CVE-2026-69378: Microsoft Exchange Server Denial of Service Vulnerability

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 1.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-09-08 · updated 2026-10-31
Weakness: CWE-674

Description

Uncontrolled recursion in Microsoft Exchange Server allows an unauthorized attacker to deny service over a network.

Affected products

Microsoft Exchange Server: 2016-cumulative_update_23, 2019-cumulative_update_14, 2019-cumulative_update_15
Microsoft Exchange Server Subscription Edition: before 15.02.2562.049 — fixed in 15.02.2562.049
Microsoft Exchange Server 2016 Cumulative Update 23: from 15.01.0.0 before 15.01.2507.073 — fixed in 15.01.2507.073
Microsoft Exchange Server 2019 Cumulative Update 14: from 15.02.0.0 before 15.02.1544.046 — fixed in 15.02.1544.046
Microsoft Exchange Server 2019 Cumulative Update 15: from 15.02.0.0 before 15.02.1748.051 — fixed in 15.02.1748.051
Microsoft Exchange Server Subscription Edition RTM: from 15.02.0.0 before 15.02.2562.049 — fixed in 15.02.2562.049

How to fix

Upgrade to a fixed version: Microsoft Exchange Server Subscription Edition 15.02.2562.049; Microsoft Exchange Server 2016 Cumulative Update 23 15.01.2507.073; Microsoft Exchange Server 2019 Cumulative Update 14 15.02.1544.046; Microsoft Exchange Server 2019 Cumulative Update 15 15.02.1748.051; Microsoft Exchange Server Subscription Edition RTM 15.02.2562.049. Check the vendor advisory for mitigations.

References

https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-69378

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