What is CVE-2026-55009?

CVE-2026-55009 is a high-severity vulnerability in Microsoft Exchange Server 2016 Cumulative Update 23, classified under Deserialization of Untrusted Data. CVSS score: 7.8/10. Published 2026-07-14.

How severe is CVE-2026-55009?

High severity. CVSS v3 base score is 7.8 out of 10.

Deserialization in Microsoft Exchange Server 2016 Cumulative Update 23

CVE-2026-55009

Deserialization of untrusted data in Microsoft Exchange Server allows an authorized attacker to elevate privileges locally.

Vulnerability class: Insecure Deserialization

CVSS v3 metric

CVSS v3 base score 7.8 (High). Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H.

Affected products

Microsoft Exchange Server 2016 Cumulative Update 23 — versions 15.01.0.0
Microsoft Exchange Server 2019 Cumulative Update 14 — versions 15.02.0.0
Microsoft Exchange Server 2019 Cumulative Update 15 — versions 15.02.0.0
Microsoft Exchange Server Subscription Edition Rtm — versions 15.02.0.0

Weakness classification (CWE)

CWE-502 — Deserialization of Untrusted Data

References

secure@microsoft.com (vendor-advisory, patch)

Frequently asked questions

What is CVE-2026-55009?: CVE-2026-55009 is a high-severity vulnerability in Microsoft Exchange Server 2016 Cumulative Update 23, classified under Deserialization of Untrusted Data. CVSS score: 7.8/10. Published 2026-07-14.
How severe is CVE-2026-55009?: High severity. CVSS v3 base score is 7.8 out of 10.