• Sublime Core Feed
Medium Severity

Link to auto-download of a suspicious file type (unsolicited)

Labels

Malware/Ransomware
Encryption
Evasion
LNK
Social engineering
Archive analysis
File analysis
Sender analysis
URL analysis
YARA

Description

A link in the body of the email downloads a suspicious file type (or embedded file) such as an LNK, JS, or VBA.

Recursively explodes auto-downloaded files within archives to detect these file types.

This technique has been used by known threat actors in the wild.

References

Sublime Security
Created Aug 17th, 2023 • Last updated Nov 20th, 2024
Feed Source
Sublime Core Feed
Source
GitHub
type.inbound
and any(body.links,
        any(ml.link_analysis(.).files_downloaded,
            any(file.explode(.),
                (
                  // look for files in encrypted zips.
                  // if password cracking the zip wasn't
                  // successful, our only opportunity to look
                  // for suspicious file types is here under
                  // .zip.attempted_files
                  "encrypted_zip" in .flavors.yara
                  and any(.scan.zip.attempted_files,
                          strings.ilike(., "*.dll", "*.html", "*.exe", "*.lnk", "*.js", "*.vba", "*.vbs", "*.vbe")
                  )
                )
                // for both non-encrypted zips and encrypted zips
                // that were successfully cracked
                or .file_extension in ("dll", "exe", "html", "lnk", "js", "vba", "vbs", "vbe", "bat")
                or strings.ilike(.file_name, "*.exe")
                or (
                  .file_extension not in ("dll", "exe")
                  and (
                    .flavors.mime in ("application/x-dosexec")
                    or any(.flavors.yara, . in ('mz_file'))
                  )
                )
                or any(.flavors.yara, . == "macho_file")
            )
            and not (
              ml.link_analysis(..).effective_url.domain.root_domain == "zoom.us"
              and .file_extension == "exe"
          )
        )
)
and (
  not profile.by_sender().solicited
  or (
    profile.by_sender().any_messages_malicious_or_spam
    and not profile.by_sender().any_false_positives
  )
)
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