sublimemediumRule

Link: Unicode character obfuscation in display name with base64-encoded URL fragment

Detects inbound messages where the sender's display name contains Unicode format characters (such as word joiners or invisible separators) interspersed between letters to evade text-based detection. These messages carry links whose URL fragments are unusually long (over 100 characters) and match a base64url-encoded pattern, a technique used to smuggle encoded payloads past URL scanners.

MITRE ATT&CK

T1566 T1566.001 T1566.002 T1598 T1036 T1027

defense-evasioninitial-access

Detection Query

type.inbound
and regex.icontains(sender.display_name, '[a-z]\p{Cf}+[a-z]')
and any(body.links,
        length(.href_url.fragment) > 100
        and regex.contains(.href_url.fragment, '^[A-Za-z0-9_-]+={0,2}$')
)

Data Sources

Email MessagesEmail HeadersEmail Attachments

Platforms

Raw Content

name: "Link: Unicode character obfuscation in display name with base64-encoded URL fragment"
description: "Detects inbound messages where the sender's display name contains Unicode format characters (such as word joiners or invisible separators) interspersed between letters to evade text-based detection. These messages carry links whose URL fragments are unusually long (over 100 characters) and match a base64url-encoded pattern, a technique used to smuggle encoded payloads past URL scanners."
type: "rule"
severity: "medium"
source: |
  type.inbound
  and regex.icontains(sender.display_name, '[a-z]\p{Cf}+[a-z]')
  and any(body.links,
          length(.href_url.fragment) > 100
          and regex.contains(.href_url.fragment, '^[A-Za-z0-9_-]+={0,2}$')
  )
attack_types:
  - "Credential Phishing"
tactics_and_techniques:
  - "Evasion"
  - "Social engineering"
detection_methods:
  - "URL analysis"
  - "Sender analysis"
  - "Content analysis"
id: "8619dce9-142e-5b3a-981a-1ac58960b221"