What is the CVSS score of CVE-2021-34523?

CVE-2021-34523 has a CVSS 3.1 base score of 9.0 (Critical). CVSS vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N. EPSS exploitation probability: 93.9%.

Is there a public PoC exploit available for CVE-2021-34523?

Yes, a public proof-of-concept exploit is available for CVE-2021-34523. Prioritise patching immediately. EPSS: 93.9%.

How to fix or mitigate CVE-2021-34523?

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

Exchange Server CVE-2021-34523

CRITICAL

2021-07-14 secure@microsoft.com

Information Disclosure Microsoft Exchange Server

9.0

CVSS 3.1 · Vendor: microsoft

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Vulnerability Assessment

Impact	Critical risk (CVSS 9.0, EPSS 93.9%). Essential component of the ProxyShell chain. Without this escalation, the SSRF alone provides limited impact; with it, attackers gain full Exchange administrative control.
Exploit ScenarioDerived	After exploiting the ProxyShell SSRF, the attacker uses this privilege escalation to gain Exchange admin rights, then uses the Export Mailbox cmdlet to write a web shell disguised as a PST export to a web-accessible directory.

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

Exploited in the wild (CISA KEV) Patch available 93.9% exploitation probability (EPSS)

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AnalysisAIv1 – Mar 26, 2026

Microsoft Exchange Server contains a privilege escalation vulnerability in the PowerShell backend that allows authenticated Exchange users to escalate to Exchange admin, the second component of the ProxyShell attack chain.

Earlier versions of this analysis (1)

v1 Mar 26, 2026 - 11:19 CVSS 9.0

A privilege escalation vulnerability in Exchange's PowerShell backend allows any authenticated user to escalate to Exchange admin privileges. This is the second link in the ProxyShell chain, enabling administrative operations after the SSRF bypass.

RemediationAI

Apply Microsoft security updates. Monitor Exchange PowerShell audit logs for unauthorized cmdlet execution. Restrict Exchange PowerShell access to authorized administrators.

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 vulnerability in Exchange's PowerShell Remoting endpoint allows an attacker who has gained access through the SSRF (CVE-2021-34473) to escalate privileges from a standard mailbox user context to Exchange administrator. This enables access to the New-MailboxExportRequest cmdlet for 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 9.0 CRITICAL

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

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

CVSS VectorVendor: microsoft

Attack Vector

Local

Attack Complexity

Low

Privileges Required

None

User Interaction

None

Scope

Changed

Confidentiality

High

Integrity

High

Availability

None

Lifecycle Timeline

Added to CISA KEV

Oct 30, 2025 - 19:13 cisa

CISA KEV

PoC Detected

Oct 30, 2025 - 19:13 vuln.today

Public exploit code

Patch released

Oct 30, 2025 - 19:13 nvd

Patch available

CVE Published

Jul 14, 2021 - 18:15 nvd

CRITICAL 9.0

DescriptionCVE.org

Microsoft Exchange Server Elevation of Privilege Vulnerability

Executive ViewAI

Why It Matters

CVE-2021-34523 presents critical security risk rated CVSS 9.0. Successful exploitation could critically impact the confidentiality, integrity, and availability of affected systems. This vulnerability is actively exploited in the wild (KEV-listed) and requires immediate remediation. A patch is available and should be applied immediately given the critical severity.

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

9.6/10

Composite threat intelligence

EPSS Score

93.9%

Top 0% exploitable

Threat Intel

1 sources

Exploit

POC

Public exploit code exists

Data Quality

6 sources

Confidence: High

NVD

CVE.org

CISA KEV

EPSS POC (1) 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 CRITICAL

CISA KEV Yes

Public POC Yes

EPSS 93.9%

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

234

Low Medium High Critical

KEV: +50

EPSS: +94

CVSS: +45

PoC: +20

Weaponized: +15

Threat intel: +10

Gov advisory: 0

Attack Technique

Information Disclosure MEDIUM

Information disclosure occurs when an application unintentionally exposes sensitive data that aids attackers in reconnaissance or directly compromises security.