SYSTEM code execution via Unified Messaging deserialization
Published Mar 2, 2021 · Updated Aug 19, 2026
Insecure deserialization in Microsoft Exchange Server Unified Messaging allows attackers with administrator access to execute code as SYSTEM. The Unified Messaging service deserializes untrusted user-controllable data, allowing attacker-supplied objects to run in its process. Exploitation requires administrator permission or a separate vulnerability that supplies equivalent access; successful execution fully controls the server.
Summary
What happened
Insecure deserialization in Microsoft Exchange Server Unified Messaging allows attackers with administrator access to execute code as SYSTEM. The Unified Messaging service deserializes untrusted user-controllable data, allowing attacker-supplied objects to run in its process. Exploitation requires administrator permission or a separate vulnerability that supplies equivalent access; successful execution fully controls the server.
The record
- CVE
- CVE-2021-26857
- Published
- Mar 2, 2021
- Updated
- Aug 19, 2026
- Vendor
- Microsoft
- Product
- Microsoft Exchange Server 2013 Cumulative Update 22
- Classifications
- CWE-502, T1068
- Attack vector
- local
- Privileges
- admin
Timeline
How it unfolded
- Mar 2, 2021CVE publishedPublication date reported by the CVE source.
- Aug 19, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=15.00.0 <15.00.1473.006
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
No public exploit references are available in this record.
Labels summarize the accepted research assessment. They do not indicate a test against your environment.
Reported exploitation
- CISA Known Exploited Vulnerabilities listingknown exploited catalog
- Ransomware campaigns using the ProxyLogon chainransomware
- HAFNIUM exploitation of on-premises Exchangethreat actor
Technologies
Your stack
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Your stack
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