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

calendar Aug 21, 2023  ·
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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.

Sublime rule (View on GitHub)

 1name: "Link to auto-download of a suspicious file type (unsolicited)"
 2description: |
 3  A link in the body of the email downloads a suspicious file type (or embedded file) such as an LNK, JS, or VBA.
 4
 5  Recursively explodes auto-downloaded files within archives to detect these file types.
 6
 7  This technique has been used by known threat actors in the wild.  
 8type: "rule"
 9references:
10  - "https://www.microsoft.com/en-us/security/blog/2021/11/11/html-smuggling-surges-highly-evasive-loader-technique-increasingly-used-in-banking-malware-targeted-attacks/"
11  - "https://twitter.com/MsftSecIntel/status/1418706920152522753"
12  - "https://sandbox.sublimesecurity.com?id=e586c888-4426-41dd-a1be-5a13852075ef"
13severity: "medium"
14source: |
15  type.inbound
16  and any(body.links,
17          any(beta.linkanalysis(.).files_downloaded,
18              any(file.explode(.),
19                  (
20                    // look for files in encrypted zips.
21                    // if password cracking the zip wasn't
22                    // successful, our only opportunity to look
23                    // for suspicious file types is here under
24                    // .zip.attempted_files
25                    "encrypted_zip" in .flavors.yara
26                    and any(.scan.zip.attempted_files,
27                            strings.ilike(., "*.lnk", "*.js", "*.vba", "*.vbs", "*.vbe")
28                    )
29                  )
30                  // for both non-encrypted zips and encrypted zips
31                  // that were successfully cracked
32                  or .file_extension in ("lnk", "js", "vba", "vbs", "vbe")
33              )
34          )
35  )
36  // unsolicited
37  and (
38    (
39      sender.email.domain.root_domain in $free_email_providers
40      and sender.email.email not in $recipient_emails
41    )
42    or (
43      sender.email.domain.root_domain not in $free_email_providers
44      and sender.email.domain.domain not in $recipient_domains
45    )
46  )  
47attack_types:
48  - "Malware/Ransomware"
49tactics_and_techniques:
50  - "Encryption"
51  - "Evasion"
52  - "LNK"
53  - "Social engineering"
54detection_methods:
55  - "Archive analysis"
56  - "File analysis"
57  - "Sender analysis"
58  - "URL analysis"
59  - "YARA"
60id: "67ae2152-ac52-52d4-bec2-6bbc4a488df9"
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