Review reviewHigh

CVE-2026-55009

Microsoft Microsoft Exchange Server 2016 Cumulative Update 23, Microsoft Exchange Server 2019 Cumulative Update 14, Microsoft Exchange Server 2019 Cumulative Update 15

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

PRIORITY ASSESSMENT

The CVSS severity warrants an early asset and exposure review.

Known exploitationNot established by KEV

Exploit probability1.61%

Technical severityCVSS 7.8

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

Product: Microsoft Microsoft Exchange Server 2016 Cumulative Update 23, Microsoft Exchange Server 2019 Cumulative Update 14, Microsoft Exchange Server 2019 Cumulative Update 15
Affected versions: >= 15.01.0.0 < 15.01.2507.071, >= 15.02.0.0 < 15.02.1544.043, >= 15.02.0.0 < 15.02.1748.048, >= 15.02.0.0 < 15.02.2562.045, 2016, 2019, < 15.02.2562.045
Fixed versions: 15.02.2562.045

Confirm exposure before applying a vendor-supported change.

1
Identify
Confirm that Microsoft Microsoft Exchange Server 2016 Cumulative Update 23, Microsoft Exchange Server 2019 Cumulative Update 14, Microsoft Exchange Server 2019 Cumulative Update 15 and an affected version are present.
2
Prioritize
Combine exploitation signals with asset exposure and business criticality.
3
Remediate
Follow the vendor advisory or supported update path and preserve rollback options.
4
Verify
Recheck the version, service health, access paths, and relevant logs.