Microsoft today posted an advisory to inform users of active attacks targeting unpatched flaws in Adobe Type Manager Library. The vulnerabilities affect all supported versions of Windows.

Two remote code execution vulnerabilities exist in Microsoft Windows when the Adobe Type Manager Library improperly handles a specially crafted multimaster font called Adobe Type 1 PostScript format, Microsoft explains in the advisory. The company is aware of "limited targeted attacks" that could leverage the unpatched vulnerabilities, which the company ranked as Critical.

There are several ways an attacker could successfully take advantage of these flaws. For example, they could convince a user to open a specially crafted document or view it in the Windows Preview pane. Opening or viewing the file would let the attacker remotely run malicious code on the target machine. While the Windows Preview pane could be an attack vector, Microsoft says Outlook Preview Pane is not an attack vector for these vulnerabilities.

"For systems running supported versions of Windows 10, a successful attack could only result in code execution within an AppContainer sandbox context with limited privileges and capabilities," officials explain of the potential effects on supported Windows 10 machines.

Microsoft is working on a fix for the vulnerabilities. The company usually releases patches on the second Tuesday of each month, a schedule it says allows for both quality assurance and IT planning. It has been known to issue out-of-band patches for urgent vulnerabilities as needed. Microsoft did not provide details on when a patch for these vulnerabilities will be released. 

In the meantime, its advisory offers workarounds for companies vulnerable to these attacks. For example, admins can disable the Preview and Details panes in Windows Explorer to prevent the automatic display of OTF fonts in Windows Explorer. This prevents malicious files from being views in Windows Explorer, Microsoft says, but it does not block a local authenticated user from running a specially crafted program to exploit the flaw.

Another workaround involves disabling the WebClient service, which helps protect against potential exploits by blocking the most likely remote attack vector via the Web Distributed Authoring and Versioning (WebDAV) client service. With this workaround, it would still be possible for attackers to cause the system to run programs on the target machine or local area network; however, users will be asked for confirmation before opening malicious programs.

Microsoft provides guidance for completing these workarounds and others, as well as how to undo them, for different affected versions of Windows in its advisory on the vulnerabilities.

To protect against the attacks that exploit these flaws, Synopsys senior security strategist Jonathan Knudsen emphasizes the importance of not clicking links or attachments in unexpected emails:

"You should never, ever, ever click on links in emails or open documents whose origin is uncertain," he says. "The attack that exploits this vulnerability depends on tricking users into opening specially crafted malicious documents. Every time you are tempted to click a link or open an attachment, take a moment and think about what you're doing."

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