introduced in Elastic Stack version 8.5.0. Older Elastic Stack versions will display unrendered Markdown in this guide.

#### Possible investigation steps

- Investigate other alerts associated with the user/host during the past 48 hours.
- Investigate the process execution chain (parent process tree) for unknown processes. Examine their executable files for prevalence, whether they are located in expected locations, and if they are signed with valid digital signatures.
  - Examine the command lines for suspicious activities.
  - Retrieve `.chm`, `.ps1`, and other files that were involved for further examination.
  - Investigate any abnormal behavior by the subject process such as network connections, registry or file modifications, and any spawned child processes.
  - Investigate the file digital signature and process original filename, if suspicious, treat it as potential malware.
- Investigate the target host that the signed binary is communicating with.
  - Check if the domain is newly registered or unexpected.
  - Check the reputation of the domain or IP address.
- Examine the host for derived artifacts that indicate suspicious activities:
  - Analyze the process executables, scripts and help files retrieved from the system using a private sandboxed analysis system.
  - Observe and collect information about the following activities in both the sandbox and the alert subject host:
    - Attempts to contact external domains and addresses.
      - Use the Elastic Defend network events to determine domains and addresses contacted by the subject process by filtering by the process' `process.entity_id`.
      - Examine the DNS cache for suspicious or anomalous entries.
        - !{osquery{"label":"Osquery - Retrieve DNS Cache","query":"SELECT * FROM dns_cache"}}
    - Use the Elastic Defend registry events to examine registry keys accessed, modified, or created by the related processes in the process tree.
    - Examine the host services for suspicious or anomalous entries.
      - !{osquery{"label":"Osquery - Retrieve All Services","query":"SELECT description, display_name, name, path, pid, service_type, start_type, status, user_account FROM services"}}
      - !{osquery{"label":"Osquery - Retrieve Services Running on User Accounts","query":"SELECT description, display_name, name, path, pid, service_type, start_type, status, user_account FROM services WHERE\nNOT (user_account LIKE '%LocalSystem' OR user_account LIKE '%LocalService' OR user_account LIKE '%NetworkService' OR\nuser_account == null)\n"}}
      - !{osquery{"label":"Osquery - Retrieve Service Unsigned Executables with Virustotal Link","query":"SELECT concat('https://www.virustotal.com/gui/file/', sha1) AS VtLink, name, description, start_type, status, pid,\nservices.path FROM services JOIN authenticode ON services.path = authenticode.path OR services.module_path =\nauthenticode.path JOIN hash ON services.path = hash.path WHERE authenticode.result != 'trusted'\n"}}
  - Retrieve the files' SHA-256 hash values using the PowerShell `Get-FileHash` cmdlet and search for the existence and reputation of the hashes in resources like VirusTotal, Hybrid-Analysis, CISCO Talos, Any.run, etc.

### False positive analysis

- If this activity is expected and noisy in your environment, consider adding exceptions — preferably with a combination of user and command line conditions.

### Response and remediation

- Initiate the incident response process based on the outcome of the triage.
- Isolate the involved host to prevent further post-compromise behavior.
- If the triage identified malware, search the environment for additional compromised hosts.
  - Implement temporary network rules, procedures, and segmentation to contain the malware.
  - Stop suspicious processes.
  - Immediately block the identified indicators of compromise (IoCs).
  - Inspect the affected systems for additional malware backdoors like reverse shells, reverse proxies, or droppers that attackers could use to reinfect the system.
- If the malicious file was delivered via phishing:
  - Block the email sender from sending future emails.
  - Block the malicious web pages.
  - Remove emails from the sender from mailboxes.
  - Consider improvements to the security awareness program.
- Remove and block malicious artifacts identified during triage.
- Run a full antimalware scan. This may reveal additional artifacts left in the system, persistence mechanisms, and malware components.
- Determine the initial vector abused by the attacker and take action to prevent reinfection through the same vector.
- Using the incident response data, update logging and audit policies to improve the mean time to detect (MTTD) and the mean time to respond (MTTR).