Microsoft has pulled a Windows 7 security update released as part of this month's Patch Tuesday after discovering it caused some machines to become unbootable.

Update 2823324, which was included in the MS13-036 bulletin, fixed a "moderate-level vulnerability" that requires an attacker to have physical computer access to be able to exploit a targeted computer, Dustin Childs, a group manager in the Microsoft Trustworthy Computing group, wrote in a blog post published Thursday evening. The company has now pulled it from the bulletin and is advising at least some Windows users who have installed it to uninstall the update following the guidance here. MS130-26 was one of nine bulletins released on Monday to fix 13 separate vulnerabilities.

"We’ve determined that the update, when paired with certain third-party software, can cause system errors," Childs wrote. "As a precaution, we stopped pushing 2823324 as an update when we began investigating the error reports, and have since removed it from the download center."

Childs didn't say what third-party software was involved, but the removal instructions his post linked to said update 2823324 may cause Kaspersky Anti-Virus for Windows Workstations and Kaspersky Anti-Virus for Windows Servers versions 6.0.4.1424 and 6.0.4.1611 to display an error message. Antivirus provider Sophos said Brazilian users are being hit particularly hard, and some Twitter messages reported that banking software in that country may be the reason.

Microsoft's advisories don't make it clear if Windows 7 users should uninstall the faulty update even if they've rebooted after installing it and aren't experiencing any problems. This lack of clarity is surprising considering the tremendous resources at Microsoft's disposal and the huge number of customers who depend on unambiguous instructions to keep their systems running reliably. Ars contacted the company's outside PR firm for clarification but the statement we received didn't answer our question.

Update: Childs' post has been updated to remove the ambiguity. It now reads: "However, all customers who have installed security update 2823324 should follow the guidance that we have provided in KB2839011 to uninstall it."