Microsoft's patches for the Meltdown vulnerability have had a fatal flaw all these past months, according to Alex Ionescu, a security researcher with cyber-security firm Crowdstrike.

Only patches for Windows 10 versions were affected, the researcher wrote today in a tweet. Microsoft quietly fixed the issue on Windows 10 Redstone 4 (v1803), also known as the April 2018 Update, released on Monday.

"Welp, it turns out the Meltdown patches for Windows 10 had a fatal flaw: calling NtCallEnclave returned back to user space with the full kernel page table directory, completely undermining the mitigation," Ionescu wrote.

Welp, it turns out the #Meltdown patches for Windows 10 had a fatal flaw: calling NtCallEnclave returned back to user space with the full kernel page table directory, completely undermining the mitigation. This is now patched on RS4 but not earlier builds -- no backport?? pic.twitter.com/VIit6hmYK0 — Alex Ionescu (@aionescu) May 2, 2018

Ionescu pointed out that older versions of Windows 10 are still running with outdated and bypass-able Meltdown patches.

Microsoft patched another —unrelated— vulnerability today

Microsoft issued today an security update, but it wasn't to backport the "fixed" Meltdown patches for older Windows 10 versions.

Instead, the emergency update fixed a vulnerability in the Windows Host Compute Service Shim (hcsshim) library (CVE-2018-8115) that allows an attacker to remotely execute code on vulnerable systems.

Microsoft classified CVE-2018-8115 as a "critical" issues. A patched hcsshim file is available for download from GitHub.

Backported patches are on the way

"We are aware and are working to provide customers with an update," a Microsoft spokesperson told Bleeping Computer today in an email.

It may be that if Microsoft doesn't bundle these fixes in an out-of-band update, they will most likely arrive in Microsoft's May 2018 Patch Tuesday, but this is only our speculation.

Microsoft released its Meltdown and Spectre patches on January 4, a day after security researchers disclosed the two flaws, vulnerabilities that allow attackers to retrieve data from protected areas of modern CPUs.

The Redmond-based OS maker has had a hard time patching the two flaws, and the company recently issued additional security updates to fix the original Spectre mitigations, and also deliver Intel CPU microcode updates, as a favor to Intel.

Article updated with comment from Microsoft. Title updated accordingly.