KB5121609: what it fixes and who gets it
KB5121609 is a security update from Microsoft's September 2026 release for Microsoft Exchange Server 2019 Cumulative Update 15.
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Should you install it?
Yes. It is a routine security update; none of its fixes are rated Critical or reported as exploited. Windows Update installs it automatically on supported home PCs.
If the installation fails or the PC misbehaves afterwards, note the exact error code first: the fix depends on it. Our update troubleshooting guides are linked on this page.
Security fixes in KB5121609
| CVE | Type | CVSS | Severity |
|---|---|---|---|
| CVE-2026-69356Microsoft Exchange Server Spoofing Vulnerability | Spoofing | 9.3 | Important |
| CVE-2026-69641Microsoft Exchange Server Elevation of Privilege Vulnerability | Elevation of Privilege | 9.1 | Important |
| CVE-2026-69355Microsoft Exchange Server Remote Code Execution Vulnerability | Remote Code Execution | 8.8 | Important |
| CVE-2026-55007Microsoft Exchange Server Remote Code Execution Vulnerability | Remote Code Execution | 8.1 | Important |
| CVE-2026-69380Microsoft Exchange Server Elevation of Privilege Vulnerability | Elevation of Privilege | 8.1 | Important |
| CVE-2026-69378Microsoft Exchange Server Denial of Service Vulnerability | Denial of Service | 7.5 | Important |
| CVE-2026-69361Microsoft Exchange Server Spoofing Vulnerability | Spoofing | 6.5 | Important |
| CVE-2026-69375Microsoft Exchange Server Tampering Vulnerability | Tampering | 6.5 | Important |
| CVE-2026-69382Microsoft Exchange Server Information Disclosure Vulnerability | Information Disclosure | 5.9 | Important |
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