Microsoft today issued one of its sporadic emergency, or "out-of-band," security updates to patch a vulnerability in Windows -- including the yet-to-be-released Windows 10 -- that was uncovered by researchers sifting through the massive cache of emails leaked after a breach of Italian surveillance vendor Hacking Team.

The Milan-based vendor sells surveillance software to governments and corporations, and markets zero-day vulnerabilities that its clients can use to silently infect targets with the firm's software. Researchers have found several zero-days -- flaws that were not fixed before they went public -- in the gigabytes of pilfered documents and messages, including three in Adobe's Flash Player, since July 5.

The Microsoft vulnerability adds to the growing tally.

The Redmond, Wash. company's update, labeled MS15-078, fixed a flaw in the Windows Adobe Type Manager Library, which handles the rendering of OpenType fonts, a format co-created by Microsoft and Adobe.

Microsoft credited FireEye's Genwei Jiang and Google Project Zero's Mateusz Jurczyk with reporting the vulnerability.

"CVE-2015-2426 is a straight-to-kernel remote code execution vulnerability," a FireEye spokesman said in an email reply to questions, using the flaw's Common Vulnerabilities and Exposure identifier. "The vulnerability was leaked with the Hacking Team email breach."

FireEye added that the bug was in the way the Adobe Type Manager Library font driver -- the file "ATMFD.dll" -- parses OpenType fonts.

Microsoft classified the vulnerability as "critical," its most serious threat level, because a successful attack could hijack a vulnerable Windows device. "An attacker could then install programs; view, change, or delete data; or create new accounts with full user rights," Microsoft's write-up said.

Cyber criminals could exploit the bug by duping victims into opening a document that included malformed OpenType fonts, or by luring them to malicious websites with embedded OpenType.

While the vulnerability had gone public before today, Microsoft asserted that it knew of no actual in-progress attacks. "[But] our analysis has shown that exploit code could be created in such a way that an attacker could consistently exploit this vulnerability," the company added.

"Looks as if it is 'easy' to exploit reliably, [so] that's why they are going out-of-band," said Wolfgang Kandek, CTO of security vendor Qualys, in an interview over instant message.

Microsoft may have also pulled the trigger because of Windows 10's looming launch: The operating system is to reach beta testers Thursday, July 29, then begin rolling out to customers who have "reserved" a copy of the free upgrade from Windows 7 or Windows 8.1. Sans a patch -- and with the vulnerability out -- Microsoft would have been mocked for claiming Windows 10 was more secure than previous versions of Windows.

Microsoft did patch Windows 10's preview build 10240, the code expected to be the final release and handed to testers six days ago. Computerworld triggered a manual check for updates on Windows 10 build 10240 within minutes of Microsoft sounding the alert; the PC found the update, then automatically downloaded and installed it.

Today's sudden update was the first since January, when Microsoft shut down its public advance notification service for pending security updates, including out-of-band patches like MS15-078. At the time, Microsoft said it would use other ways to communicate the urgency of an out-of-band update to customers, but it did not elaborate.

Microsoft used the Twitter account of its security response center and that group's blog to announce the availability of MS15-078 today.

The last out-of-band security update from Microsoft was in November 2014, when it issued a patch for a bug hackers were already exploiting in its Windows Server software.

The MS15-078 update can be downloaded and installed via the Windows Update service, as well as through Windows Server Update Services (WSUS) to patch Windows Vista, Windows 7, Windows RT and RT 8.1, Windows 8 and 8.1, Windows 10, Windows Server 2008 and 2008 R2, and Windows Server 2012 and 2012 R2.