What is the CVSS score of CVE-2021-26858?

CVE-2021-26858 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: 53.0%.

Is CVE-2021-26858 being actively exploited?

Yes, CVE-2021-26858 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-2021-26858?

Within 24 hours: Identify all affected systems and apply vendor patches immediately. Vendor patch is available.

Exchange Server CVE-2021-26858

HIGH

2021-03-03 secure@microsoft.com

RCE Microsoft Exchange Server

7.8

CVSS 3.1 · Vendor: microsoft

View

Vulnerability Assessment

Impact	High risk as part of the ProxyLogon chain. The file write capability enables persistent access through web shells even after the SSRF vulnerability is patched.
Exploit ScenarioDerived	Following SSRF exploitation and deserialization, the attacker uses the file write to deploy an ASPX web shell to the OWA or ECP directory. This web shell provides persistent access independent of the original vulnerability chain.

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

Exploited in the wild (CISA KEV) Patch available 53.0% exploitation probability (EPSS)

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AnalysisAIv1 – Mar 26, 2026

Microsoft Exchange Server allows authenticated attackers to write arbitrary files to the server filesystem, the third component of the ProxyLogon exploit chain enabling web shell deployment.

Earlier versions of this analysis (1)

v1 Mar 26, 2026 - 11:19 CVSS 7.8

An arbitrary file write vulnerability in Exchange Server allows authenticated attackers to write files to any location on the server. This is the third link in the ProxyLogon chain, enabling web shell deployment.

RemediationAI

Apply security updates. Scan for web shells in Exchange web directories. Monitor file creation events in IIS content directories.

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 post-authentication file write vulnerability allows writing arbitrary content to any filesystem location accessible by the Exchange service account (SYSTEM). Combined with the SSRF (CVE-2021-26855) for authentication bypass, this enables unauthenticated web shell deployment.

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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-2021-26858 presents notable security risk rated CVSS 7.8. Exploitation could compromise the security of affected deployments. 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) Implement network-level access controls to limit exposure; (2) Enable enhanced monitoring and alerting; (3) Apply principle of least privilege.

Threat Intelligence Overview

Threat Score

6.7/10

Composite threat intelligence

EPSS Score

53.0%

Top 2% 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 53.0%

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

152

Low Medium High Critical

KEV: +50

EPSS: +53

CVSS: +39

PoC: 0

Weaponized: 0

Threat intel: +10

Gov advisory: 0

Attack Technique

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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CVE-2015-5119 CRITICAL POC

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CVE-2015-3113 CRITICAL POC

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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-2021-26858 REF https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-26858 REF https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-26858

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GitHub PoC

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