macOS Backdoor Uses Innovative Disguise Technique

A variant of the macOS-targeting OceanLotus backdoor is using an innovative technique to disguise the fact that it is an executable in order to avoid alerting users on its execution, Malwarebytes warns.

Dubbed HiddenLotus, the backdoor is distributed via an application named Lê Thu Hà (HAEDC).pdf, which masquerades as an Adobe Acrobat file. The app uses an old method for this behavior, one that inspired the file quarantine feature introduced in Leopard (Mac OS X 10.5), where files downloaded from the Internet are tagged as quarantined.

Should the downloaded file be an executable, such as an application, a pop-up notification warns the user on the fact when they attempt to open the file. The quarantine feature has been around for nearly a decade, but malware continues to masquerade as documents, Malwarebytes says.

HiddenLotus, a new variant of the OceanLotus backdoor that was last seen this summer posing as a Microsoft Word document and targeting users in Vietnam, takes the disguise to a new level. While older malware had a hidden .app extension to indicate that it was an application, HiddenLotus actually has a .pdf extension. There was no .app extension included.

This is possible because the threat uses a hidden extension, where the ‘d’ in .pdf is actually the Roman numeral ‘D’ (representing the number 500) in lowercase, as Arnaud Abbati has discovered.

“An application does not need to have a .app extension to be treated like an application. An application on macOS is actually a folder with a special internal structure called a bundle. A folder with the right structure is still only a folder, but if you give it an .app extension, it instantly becomes an application,” Malwarebytes explains.

Because of that, the Finder treats the folder as a single file and launches it as an application when double-clicked, instead of opening the folder.

When the user double-clicks a file or a folder, LaunchServices considers the extension first and opens the item accordingly, if it knows the extension. A file with a .txt extension will be opened with TextEdit by default. Thus, a folder with the .app extension will be launched as an application, should it have the right internal structure.

If the extension isn’t known, the user is consulted when attempting to open the file, and they can choose an application to open the file or search the Mac App Store.

When double-clicking a folder with an unknown extension, however, LaunchServices falls back on looking at the folder’s bundle structure.

This is the behavior that HiddenLotus’ author leverages: the dropper is a folder that has the internal bundle structure of an application. Because of the use of a Roman numeral in the .pdf extension and because there is no application registered to open it, the system treats it as an application even though it does not have a telltale .app extension.

“There is nothing particularly special about this .pdf extension (using a Roman numeral ‘d’) except that it is not already in use. Any other extension that is not in use will work just as well,” Malwarebytes says.

The security researchers also point out that there is an enormously large list of possible extensions that malicious actors could abuse, especially when using Unicode characters. Because of that, users could be tricked into opening files that mimic Word documents (.doc), Excel spreadsheets (.xls), Pages documents (.pages), and the like.

“This is a neat trick, but it’s still not going to get past file quarantine. The system will alert you that what you’re trying to open is an application. Unless, of course, what you are opening was downloaded via an application that does not use the APIs that properly set the quarantine flag on the file, as is the case for some torrent apps,” the researchers also note.

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