Suspicious Startup Shell Folder Modification

## Triage and analysis

### Investigating Suspicious Startup Shell Folder Modification

Techniques used within malware and by adversaries often leverage the Windows registry to store malicious programs for
persistence. Startup shell folders are often targeted as they are not as prevalent as normal Startup folder paths so this
behavior may evade existing AV/EDR solutions. These programs may also run with higher privileges which can be ideal for
an attacker.

#### Possible investigation steps

- 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.
- Review the source process and related file tied to the Windows Registry entry.
- Validate the activity is not related to planned patches, updates, network administrator activity or legitimate software
installations.
- Assess whether this behavior is prevalent in the environment by looking for similar occurrences across hosts.
- Retrieve the file and determine if it is malicious:
  - Use a private sandboxed malware analysis system to perform analysis.
    - Observe and collect information about the following activities:
      - Attempts to contact external domains and addresses.
      - File and registry access, modification, and creation activities.
      - Service creation and launch activities.
      - Scheduled task creation.
  - Use the PowerShell Get-FileHash cmdlet to get the files' SHA-256 hash values.
    - Search for the existence and reputation of the hashes in resources like VirusTotal, Hybrid-Analysis, CISCO Talos, Any.run, etc.

### False positive analysis

- There is a high possibility of benign legitimate programs being added to shell folders. This activity could be based
on new software installations, patches, or other network administrator activity. Before undertaking further investigation,
it should be verified that this activity is not benign.

### Related rules

- Startup or Run Key Registry Modification - 97fc44d3-8dae-4019-ae83-298c3015600f
- Persistent Scripts in the Startup Directory - f7c4dc5a-a58d-491d-9f14-9b66507121c0

### 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.
- Determine the initial vector abused by the attacker and take action to prevent reinfection through the same vector.
- 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.
- Run a full antimalware scan. This may reveal additional artifacts left in the system, persistence mechanisms, and
malware components.
- 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).