sublimemediumRule

Attachment: Encrypted Microsoft Office file (unsolicited)

Encrypted OLE2 (eg Microsoft Office) attachment from an unsolicited sender. Attachment encryption is a common technique used to bypass malware scanning products. Use if receiving encrypted attachments is not normal behavior in your environment.

MITRE ATT&CK

T1566.001 T1204.002 T1486 T1027 T1573 T1059.005 T1059

defense-evasionexecution

Detection Query

type.inbound
and any(attachments,
        (
          .file_extension in~ $file_extensions_macros
          or (
            .file_extension is null
            and .file_type == "unknown"
            and .content_type == "application/octet-stream"
            and .size < 100000000
          )
        )
        and file.oletools(.).indicators.encryption.exists
)
and (
  not profile.by_sender().solicited
  or (
    profile.by_sender().any_messages_malicious_or_spam
    and not profile.by_sender().any_messages_benign
  )
)

Data Sources

Email MessagesEmail HeadersEmail Attachments

Platforms

References

https://www.cyren.com/blog/articles/anatomy-of-an-attack-password-protected-files-attached-to-emails

Tags

Attack surface reduction

Raw Content

name: "Attachment: Encrypted Microsoft Office file (unsolicited)"
description: |
  Encrypted OLE2 (eg Microsoft Office) attachment from an unsolicited sender. Attachment encryption is a common technique used to bypass malware scanning products.
  Use if receiving encrypted attachments is not normal behavior in your environment.
references:
  - "https://www.cyren.com/blog/articles/anatomy-of-an-attack-password-protected-files-attached-to-emails"
type: "rule"
severity: "medium"
source: |
  type.inbound
  and any(attachments,
          (
            .file_extension in~ $file_extensions_macros
            or (
              .file_extension is null
              and .file_type == "unknown"
              and .content_type == "application/octet-stream"
              and .size < 100000000
            )
          )
          and file.oletools(.).indicators.encryption.exists
  )
  and (
    not profile.by_sender().solicited
    or (
      profile.by_sender().any_messages_malicious_or_spam
      and not profile.by_sender().any_messages_benign
    )
  )
tags:
  - "Attack surface reduction"
attack_types:
  - "Malware/Ransomware"
tactics_and_techniques:
  - "Encryption"
  - "Macros"
  - "Scripting"
detection_methods:
  - "Archive analysis"
  - "File analysis"
  - "OLE analysis"
  - "Sender analysis"
id: "1e47e953-576c-5ba9-b84e-b72a1a89de87"