CVE-2026-55009: Microsoft Exchange Server deserialization privilege escalation

CVE-2026-55009 · Severity: high · CVSS 7.8 · Published 2026-07-14

A security vulnerability in Microsoft Exchange Server, the platform used by organizations for email and calendaring, could allow an attacker with existing low-level access to gain full administrative control. By exploiting how the server processes certain data, an authorized user can bypass security restrictions to perform unauthorized actions. This could lead to the theft of sensitive emails, disruption of communication services, or further compromise of the corporate network.

Technologies: Microsoft Exchange Server 2016, Microsoft Exchange Server 2019, Microsoft Exchange Server Subscription Edition. Vendors: Microsoft.

Quick answers

Is CVE-2026-55009 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-55009 being exploited in the wild.
How severe is CVE-2026-55009?
High severity, with a CVSS score of 7.8, published 14 July 2026.
Which products and versions are affected?
Microsoft Exchange Server 2016 < 15.01.2507.071; Microsoft Exchange Server 2019 < 15.02.1544.043; Microsoft Exchange Server Subscription Edition < 15.02.2562.045.
Is there a fix for CVE-2026-55009?
No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.
How is CVE-2026-55009 attacked?
It is attacked with local access, in a network appliance.
Am I affected by CVE-2026-55009?
Check whether you run Microsoft Exchange Server 2016, Microsoft Exchange Server 2019 or Microsoft Exchange Server Subscription Edition at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

A deserialization vulnerability (CWE-502) exists in Microsoft Exchange Server due to the insecure handling of untrusted data. An attacker with local access and low-level privileges can exploit this flaw by providing specially crafted serialized data to the server, which, when processed, allows for arbitrary code execution or privilege escalation. The attack vector is local, meaning the attacker must already have a foothold on the system or be an authenticated user with local access. Successful exploitation grants the attacker high-level system privileges, potentially leading to full compromise of the Exchange environment. Microsoft has released security updates to address this issue across affected versions of Exchange Server 2016, 2019, and the Subscription Edition.

Affected products

  • 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

Timeline

  • 2026-07-14: disclosed
  • 2026-07-14: advisory

References

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