Malware that is exploiting a bug in Microsoft Word is programmed to download additional malware onto Windows PCs, according to a warning issued by Microsoft's Malware Protection Center.
The problem was actually patched in November's Patch Tuesday, but after detecting the first pieces of malware in the wild that exploited the flaw, the security team is reminding users about the update to fix the RTF vulnerability, the company said on Dec. 28. It had also updated the bulletin several times since November to include updates for more applications, according to the FAQ.
The patch exists for Microsoft Word 2002, 2003, 2007 and 2010 (both 32-bit and 64-bit versions) for Windows. There is no fix as yet for Word 2004. Mac versions of Office have also been patched, according to the update, named MS10-087.
The exploitation involves using a malicious Rich Text Format file to generate a stack overflow in Word, MMPC said. Once the user's Word installation has been compromised, the attack code downloads and executes additional Trojans onto the computer, according to the security warning.
The exploit code is particularly efficient, as users don't need to manually open the malicious RTF file or the e-mail with the file attached to be infected, Symantec said. The malware executes automatically if users running Office 2007 or 2010 preview the e-mail in Outlook in its Reading Pane, according to the security company. Alternatively, if the attacking e-mail is the most recent message received, it opens automatically in the Reading Pane when Outlook starts, said Symantec.
"In this scenario, this attack vector requires minimal user action," said Microsoft. Even if users are careful and not opening suspicious messages, a majority of the users have Reading Pane enabled, the company said.
Microsoft also said that Outlook is not "directly" affected because the malware executes only through Microsoft Word. However, because Word is the default e-mail reader in Outlook 2007 and 2010, criminals using e-mail to spread the malicious RTF files, present a particularly dangerous threat, the company said.
The attacker gains the "same user rights as a local user," according to the alert, so the attack's impact would vary depending on the configuration of a user's account on a given system. While all systems are vulnerable to remote-code execution, users with restricted rights would be affected less severely than a user with administrative rights, according to Microsoft.
The vulnerability was rated as "1" on Microsoft's exploitability index, meaning the team expected an exploit to appear within 30 days. The RTF bug is "critical" for Office 2007 and 2010 installations, but "important" for Office XP and Office 2003, according to the Patch Tuesday update. It is also "important" for the Open XML File format Converter for the Mac as well as the Mac versions of Office - 2004, 2008 and 2011.
The patch fixes the bug by ensuring Office uses "a more appropriate and secure search order" when loading libraries, according to the warning.
The various viewer programs for Word, Excel, PowerPoint and Visio are not vulnerable at this time, according to the MS10-087 page.