Anti-Virus Protection for WMF Flaw Still Inconsistent

Updated: Many products provide complete protection against current WMF exploits, but others are less successful; Microsoft's workaround works, but has limits.

Days after the revelation of a flaw in Windows handling of WMF graphics files, dozens of exploits are being spread from thousands of adware sites. But good protection is available.

At the same time, further testing confirms that a workaround issued by third parties and endorsed by Microsoft Corp. is effective in most regards, and in the most important circumstances, but not in all. Also, the workaround has side effects that could prove troublesome.

AV-Test, which tests anti-malware products, has been tracking the situation closely and has, so far, analyzed 73 variants of malicious WMF files. Products from the following companies have identified all 73:

  • Alwil Software (Avast)
  • Softwin (BitDefender)
  • ClamAV
  • F-Secure Inc.
  • Fortinet Inc.
  • McAfee Inc.
  • ESET (Nod32)
  • Panda Software
  • Sophos Plc
  • Symantec Corp.
  • Trend Micro Inc.
  • VirusBuster
These products detected fewer variants:
  • 62 — eTrust-VET
  • 62 — QuickHeal
  • 61 — AntiVir
  • 61 — Dr Web
  • 61 — Kaspersky
  • 60 — AVG
  • 19 — Command
  • 19 — F-Prot
  • 11 — Ewido
  • 7 — eSafe
  • 7 — eTrust-INO
  • 6 — Ikarus
  • 6 — VBA32
  • 0 — Norman

The difference for the more effective products is likely to be heuristic detection, tracking the threat by identifying the basic techniques of the exploit, rather than looking for specific patterns for specific exploits.

The latter technique leaves users vulnerable to threats that the vendor has not yet identified and protected against. Mikko Hypponen of F-Secure, when asked about the matter, said, "Heuristic detection rocks."

After some concern was expressed about the efficacy of the workaround proposed by third parties and endorsed by Microsoft, it appears that it is basically effective at preventing exploitation in the most common circumstances, but not in all.

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The registry fix discussed in a previous article does not work effectively, however, and users who have been relying on it will need to switch to other measures.

/zimages/3/28571.gifWhy does Security Editor Larry Seltzer say WMF stands for "Windows Major Foul-Up." Click here to read more.

The effective fix de-registers a DLL from the system relied on by the Windows Picture and Fax Viewer program. To effect the change, click Start, then Run, then enter the following command:

regsvr32 /u %windir%\system32\shimgvw.dll To re-enable the same DLL, click Start, then Run, then enter the following command: regsvr32 %windir%\system32\shimgvw.dll

This fix prevents exploitation when a WMF file is loaded from Windows Explorer or Internet Explorer.

Enterprises looking for a more manageable solution may want to investigate using an Active Directory Software Restriction Policy to set a path restriction, blocking all execution of the shimgvw.dll file. Click here for background and instructions on Software Restriction Policies on Windows Server 2003.

Some sources are recommending this, although nobody will admit to actually having tested it with the WMF vulnerability.

Next Page: Problems with the Windows fix.