In September 2014, we wrote about a improvement in VBA malware.
VBA stands for Visual Basic for Applications: it is a able and actual widely-used programming apparatus that can be acclimated appropriate central applications such as Microsoft Office.
That makes it common, and absolutely altogether usual, in accepted files.
But, as we we wrote aftermost time:
Visual Basic cipher is accessible to write, adjustable and accessible to refactor. Agnate functionality can generally be bidding in abounding altered agency which gives malware authors added options for bearing distinct, applicable versions of their software than they accept with exploits.
In short, what is acceptable for the ankle is appropriately acceptable for the goose.
Indeed, over the accomplished six months, malware that arrives as a VBA affairs central an innocent-looking certificate has become an all-too-common accident in the blackmail landscape, and an capital weapon in spam campaigns.
Obviously, attackers who use VBA await on their victims accepting some adaptation of Office installed.
As you can see, SophosLabs statistics appearance that malware writers adopt Word and Excel to PowerPoint.
The acumen for this is acceptable because malware delivered in spam actual frequently pretends to be a bagman commitment apprehension or an invoice, or similar, and these are about stored as Word abstracts or Excel spreadsheets.
But the crooks additionally abundantly adopt the earlier “1997-2003” Office book format.
Files in the 1997-2003 architecture are stored in what Microsoft calls the Object Linking and Embedding (OLE) Compound Book format, generally aloof alleged OLE2 for short.
OLE2 uses a FAT-like anatomy to ascertain assorted streams (which you can anticipate of as files in a deejay image) consisting of fixed-size blocks; these streams acknowledge the anatomy and agreeable of the document.
The blow of the VBA malware we see is in the added contempo “2007 and later” format.
These files are denoted with an -X added to the book addendum (e.g. DOCX instead of DOC, XLSX instead of XLS).
Dash-X files are stored in a book blueprint accepted as Office Accessible XML (OOXML).
Files of this blazon booty the anatomy of a ZIP annal absolute a alternation of XML files that ascertain the document’s agreeable and presentation.
We can alone assumption why malware writers accept been afraid to accomplish to the 2007 format, but a acceptable bet would be the added likelihood of a acknowledged infection.
Newer versions of Office can accessible both new and old book types, acknowledgment to astern compatibility, but the old Office versions were never patched to let them handle the new formats.
Interestingly, there is another, little-used book architecture that was alien way aback in Office 2003.
Files in this architecture abide of a standalone XML file, and they are abundantly abnormal that they don’t arise at all in the pie blueprint above.
To our surprise, however, we accept afresh apparent a billow in cast new VBA malware packaged in this old and abnormal format.
Once again, we accept to assumption why the crooks accept absitively to animate this format, which ability artlessly be bottomward to the actuality it is little used, and appropriately not frequently associated with attacks.
Perhaps, also, malware authors achievement that the aberration of XML-type files agency that some aegis articles are clumsy to deconstruct it properly.
→ Sophos articles can decompose OLE2, XML and OOXML blazon files and abstract their capacity in a agnate way. In added words, the aforementioned malware adored in three altered formats will be detected identically.
The action of extracting a VBA affairs from an Office book depends on the alembic architecture that is used.
In “1997-2003” files, VBA cipher is stored in a cardinal of streams which are amid aural the aforementioned OLE2 alembic as the added certificate streams, such as the WordDocument beck which contains the document’s text.
Office 2007 files additionally abundance their VBA cipher as streams in an OLE2 file, but the added certificate abstracts is alone into abstracted XML files in the capital alembic file, which is in the ZIP format.
So the OLE2 alembic that holds the VBA cipher is artlessly a book alleged vbaProject.bin inamongst the XML files in the alien ZIP file.
And the Office 2003 XML architecture additionally uses a committed OLE2 alembic to abundance VBA code, with the structural aberration that the abstracts is aeroemism into MSO architecture (a proprietary Microsoft architecture additionally acclimated for email attachments) and afresh text-encoded into Base64.
If we abstract the Base64 abstracts and break it, we access the MSO file, adumbrated by the argument “ActiveMime” at the start.
Unpacking the MSO book leaves us with an OLE2 alembic with the VBA progam.
Using a contempo malware example, we extracted the VBA cipher from its XML wrapper.
Here’s what we found:
At aboriginal glance the cipher ability arise circuitous but it is absolutely actual simple cipher that has been advisedly bedlam out in an advance to beard its accurate intentions.
This subroutine is the access point of the VBA and the aboriginal credibility of absorption are the acutely nonsense strings declared at the alpha of the book and what appears to be the aforementioned four curve of cipher afresh in groups of three.
We will attending at the strings in abyss after but aboriginal let’s attending at the bifold cipher (highlighted in red).
These four curve arise to accept no aftereffect on the final aftereffect of the subroutine.
The cipher declares a capricious that is never agreeably referenced, a for bend whose abortion action assures that it is never accomplished and a codicillary if account that is consistently false.
Programming like this is generally referred to as asleep code, apparently created automatically by a cipher bearing engine.
Removing this asleep cipher leaves us with a abundant smaller, added bright subroutine, although it is still not bright what the cipher absolutely does:
A apparent affection is the afresh action calls to “ho3NnG”.
Each alarm is accompanied with one of a cardinal of hardcoded cord constants declared at the top of the file.
Jumping to the “ho3NnG” function, independent in a abstracted cipher module, already afresh seems to attempt us into complexity.
But apprehension that there are abundant GoTo statements broadcast amidst the function’s body:
Since these all-overs are non-conditional, and there are no labels amid anniversary jump and its destination, the cipher sandwiched amid them can never be triggered.
Code like this is accepted as aloof code, we can artlessly aish it from consideration.
Without the aloof cipher noise, and with a little bit of re-arrangement, we are larboard with a abundant simpler function:
The cipher aloft loops through the passed-in cord and XORs anniversary appearance with the decimal amount 255. (This has the aftereffect of flipping anniversary bit in anniversary byte.)
The aftereffect of anniversary XOR is added to a new cord which is alternate to the caller.
This array of text-unscrambling action is actual accepted in malware, because it is a simple way of camouflage abstracts such as filenames, letters and URLs that would contrarily be both accessible and suspicious.
We can now artlessly alter the aboriginal calls to “ho3NnG” with the unscrambled abstracts that comes aback anniversary time.
Now it looks added like malware:
With the formatting bankrupt up a little and the variables renamed, the accurate intentions of this book are clear.
Simply put, this code:
The crooks could artlessly accept anchored the agreeable of abs5ajsu.exe as accolade abstracts in the VBA code, so that the malware would assignment alike back offline.
But by application a downloader, they adjournment assuming their duke until the aftermost moment.
Only back the Office book is opened (rather than back it is received) do they acknowledge what malware they are absolutely application in the attack.
That gives them added flexibility: they can change the malware at any any time; acclimate it depending on the geolocation of the victim; or alike download apple-pie files as decoys.
In this example, the malware that was downloaded aing was a alternative of Dridex, a cyberbanking Trojan acquired from Cridex.
This accurate array of VBA downloader is frequently associated with Dridex payloads, accounting for about 70% of all VBA-based malware in the accomplished three months.
What’s old is new again!
→ Sophos detects and blocks the malware declared aloft as Troj/DocDl-GO (VBA downloader part) and Troj/Dridex-AZ (dropped malware part).
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