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%.

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

RCE Deserialization Microsoft Exchange Server

7.8

CVSS 3.1 · Vendor: microsoft

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Vulnerability Assessment

Weakness	CWE CWE-502 OWASP Top 10:2025 A08:2021 - Software and Data Integrity Failures ASVS 5.0 V10 - Malicious Code Integrity verification, supply chain security MITRE ATT&CK 2 techniques across 2 tactics: Execution, Initial Access The technique list with ATT&CK links and the kill-chain view is part of the Basic plan and up. Sign in to read
Impact	High risk. While requiring chaining with the SSRF vulnerability, this was a critical component of the ProxyLogon exploit chain that compromised hundreds of thousands of Exchange servers.
Exploit ScenarioDerived	As part of the ProxyLogon chain: after exploiting the SSRF (CVE-2021-26855) to access internal services, the attacker sends a crafted serialized payload to the Unified Messaging endpoint. The deserialization executes code as SYSTEM, enabling subsequent file write operations.

Act Now Prioritize within 24-48h. Why: In CISA KEV with patch available.

Exploited in the wild (CISA KEV) Patch available 40.5% exploitation probability (EPSS)

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AnalysisAIv1 – Mar 26, 2026

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.

Earlier versions of this analysis (1)

v1 Mar 26, 2026 - 11:19 CVSS 7.8

A deserialization vulnerability in Exchange's Unified Messaging service enables code execution when processing crafted serialized data. Used as the second link in the ProxyLogon exploit chain after the SSRF (CVE-2021-26855).

RemediationAI

Apply Microsoft security updates. Disable Unified Messaging if not required. Monitor for deserialization attack indicators.

Affects your stack

Find out whether the stacks and servers you run are on an affected version.

Checking this CVE against your own stacks and servers is part of the Basic plan and up.

Technical ContextAI · show details

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.

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Exploit and PoC references, threat intelligence from 22 sources

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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

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

Executive ViewAI

Why It Matters

CVE-2021-26857 presents notable security risk, a insecure deserialization vulnerability rated CVSS 7.8. Insecure deserialization could allow attackers to execute arbitrary code by providing crafted serialized data. This vulnerability is actively exploited in the wild (KEV-listed) and requires immediate remediation. A patch is available and should be applied as part of regular vulnerability management.

Business Impact: Severe

Compensating Controls

Apply vendor patch. If immediate patching is not feasible: (1) Restrict deserialization endpoints; (2) Implement integrity checks on serialized data; (3) Use type-safe alternatives.

Threat Intelligence Overview

Threat Score

6.3/10

Composite threat intelligence

EPSS Score

40.5%

Top 3% exploitable

Threat Intel

1 source

Data Quality

5 sources

Confidence: Medium

NVD

CVE.org

CISA KEV

EPSS THREAT

How this decision was reached

The engine reads each signal below and applies a fixed list of rules from top to bottom. The first rule that matches sets the recommended action – like a statement, every line is shown.

SignalValue

Severity HIGH

CISA KEV Yes

Public POC No

EPSS 40.5%

Patch available Yes

Active threat Yes

Deciding rule

In CISA KEV with patch available

What every action level means

Emergency Immediate action required. Criteria: CRITICAL + in CISA KEV (actively exploited) without patch, OR CRITICAL + EPSS > 30% without patch, OR CRITICAL + active threat signals without patch.

Act Now Prioritize within 24-48h. Criteria: in KEV with patch available, OR CRITICAL + public POC, OR HIGH + EPSS > 10%, OR CRITICAL + active threat, OR any CRITICAL severity.

This Week Schedule remediation this week. Criteria: HIGH severity (CVSS 7.0-8.9). Patch within normal change window.

This Month Plan within 30 days. Criteria: MEDIUM severity (CVSS 4.0-6.9), lower exploitation probability. Include in regular patch cycle.

Monitor Track and reassess. Criteria: LOW severity (CVSS 0.1-3.9), minimal exploitation risk. No immediate action needed.

SSVC AssessmentENISA

SSVC Decision· derived

Out-of-Cycle

Exploitation· derived

Active

System Exposure· derived

Small

Automatable· derived

Human Impact

Very High

Priority Score

139

Low Medium High Critical

KEV: +50

EPSS: +40

CVSS: +39

PoC: 0

Weaponized: 0

Threat intel: +10

Gov advisory: 0

Attack Technique

Insecure Deserialization CRITICAL

Insecure deserialization occurs when an application converts serialized data (a stream of bytes representing an object's state) back into a living object without proper validation.

Remote Code Execution CRITICAL

Remote Code Execution represents the critical moment when an attacker successfully runs arbitrary code on a target system without physical access.

Affected Products (CPE)CVE.org

cpe:2.3:a:microsoft:exchange_server:2010:sp3:*:*:*:*:*:*

cpe:2.3:a:microsoft:exchange_server:2013:cumulative_update_22:*:*:*:*:*:*

cpe:2.3:a:microsoft:exchange_server:2013:cumulative_update_23:*:*:*:*:*:*

cpe:2.3:a:microsoft:exchange_server:2013:sp1:*:*:*:*:*:*

cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_10:*:*:*:*:*:*

cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_11:*:*:*:*:*:*

cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_12:*:*:*:*:*:*

cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_13:*:*:*:*:*:*

cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_14:*:*:*:*:*:*

cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_15:*:*:*:*:*:*

cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_16:*:*:*:*:*:*

cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_17:*:*:*:*:*:*

cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_18:*:*:*:*:*:*

cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_19:*:*:*:*:*:*

cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_8:*:*:*:*:*:*

cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_9:*:*:*:*:*:*

cpe:2.3:a:microsoft:exchange_server:2019:-:*:*:*:*:*:*

cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_1:*:*:*:*:*:*

cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_2:*:*:*:*:*:*

cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_3:*:*:*:*:*:*

+ 5 more

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Threat IntelligenceMISP

Threat Actors

Threat Group-3390 Magic Hound

ATT&CK Techniques (observed)

T1190 - Exploit Public-Facing Application T1595.002 - Vulnerability Scanning

Sources: MITRE ATT&CK

References

Advisories & patches

PATCH https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26857 REF https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-26857 REF https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-26857

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