CVE-2026-55009: Microsoft Exchange Server Elevation of Privilege Vulnerability

Severity: high (CVSS 7.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 2.5% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-07-14 · updated 2026-10-28
Weakness: CWE-502

Description

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

Affected products

Microsoft Exchange Server: 2016-cumulative_update_23, 2019-cumulative_update_14, 2019-cumulative_update_15
Microsoft Exchange Server Subscription Edition: before 15.02.2562.045 — fixed in 15.02.2562.045
Microsoft Exchange Server 2016 Cumulative Update 23: from 15.01.0.0 before 15.01.2507.071 — fixed in 15.01.2507.071
Microsoft Exchange Server 2019 Cumulative Update 14: from 15.02.0.0 before 15.02.1544.043 — fixed in 15.02.1544.043
Microsoft Exchange Server 2019 Cumulative Update 15: from 15.02.0.0 before 15.02.1748.048 — fixed in 15.02.1748.048
Microsoft Exchange Server Subscription Edition RTM: from 15.02.0.0 before 15.02.2562.045 — fixed in 15.02.2562.045

How to fix

Upgrade to a fixed version: Microsoft Exchange Server Subscription Edition 15.02.2562.045; Microsoft Exchange Server 2016 Cumulative Update 23 15.01.2507.071; Microsoft Exchange Server 2019 Cumulative Update 14 15.02.1544.043; Microsoft Exchange Server 2019 Cumulative Update 15 15.02.1748.048; Microsoft Exchange Server Subscription Edition RTM 15.02.2562.045. Check the vendor advisory for mitigations.

References

https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-55009

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