CVE-2026-55009 Microsoft Exchange Server Elevation of Privilege Vulnerability
An authorized attacker can elevate privileges locally via untrusted data deserialization in Microsoft Exchange Server.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-55009?
CVE-2026-55009 is a high-severity unsafe deserialization vulnerability in Microsoft Exchange Server, published on 14 Jul 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
Deserialization of untrusted data in Microsoft Exchange Server allows an authorized attacker to elevate privileges locally.
Weakness: CWE-502
Which versions are affected by CVE-2026-55009?
It affects 2 products.
Products include Microsoft Exchange Server, Microsoft Exchange Server Subscription Edition.
How do I fix CVE-2026-55009?
No fix has been published for CVE-2026-55009 yet. Limit exposure of the affected software until one is available.
Upgrade, and only deserialise data from trusted sources or with an allow-list of types. More about Unsafe deserialization.
Is CVE-2026-55009 being exploited?
There is no known exploitation of CVE-2026-55009 in the wild and no public exploit code.
Other vulnerabilities in Microsoft Exchange Server
- CVE-2024-21410 Microsoft Exchange Server Privilege Escalation
- CVE-2021-34473 Microsoft Exchange Server Remote Code Execution
- CVE-2021-26855 Microsoft Exchange Server Remote Code Execution
- CVE-2021-34523 Microsoft Exchange Server Privilege Escalation
- CVE-2023-21529 Microsoft Exchange Server Deserialization of Untrusted Data
Sources
Also known as GHSA-78c8-h6hm-94hg.