With a patch for the worsening zero-day Windows vulnerability perhaps weeks — or more — away, security companies and Microsoft on Thursday recommended workarounds and other ad hoc defenses.
Several firms, Microsoft included, told users to disable the Windows Picture and Fax Viewer, the application that Internet Explorer automatically launches to display WMF image files. Microsoft’s advisory instructed users to click the Start menu, choose Run, then enter “regsvr32 -u %windir%system32shimgvw.dll” (without the quote marks), and click OK. Doing so, however, breaks the viewer so that it won’t display other associated image file formats, such as those with the .jpg extension, a popular format used by most digital cameras.
And it might not solve the problem. “Any application which automatically displays or renders WMF files is vulnerable,” wrote Chris Carboni, an analyst with the Internet Storm Center, in a blog entry Thursday.
Another tactic, said some security vendors, is to block all WMF image files at the network perimeter. Symantec, for instance, listed that advice in its latest bulletin about the vulnerability. Unfortunately, hackers can simply rename a malicious WMF file with a different extension — .gif or .jpg, for example — to pass through an exploit. Windows parses WMF files based not on the extension it reads, but on the content of the file, making such blocking strategies ineffective. On Wednesday, several security companies recommended that users and companies also block access to the sites known to be using the exploit. Sunbelt Software posted a list of some of the sites — which included the most prominent, iFrameurl [dot] biz — but with the exploit being used by an ever-increasing number of malicious and/or spyware sites, the technique will soon be impossible to implement manually.
“Yesterday only a few of the sites we monitor used this exploit,” wrote Eric Sites, vice president of research at Sunbelt, “but now that number is exploding.” (Another security vendor, San Diego-based Websense, said Thursday that “thousands of sites” were distributing exploit code from iFramecash [dot] biz.)
Users can also ditch Internet Explorer for Firefox or Opera. The vulnerability isn’t within IE itself, but that browser does open WMF files automatically without asking permission from the user. Firefox and Opera at least put up a dialog box asking the user if he or she wants to open the file with Windows Picture and Fax Viewer. Using Firefox or Opera, however, doesn’t guarantee that a PC is immune, since a malicious WMF file could still be introduced via e-mail.
Finally, said Microsoft, users should keep their anti-virus defenses up to date, since most are or soon will provide signatures for the exploits taking advantage of the vulnerability. As of mid-day Thursday, for example, all the major anti-virus vendors had released some signatures.
But that, too, may not completely defend against the threat. By late Wednesday, Sunbelt Software had detected more than 50 exploit variants.