## Triage and analysis

### Investigating Suspicious Print Spooler SPL File Created

Print Spooler is a Windows service enabled by default in all Windows clients and servers. The service manages print jobs
by loading printer drivers, receiving files to be printed, queuing them, scheduling, etc.

The Print Spooler service has some known vulnerabilities that attackers can abuse to escalate privileges to SYSTEM, like
CVE-2020-1048 and CVE-2020-1337. This rule looks for unusual processes writing SPL files to the location
`?:\Windows\System32\spool\PRINTERS\`, which is an essential step in exploiting these vulnerabilities.

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

#### 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.
- Investigate any abnormal behavior by the subject process such as network connections, registry or file modifications,
and any spawned child processes.
- Investigate other alerts associated with the user/host during the past 48 hours.
- Inspect the host for suspicious or abnormal behavior in the alert timeframe.
- Examine the host for derived artifacts that indicates suspicious activities:
  - Analyze the process executable 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{"query":"SELECT * FROM dns_cache", "label":"Osquery - Retrieve 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{"query":"SELECT description, display_name, name, path, pid, service_type, start_type, status, user_account FROM services","label":"Osquery - Retrieve All Services"}}
      - !{osquery{"query":"SELECT description, display_name, name, path, pid, service_type, start_type, status, user_account FROM services WHERE NOT (user_account LIKE "%LocalSystem" OR user_account LIKE "%LocalService" OR user_account LIKE "%NetworkService" OR user_account == null)","label":"Osquery - Retrieve Services Running on User Accounts"}}
      - !{osquery{"query":"SELECT concat('https://www.virustotal.com/gui/file/', sha1) AS VtLink, name, description, start_type, status, pid, services.path FROM services JOIN authenticode ON services.path = authenticode.path OR services.module_path = authenticode.path JOIN hash ON services.path = hash.path WHERE authenticode.result != "trusted"","label":"Osquery - Retrieve Service Unsigned Executables with Virustotal Link"}}
  - 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 process executable and file 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.
- 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.
- Investigate credential exposure on systems compromised or used by the attacker to ensure all compromised accounts are
identified. Reset passwords for these accounts and other potentially compromised credentials, such as email, business
systems, and web services.
- Ensure that the machine has the latest security updates and is not running legacy Windows versions.
- 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).