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 (
23    (
24      sender.email.domain.root_domain in $high_trust_sender_root_domains
25      and (
26        any(distinct(headers.hops, .authentication_results.dmarc is not null),
27            strings.ilike(.authentication_results.dmarc, "*fail")
28        )
29      )
30    )
31    or sender.email.domain.root_domain not in $high_trust_sender_root_domains
32  )
33  and (
34    not profile.by_sender().solicited
35    or (
36      profile.by_sender().any_messages_malicious_or_spam
37      and not profile.by_sender().any_false_positives
38    )
39  )  
40
41attack_types:
42  - "Spam"
43tactics_and_techniques:
44  - "Free email provider"
45  - "Free file host"
46  - "Open redirect"
47  - "Social engineering"
48detection_methods:
49  - "Content analysis"
50  - "Sender analysis"
51  - "URL analysis"
52id: "bb4f1ea9-0888-5f93-bbfe-fcb7f08ba500"