File Formats: A Moving Target

PDF attacks? Please. That's soooo mid-July.

Like teenage fashion, the most favored format for hiding malware seems to be changing on a nearly-weekly basis. And vendors, like IT and security managers, are struggling to keep up.

Just two weeks ago, security vendors were alerting users to a new class of malware, worms, and viruses buried in PDF files sent over email. The PDF file format was an effective disguise for executable code and image-based spam that could escape most antivirus and content-filtering tools, they said. (See Spam Changes Direction, Marshal Warns of New Spam Tactics, and BorderWare Finds New Trend: PDF Image Spam.)

The PDF craze appeared to be "the new image spam," until last week, when McAfee reported an uptick in the use of Excel file formats to carry malware and spam.

"The spam shares the same traits as the PDF spam, so although the filetype is different, this spam is otherwise similar to the previous PDF spam," said McAfee's Nick Kelly in a blog last week. "It's not unlikely that the spammers will try to embed other attachment types, such as Microsoft Word and PowerPoint documents in the future."

While Excel and PDF spam are increasing, GIF and JPG-based image spam concerning the stock market has decreased from about 20 percent of all spam to less than 10 percent, Kelly said.

So first it was images, then it was PDFs, and then it was Excel files. Are we done yet? Not according to a blog issued yesterday by Symantec, and confirmed by McAfee earlier today.

Symantec is warning users of a slow increase in the deployment of malware and other exploits via older file formats such as Zip and RAR. While newer file formats such as Word and Excel are easily or automatically updated, less pervasive programs "are often harder to keep current," Symantec warns.

"A prime example of this is the archive format, with extensions such as .zip, .rar, etc.," Symantec says. "There are a wide number of different programs available for different platforms; more importantly, they have historically been quite vulnerable to exploits." Employees often use differing versions of these older programs, the company says, so an enterprise-wide update can be difficult.

McAfee researcher Pedro Bueno confirmed the RAR trend -- and that file formats have become a moving target -- in his blog earlier today.

"There was a time when we used to receive spam through regular email," Bueno recalls. "Then, the spammers learned about image spam, putting the message on picture files. Recently, we saw that they have moved to PDF spam, and later to encrypted PDF files. A couple of days ago, we noticed spam in Excel files, and now we find another round with a different vector: RAR-compacted archives with spam inside text files! Whatever will we see next?"

Many enterprise security pros are likely wondering the same thing.

— Tim Wilson, Site Editor, Dark Reading