Should you update to KB5099539?
No: install KB5126657 instead. KB5099539 is a security update Microsoft released on 14 Jul 2026 for Windows PCs. It fixes 309 security flaws, 21 rated critical and 288 important by Microsoft.[1] None of them is known to be used in attacks.
It has since been replaced by KB5120249, released 11 Aug 2026, which includes all of its fixes. If you’re on an older version, install that instead.
Should you update?
No: install KB5126657 instead. KB5126657 came out on 13 Oct 2026 and includes every fix in KB5099539, so there’s no reason to stop at this one. Software Update offers the newest version your device can run.
Is KB5099539 still safe?
Not any more. The 3 updates since, up to KB5126657, fix 1,624 security flaws that KB5099539 doesn’t, and each became public the day its fix shipped. Installing it is safe: get it only from Start → Settings → Windows Update, never from a link in an email, text or pop-up.
What it fixes
Microsoft lists 309 security flaws, 21 rated critical and 288 important by Microsoft.[1] The 309 critical and important ones are:
| Flaw | Severity | What Microsoft calls it |
|---|---|---|
CVE-2026-42982 | Critical | Windows Secure Kernel Mode Elevation of Privilege Vulnerability |
CVE-2026-49164 | Critical | Windows Active Directory Domain Services Remote Code Execution Vulnerability |
CVE-2026-49796 | Critical | Windows GDI+ Remote Code Execution Vulnerability |
CVE-2026-50380 | Critical | Windows GDI+ Remote Code Execution Vulnerability |
CVE-2026-50382 | Critical | DirectX Graphics Kernel Remote Code Execution Vulnerability |
CVE-2026-50474 | Critical | Remote Desktop Client Remote Code Execution Vulnerability |
CVE-2026-50655 | Critical | Microsoft Windows Media Foundation Remote Code Execution Vulnerability |
CVE-2026-50680 | Critical | Windows Hyper-V Elevation of Privilege Vulnerability |
CVE-2026-50694 | Critical | Windows Secure Socket Tunneling Protocol (SSTP) Remote Code Execution Vulnerability |
CVE-2026-54128 | Critical | Windows DHCP Client Remote Code Execution Vulnerability |
CVE-2026-54982 | Critical | Windows Reliable Multicast Transport Driver (RMCAST) Remote Code Execution Vulnerability |
CVE-2026-54992 | Critical | Microsoft Message Queuing Queue Manager Remote Code Execution Vulnerability |
CVE-2026-54995 | Critical | Windows Reliable Multicast Transport Driver (RMCAST) Remote Code Execution Vulnerability |
CVE-2026-54999 | Critical | Windows TCP/IP Remote Code Execution Vulnerability |
CVE-2026-56188 | Critical | Windows Server Network driver Remote Code Execution Vulnerability |
CVE-2026-56189 | Critical | Microsoft Windows Media Foundation Remote Code Execution Vulnerability |
CVE-2026-57087 | Critical | Microsoft Windows Media Foundation Remote Code Execution Vulnerability |
CVE-2026-57090 | Critical | Microsoft Windows Media Foundation Remote Code Execution Vulnerability |
CVE-2026-57092 | Critical | Microsoft Windows VMSwitch Elevation of Privilege Vulnerability |
CVE-2026-57094 | Critical | Microsoft Windows Media Foundation Remote Code Execution Vulnerability |
CVE-2026-58608 | Critical | Windows Print Spooler Remote Code Execution Vulnerability |
CVE-2026-33842 | Important | Windows File Explorer Information Disclosure Vulnerability |
CVE-2026-34328 | Important | Windows Audio Service Information Disclosure Vulnerability |
CVE-2026-34346 | Important | Windows Ancillary Function Driver for WinSock Information Disclosure Vulnerability |
CVE-2026-34348 | Important | Windows Event Logging Service Information Disclosure Vulnerability |
CVE-2026-34349 | Important | Windows Media Information Disclosure Vulnerability |
CVE-2026-40378 | Important | Windows Local Security Authority Subsystem Service (LSASS) Denial of Service Vulnerability |
CVE-2026-40422 | Important | Windows File Explorer Information Disclosure Vulnerability |
CVE-2026-41087 | Important | Windows File Explorer Information Disclosure Vulnerability |
CVE-2026-42900 | Important | Microsoft Windows App Store Elevation of Privilege Vulnerability |
| and 279 more | ||
How to update, and how long it takes
Usually 15 to 60 minutes in all, with one or two restarts. You can keep working while it downloads.
- Save your work first.Windows restarts to finish the update, once or twice.
- Open Start → Settings → Windows Update.Select Check for updates. Windows downloads the month’s cumulative update; it includes every earlier fix.
- Restart when it asks.Select Restart now, or schedule it for tonight. Until it restarts, the fixes aren’t in place.
- Leave automatic updates on.In the same place, under Advanced options, turn on “Get me up to date” so restarts don’t wait for days.
To check it worked: Press Windows key + R, type winver and press Enter: it shows the OS build. It should say 19045.7663 or later.
Older devices
The same day, Microsoft released KB5101649 (376 security flaws), KB5101650 (373 security flaws) and KB5099414 (18 security flaws) for devices that stay on an older version. Each fixes the same holes where they apply.
KB5094127 vs KB5099539
KB5094127, released 9 Jun 2026, fixed 93 security flaws. KB5099539 fixes 309 security flaws and includes the fixes before it, so going straight to KB5099539 gets you both.
- Before: KB5094127, 9 Jun 2026, 93 security flaws
- After: KB5120249, 11 Aug 2026, 150 security flaws
- Windows updates, every release and the latest version
- Updates, the latest version of everything HARD tracks
References
- ^ Microsoft. “KB5099539”, 14 Jul 2026.