Link: Google Calendar invite linking to an open redirect from an untrusted freemail sender

 1name: "Link: Google Calendar invite linking to an open redirect from an untrusted freemail sender"
 2description: "Attackers invite users to view a Google Calendar whose name contains a suspicious link, generally linking to spam content such as crypto giveaways, using open redirects to mask the true destination."
 3type: "rule"
 4severity: "high"
 5source: |
 6  type.inbound
 7  and strings.iends_with(subject.subject, "has shared a calendar with you")
 8  and strings.ilike(headers.message_id, "<calendar*@google.com>")
 9  and sender.email.domain.domain in $free_email_providers
10  and any(body.links,
11          any(.href_url.rewrite.encoders, strings.icontains(., "open_redirect"))
12          or (
13            any(ml.link_analysis(.).final_dom.links,
14                any(.href_url.rewrite.encoders,
15                    strings.icontains(., "open_redirect")
16                )
17            )
18          )
19  )
20  and profile.by_sender().prevalence in ("new", "outlier")
21  // negate highly trusted sender domains unless they fail DMARC authentication
22  and not (
23    sender.email.domain.root_domain in $high_trust_sender_root_domains
24    and coalesce(headers.auth_summary.dmarc.pass, false)
25  )
26  and (
27    not profile.by_sender().solicited
28    or (
29      profile.by_sender().any_messages_malicious_or_spam
30      and not profile.by_sender().any_messages_benign
31    )
32  )  
33
34attack_types:
35  - "Spam"
36tactics_and_techniques:
37  - "Free email provider"
38  - "Free file host"
39  - "ICS Phishing"
40  - "Open redirect"
41  - "Social engineering"
42detection_methods:
43  - "Content analysis"
44  - "Sender analysis"
45  - "URL analysis"
46id: "bb4f1ea9-0888-5f93-bbfe-fcb7f08ba500"