sublimehighRule

Attachment: HTML smuggling with RC4 decryption

Potential HTML smuggling. The RC4 algorithm is used within inline JavaScript to decrypt the payload on-the-fly.

MITRE ATT&CK

T1566 T1566.001 T1566.002 T1598 T1204.002 T1486 T1027 T1573 T1036 T1059

defense-evasionexecution

Detection Query

type.inbound
and any(attachments,
        (
          .file_extension in~ ("html", "htm", "shtml", "dhtml")
          or .file_extension in~ $file_extensions_common_archives
          or .file_type == "html"
        )
        and any(file.explode(.),
                any(.scan.strings.strings,
                    strings.ilike(., "*?[(?[?]+?[?])%256]*")
                )
        )
)

Data Sources

Email MessagesEmail HeadersEmail Attachments

Platforms

References

Raw Content

name: "Attachment: HTML smuggling with RC4 decryption"
description: |
  Potential HTML smuggling.
  The RC4 algorithm is used within inline JavaScript to decrypt the payload on-the-fly.
references:
  - "https://delivr.to/payloads?id=0dcec496-0130-465c-bf3d-c9714e17d4d4"
  - "https://playground.sublimesecurity.com?id=6b219754-2b0b-47c3-ba53-ec080d946499"
type: "rule"
severity: "high"
source: |
  type.inbound
  and any(attachments,
          (
            .file_extension in~ ("html", "htm", "shtml", "dhtml")
            or .file_extension in~ $file_extensions_common_archives
            or .file_type == "html"
          )
          and any(file.explode(.),
                  any(.scan.strings.strings,
                      strings.ilike(., "*?[(?[?]+?[?])%256]*")
                  )
          )
  )
attack_types:
  - "Credential Phishing"
  - "Malware/Ransomware"
tactics_and_techniques:
  - "Encryption"
  - "Evasion"
  - "HTML smuggling"
  - "Scripting"
detection_methods:
  - "Archive analysis"
  - "Content analysis"
  - "File analysis"
  - "HTML analysis"
  - "Javascript analysis"
id: "3a46d765-4b27-5026-9b8e-043e3817854b"