Detection rules › Sublime MQL

Link: SharePoint filename matches org name

Severity
medium
Type
rule
Source
github.com/sublime-security/sublime-rules

Detects messages claiming to share files via SharePoint or OneDrive where the shared file name pattern matches the organizational naming pattern, indicating potential abuse of legitimate file sharing services to impersonate organizations.

Threat classification

Sublime's own taxonomy (not MITRE ATT&CK).

CategoryValues
Attack typesCredential Phishing
Tactics and techniquesImpersonation: Employee, Social engineering

Event coverage

Rule body MQL

type.inbound
and strings.ilike(subject.subject, "*shared*", "*invit*")
and strings.ilike(body.current_thread.text,
                  "*shared a file with you*",
                  "*shared with you*",
                  "*invited you to access a file*"
)
and not strings.ilike(body.current_thread.text, "invited you to edit")
and (
  // use the display text of the link to determine the name of the file
  any(filter(body.current_thread.links,
             .href_url.domain.domain not in $tenant_domains
             and (
               .href_url.domain.root_domain == "sharepoint.com"
               or .href_url.domain.root_domain == "1drv.ms"
               // handle urls with mimecast rewriting
               or (
                 .href_url.domain.root_domain == 'mimecastprotect.com'
                 and strings.icontains(.href_url.query_params,
                                       '.sharepoint.com'
                 )
               )
             )
             and .display_text != "Open"
      ),
      .display_text =~ sender.email.domain.sld
      or any(regex.extract(body.current_thread.text,
                           "generated through (?P<org_name>[^']+)'s use"
             ),
             // the document name is the same as the org name as determined by the footer
             // this checks that the display_text starts with the org_name
             strings.istarts_with(.named_groups["org_name"], ..display_text)

             // this checks that the org_name is a substring of the display_text
             // it is in effect the "reverse" of the above check
             or (
               (
                 strings.istarts_with(..display_text, .named_groups["org_name"])
                 or strings.iends_with(..display_text,
                                       .named_groups["org_name"]
                 )
               )
               and (
                 length(.named_groups["org_name"]) / (
                   length(..display_text) * 1.0
                 )
               ) > 0.45
             )
      )
  )
)

Detection logic

Scope: inbound message.

Detects messages claiming to share files via SharePoint or OneDrive where the shared file name pattern matches the organizational naming pattern, indicating potential abuse of legitimate file sharing services to impersonate organizations.

  1. inbound message
  2. subject.subject matches any of 2 patterns
    • *shared*
    • *invit*
  3. body.current_thread.text matches any of 3 patterns
    • *shared a file with you*
    • *shared with you*
    • *invited you to access a file*
  4. not:
    • body.current_thread.text matches 'invited you to edit'
  5. any of filter(body.current_thread.links) where any holds:
    • .display_text is sender.email.domain.sld
    • any of regex.extract(body.current_thread.text) where any holds:
      • strings.istarts_with(.named_groups['org_name'])
      • all of:
        • any of:
          • strings.istarts_with(.display_text)
          • strings.iends_with(.display_text)
        • length(.named_groups['org_name']) / length(.display_text) * 1.0 > 0.45

Inspects: body.current_thread.links, body.current_thread.links[].display_text, body.current_thread.links[].href_url.domain.domain, body.current_thread.links[].href_url.domain.root_domain, body.current_thread.links[].href_url.query_params, body.current_thread.text, sender.email.domain.sld, subject.subject, type.inbound. Sensors: regex.extract, strings.icontains, strings.iends_with, strings.ilike, strings.istarts_with. Reference lists: $tenant_domains.

Indicators matched (11)

FieldMatchValue
strings.ilikesubstring*shared*
strings.ilikesubstring*invit*
strings.ilikesubstring*shared a file with you*
strings.ilikesubstring*shared with you*
strings.ilikesubstring*invited you to access a file*
strings.ilikesubstringinvited you to edit
body.current_thread.links[].href_url.domain.root_domainequalssharepoint.com
body.current_thread.links[].href_url.domain.root_domainequals1drv.ms
body.current_thread.links[].href_url.domain.root_domainequalsmimecastprotect.com
strings.icontainssubstring.sharepoint.com
regex.extractregexgenerated through (?P<org_name>[^']+)'s use