sublimehighRule

Link: Base64 encoded recipient address in URL fragment with hex subdomain

Detects links containing a 40-character hexadecimal subdomain with the recipient's email address base64 encoded in the URL fragment, a technique used to personalize malicious links and evade detection.

MITRE ATT&CK

T1566 T1566.001 T1566.002 T1598 T1036 T1027

defense-evasion

Detection Query

type.inbound
and any(body.links,
        length(.href_url.domain.subdomain) == 40
        and regex.match(.href_url.domain.subdomain, '^[a-f0-9]{40}')
        and any(strings.scan_base64(.href_url.fragment),
                . == recipients.to[0].email.email
        )
)

Data Sources

Email MessagesEmail HeadersEmail Attachments

Platforms

Raw Content

name: "Link: Base64 encoded recipient address in URL fragment with hex subdomain"
description: "Detects links containing a 40-character hexadecimal subdomain with the recipient's email address base64 encoded in the URL fragment, a technique used to personalize malicious links and evade detection."
type: "rule"
severity: "high"
source: |
  type.inbound
  and any(body.links,
          length(.href_url.domain.subdomain) == 40
          and regex.match(.href_url.domain.subdomain, '^[a-f0-9]{40}')
          and any(strings.scan_base64(.href_url.fragment),
                  . == recipients.to[0].email.email
          )
  )

attack_types:
  - "Credential Phishing"
tactics_and_techniques:
  - "Evasion"
detection_methods:
  - "URL analysis"
  - "Content analysis"
id: "781e86ae-0be9-5965-8b94-8f0c9ad13854"