Didier Stevens

Thursday 16 June 2022

Discovering A Forensic Artifact

Filed under: Forensics,Hacking,maldoc — Didier Stevens @ 0:00

While developing my oledump plugin plugin_olestreams.py, I noticed that the item moniker’s name field (lpszItem) values I observed while analyzing Follina RTF maldocs, had a value looking like _1715622067:

The number after the underscore (_), is derived from the timestamp when the item moniker was created. That timestamp is expressed as an epoch value in local time, to which a constant number is added: 61505155.

I figured this out by doing some tests. 61505155 is an approximation: I might be wrong by a couple of seconds.

Item name _1715622067 is the value you find in Follina maldocs created from this particular RTF template made by chvancooten. 1715622067 minus 61505155 is 1654116912. Converting epoch value 1654116912 to date & time value gives: Wednesday, June 1, 2022 8:55:12 PM. That’s when that RTF document was created.

RTF documents made from this template, can be detected by looking for string 0c0000005f3137313536323230363700 inside the document (you have to look for this hexadecimal string, not case sensitive, because OLE files embedded in RTF are represented in hexadecimal).

Notice that the newest template in that github repository is taken from a cve-2017-0199 RTF template document, and that it no longer contains a item moniker.

But it does contain another timestamp:

This hexadecimal string can also be used for detection purposes: 906660a637b5d201

I used the following YARA rules for a retrohunt (34 matches):

rule follina_rtf_template_1 {
        $a = "0c0000005f3137313536323230363700" nocase

rule follina_rtf_template_2 {
        $a = "906660a637b5d201" nocase

Notice that I do not include a test for RTF documents in my rules: these rules also detect Python program follina.py.

And if you are a bit familiar with the RTF syntax, you know that it’s trivial to modify such RTF documents to avoid detection by the above YARA rules.

Later I will spend some time to find the actual code that implements the generation of the item value _XXXXXXXXXX. Maybe you can find it, or you already know where it is located.

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