Once again, Microsoft has opted to patch the out-of-support Windows XP. Dan has written about the new patch , the circumstances around the flaws it addresses, and why Microsoft has chosen to protect Windows XP users. While Microsoft's position is a tricky one, we argue in this post first published in 2014 that patching is the wrong decision: it sends a clear message to recalcitrant corporations that they can stick with Windows XP, insecure as it is, because if anything too serious is found, Microsoft will update it anyway. Windows 10 contains a wide range of defense-in-depth measures that will never be included in Windows XP: every time an organization resists upgrading to Microsoft's latest operating system, it jeopardizes its own security. Back in 2014, it was an Internet Explorer patch that Microsoft released after Windows XP's end of support; this time around the patches are for flaws in the kernel and file sharing drivers. While this means that the situations are not quite identical, we nonetheless feel that the arguments against releasing a patch for an out-of-support operating system in 2014 hold up today. It was bad then; it's still bad now.

Microsoft officially ended support of the twelve-and-a-half-year-old Windows XP operating system a few weeks ago. Except it apparently didn't, because the company has included Windows XP in its off-cycle patch to fix an Internet Explorer zero-day that's receiving some amount of in-the-wild exploitation. The unsupported operating system is, in fact, being supported.

Explaining its actions , Microsoft says that this patch is an "exception" because of the "proximity to the end of support for Windows XP."

The decision to release this patch is a mistake, and the rationale for doing so is inadequate.

A one-off patch of this kind makes no meaningful difference to the security of a platform. Internet Explorer received security patches in 11 of the last 12 Patch Tuesdays. Other browsers such as Chrome and Firefox receive security updates on a comparable frequency.

Web browsers are complex. They're necessarily exposed to all manner of potentially hostile input that the user can't really control, and as such, they're a frequent target for attacks. They need regular updates and ongoing maintenance. The security of a browser is not contingent on any one bugfix; it's dependent on a continuous delivery of patches, fixes, and improvements. One-off "exceptions" do not make Internet Explorer on Windows XP "safe." There's no sense in which this patch means that all of a sudden it's now "OK" to use Internet Explorer on Windows XP.

And yet it seems inevitable that this is precisely how it will be received. The job of migrating away from Windows XP just got a whole lot harder. I'm sure there are IT people around the world who are now having to argue with their purse-string-controlling bosses about this very issue and IT people who have had to impress on their superiors that they need the budget to upgrade from Windows XP because Microsoft won't ship patches for it any longer. Microsoft has made these IT people into liars. "You said we had to spend all this money because XP wasn't going to get patched any more. But it is!"

Bosses who were convinced that they could stick with Windows XP because Microsoft would blink are now vindicated.

After all, if Microsoft can blink once, who's to say it won't do so again? The next Patch Tuesday patch for Internet Explorer is almost certainly going to include flaws that affect Internet Explorer on Windows XP: the nature of software means that most flaws in Internet Explorer 7 (supported for the remainder of Windows Vista's life cycle) and Internet Explorer 8 (tied to Windows 7's life cycle) will also be flaws in Internet Explorer 7 and 8 when run on Windows XP. Many of them will also hit Internet Explorer 6.

In fact, this is precisely the pattern we've seen with this flaw. The first in-the-wild exploits hit only Internet Explorer 9, 10, and 11, on Windows 7 and 8. As security firm FireEye reports, it's only later that attacks for (unsupported) Internet Explorer 8 on Windows XP materialized.

Virtually every time Microsoft updates one of its remaining supported platforms, the company will also simultaneously be disclosing a zero-day vulnerability for Windows XP ( something Apple has recently been criticized for doing ). The patch list for May's Patch Tuesday—less than two weeks away—isn't out yet, but based on Internet Explorer's track record, it's highly likely that it's going to get updated, and it's highly likely that these updates will reveal exploitable flaws on Windows XP.

By Microsoft's "proximity" argument, those flaws should be patched on Windows XP, too. In fact, it's hard to see a time when "proximity" won't be an issue. It's inevitable that Patch Tuesday will reveal exploitable flaws for the unsupported operating system, and it's similarly inevitable that at least some of those flaws will get exploited. With Windows XP's market share as high as it is, there was never any realistic chance that an exploit would not materialize in "proximity" to the end of support.

People using Windows XP are going to be exploited through known but unpatched vulnerabilities. That is what the end of support means. That is its unavoidable consequence. For as long as Windows XP has a substantial number of users, there will be calls for "one more patch" to be released. There's nothing special about this latest flaw that warrants special treatment, and the next weeks and months will see the disclosure and exploitation of many more similar flaws. If this bug was fixed, all those bugs should get fixed, too.

The zero-day flaw and its exploitation is unfortunate, and Microsoft is likely smarting from government calls for people to stop using Internet Explorer. The company had three ways it could respond. It could have done nothing—stuck to its guns, maintained that the end of support means the end of support, and encouraged people to move to a different platform. It could also have relented entirely, extended Windows XP's support life cycle for another few years and waited for attrition to shrink Windows XP's userbase to irrelevant levels. Or it could have claimed that this case is somehow "special," releasing a patch while still claiming that Windows XP isn't supported.

None of these options is perfect. A hard-line approach to the end-of-life means that there are people being exploited that Microsoft refuses to help. A complete about-turn means that Windows XP will take even longer to flush out of the market, making it a continued headache for developers and administrators alike.

But the option Microsoft took is the worst of all worlds. It undermines efforts by IT staff to ditch the ancient operating system and undermines Microsoft's assertion that Windows XP isn't supported, while doing nothing to meaningfully improve the security of Windows XP users. The upside? It buys those users at best a few extra days of improved security. It's hard to say how that was possibly worth it.