What is the CVSS score of CVE-2021-26857?

CVE-2021-26857 has a CVSS 3.1 base score of 7.8 (High). CVSS vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H. EPSS exploitation probability: 40.5%.

Which versions of Exchange Server are affected by CVE-2021-26857?

CVE-2021-26857 presents notable security risk, a insecure deserialization vulnerability rated CVSS 7.8.. A patch is available.

Is CVE-2021-26857 being actively exploited?

Yes, CVE-2021-26857 is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, confirming active in-the-wild exploitation. Immediate patching is required.

How to fix or mitigate CVE-2021-26857?

Within 24 hours: Identify all affected systems and apply vendor patches immediately. Restrict deserialization to trusted data sources and implement integrity checks.

Exchange Server CVE-2021-26857

HIGH

Deserialization of Untrusted Data (CWE-502)

2021-03-03 secure@microsoft.com

Microsoft RCE Deserialization Exchange Server

7.8

CVSS 3.1 · Vendor: microsoft

Severity by source

Vendor (microsoft) PRIMARY

7.8 HIGH

AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

Primary rating from Vendor (microsoft) · only source for this CVE.

CVSS VectorVendor: microsoft

Attack Vector

Local

Attack Complexity

Low

Privileges Required

None

User Interaction

Required

Scope

Unchanged

Confidentiality

High

Integrity

High

Availability

High

Lifecycle Timeline

Analysis Generated

Mar 26, 2026 - 11:19 vuln.today

Added to CISA KEV

Dec 18, 2025 - 02:00 cisa

CISA KEV

Patch released

Dec 18, 2025 - 02:00 nvd

Patch available

CVE Published

Mar 03, 2021 - 00:15 nvd

HIGH 7.8

DescriptionCVE.org

Microsoft Exchange Server Remote Code Execution Vulnerability

AnalysisAI

Microsoft Exchange Server Unified Messaging service contains a deserialization vulnerability that allows authenticated attackers to execute code as SYSTEM, part of the ProxyLogon exploit chain.

Technical ContextAI

The CWE-502 deserialization flaw in the Unified Messaging service processes untrusted serialized .NET objects. When chained with the ProxyLogon SSRF, an unauthenticated attacker can deliver the malicious payload to the Unified Messaging endpoint for code execution as SYSTEM.