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Encrypting Files with WinRar or 7z

Elastic Stack Serverless Security

Identifies use of WinRar or 7z to create an encrypted files. Adversaries will often compress and encrypt data in preparation for exfiltration.

Rule type: eql

Rule indices:

  • logs-endpoint.events.*
  • winlogbeat-*
  • logs-windows.*

Severity: medium

Risk score: 47

Runs every: 5m

Searches indices from: now-9m (https://www.elastic.co/guide/en/elasticsearch/reference/current/common-options.html#date-math[Date Math format], see also Additional look-back time)

Maximum alerts per execution: 100

References:

Tags:

  • Elastic
  • Host
  • Windows
  • Threat Detection
  • Collection

Version: 6

Rule authors:

  • Elastic

Rule license: Elastic License v2

Investigation guide

Attackers may compress and/or encrypt data collected before exfiltration. Compressing the data can help obfuscate the
collected data and minimize the amount of data sent over the network. Encryption can be used to hide information that is
being exfiltrated from detection or make exfiltration less apparent upon inspection by a defender.

These steps are usually done in preparation for exfiltration, meaning the attack may be in its final stages.

  • Investigate the script 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.
  • Retrieve the encrypted file.
  • Investigate other alerts associated with the user/host during the past 48 hours.
  • Check if the password used in the encryption was included in the command line.
  • Decrypt the .rar/.zip and check if the information is sensitive.
  • If the password is not available, and the format is .zip or the option used in WinRAR is not the -hp, list the
    file names included in the encrypted file.
  • Investigate if the file was transferred to an attacker-controlled server.
  • Backup software can use these utilities. Check the process.parent.executable and
    process.parent.command_line fields to determine what triggered the encryption.
  • Initiate the incident response process based on the outcome of the triage.
  • Prioritize cases that involve personally identifiable information (PII) or other classified data.
  • Isolate the involved hosts to prevent further post-compromise behavior.
  • 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.
  • 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).

If enabling an EQL rule on a non-elastic-agent index (such as beats) for versions <8.2, events will not define event.ingested and default fallback for EQL rules was not added until 8.2, so you will need to add a custom pipeline to populate event.ingested to @timestamp for this rule to work.

Rule query

process where event.type in ("start", "process_started") and
  ((process.name:"rar.exe" or process.code_signature.subject_name == "win.rar GmbH" or
      process.pe.original_file_name == "Command line RAR") and
    process.args == "a" and process.args : ("-hp*", "-p*", "-dw", "-tb", "-ta", "/hp*", "/p*", "/dw", "/tb", "/ta"))

  or
  (process.pe.original_file_name in ("7z.exe", "7za.exe") and
     process.args == "a" and process.args : ("-p*", "-sdel"))

  /* uncomment if noisy for backup software related FPs */
  /* not process.parent.executable : ("C:\\Program Files\\*.exe", "C:\\Program Files (x86)\\*.exe") */

Framework: MITRE ATT&CKTM