What is the CVSS score of CVE-2021-34473?

CVE-2021-34473 has a CVSS 3.1 base score of 9.1 (Critical). CVSS vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N. EPSS exploitation probability: 94.2%.

Is there a public PoC exploit available for CVE-2021-34473?

Yes, a public proof-of-concept exploit is available for CVE-2021-34473. Prioritise patching immediately. EPSS: 94.2%.

How to fix or mitigate CVE-2021-34473?

Within 24 hours: Identify all affected systems and apply vendor patches immediately. Implement egress filtering and URL validation for server-side requests.

Exchange Server CVE-2021-34473

CRITICAL

Server-Side Request Forgery (SSRF) (CWE-918)

2021-07-14 secure@microsoft.com

RCE SSRF Microsoft Exchange Server

9.1

CVSS 3.1 · Vendor: microsoft

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Vulnerability Assessment

Weakness	CWE CWE-918 OWASP Top 10:2025 A10:2021 - Server-Side Request Forgery ASVS 5.0 V12 - Files and Resources SSRF prevention, URL validation MITRE ATT&CK 2 techniques across 2 tactics: Initial Access, Credential 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	Critical risk (CVSS 9.1, EPSS 94.2%). ProxyShell exploitation began at scale in August 2021, targeting Exchange servers that had been patched for ProxyLogon but not for the newer vulnerabilities. LockFile ransomware was notably deployed via ProxyShell.
Exploit ScenarioDerived	An attacker exploits the SSRF to access the Exchange PowerShell Remoting endpoint, chains CVE-2021-34523 for privilege escalation to Exchange admin, then uses CVE-2021-31207 to write a web shell. The entire chain executes without authentication, compromising the Exchange server.

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

Exploited in the wild (CISA KEV) Patch available 94.2% exploitation probability (EPSS)

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AnalysisAIv1 – Mar 26, 2026

Microsoft Exchange Server contains a pre-authentication SSRF vulnerability known as 'ProxyShell' that allows unauthenticated attackers to access Exchange backend services and achieve remote code execution when chained with CVE-2021-34523 and CVE-2021-31207.

Earlier versions of this analysis (1)

v1 Mar 26, 2026 - 11:19 CVSS 9.1

A pre-authentication SSRF in Exchange Server enables unauthenticated access to backend services. Known as the first component of 'ProxyShell,' this was the successor to ProxyLogon and was massively exploited from August 2021.

RemediationAI

Apply all Exchange security updates. ProxyShell patches were included in April and May 2021 cumulative updates. Scan for web shells. Consider migrating to Exchange Online to reduce on-premises attack surface.

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-918 SSRF exploits the same Exchange Client Access architecture as ProxyLogon but through a different code path. Attackers abuse the autodiscover normalization to access the Exchange PowerShell backend without authentication, enabling mailbox access and command execution.

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Exploit and PoC references, threat intelligence from 22 sources

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Severity by source

Vendor (microsoft) PRIMARY 9.1 CRITICAL

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

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

CVSS VectorVendor: microsoft

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

None

Scope

Unchanged

Confidentiality

High

Integrity

High

Availability

None

Lifecycle Timeline

Added to CISA KEV

Oct 29, 2025 - 16:46 cisa

CISA KEV

PoC Detected

Oct 29, 2025 - 16:46 vuln.today

Public exploit code

Patch released

Oct 29, 2025 - 16:46 nvd

Patch available

CVE Published

Jul 14, 2021 - 18:15 nvd

CRITICAL 9.1

DescriptionCVE.org

Microsoft Exchange Server Remote Code Execution Vulnerability

Executive ViewAI

Why It Matters

CVE-2021-34473 presents critical security risk, a server-side request forgery vulnerability rated CVSS 9.1. Attackers could trick authenticated users into performing unintended actions, potentially modifying critical settings or data. 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 strict egress filtering; (2) Use allowlists for outbound connections; (3) Block access to cloud metadata endpoints.

Threat Intelligence Overview

Threat Score

9.6/10

Composite threat intelligence

EPSS Score

94.2%

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 (2) 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 94.2%

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

Immediate

Exploitation· derived

Active

System Exposure· derived

Open

Automatable· derived

Yes

Human Impact

Very High

Priority Score

235

Low Medium High Critical

KEV: +50

EPSS: +94

CVSS: +46

PoC: +20

Weaponized: +15

Threat intel: +10

Gov advisory: 0

Attack Technique

Server-Side Request Forgery HIGH

Server-Side Request Forgery exploits applications that fetch remote resources based on user-supplied URLs.

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:2013:cumulative_update_23:*:*:*:*:*:*

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

cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_20:*:*:*:*:*:*

cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_8:*:*:*:*:*:*

cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_9:*:*:*:*:*:*

More in Exchange Server

View all

CVE-2021-26855 CRITICAL Act Now POC

9.1 Mar 03

Microsoft Exchange Server contains a server-side request forgery (SSRF) vulnerability known as 'ProxyLogon' that allows

CVE-2021-34523 CRITICAL Act Now POC

9.0 Jul 14

Microsoft Exchange Server contains a privilege escalation vulnerability in the PowerShell backend that allows authentica

CVE-2021-27065 HIGH Act Now POC

7.8 Mar 03

Microsoft Exchange Server allows post-authentication arbitrary file write that enables web shell deployment, the primary

CVE-2021-42321 HIGH Act Now POC

8.8 Nov 10

Microsoft Exchange Server Remote Code Execution Vulnerability. Rated high severity (CVSS 8.8), this vulnerability is rem

CVE-2021-33766 HIGH Act Now POC

7.3 Jul 14

Microsoft Exchange Server Information Disclosure Vulnerability. Rated high severity (CVSS 7.3), this vulnerability is re

CVE-2022-41080 HIGH Act Now

8.8 Nov 09

Microsoft Exchange Server Elevation of Privilege Vulnerability. Rated high severity (CVSS 8.8), this vulnerability is re

CVE-2021-26858 HIGH Act Now

7.8 Mar 03

Microsoft Exchange Server allows authenticated attackers to write arbitrary files to the server filesystem, the third co

CVE-2023-21529 HIGH Act Now POC

8.8 Feb 14

Remote code execution in Microsoft Exchange Server 2013, 2016, and 2019 allows authenticated attackers to execute arbitr

CVE-2021-41349 MEDIUM This Month POC

6.5 Nov 10

Microsoft Exchange Server Spoofing Vulnerability. Rated medium severity (CVSS 6.5), this vulnerability is remotely explo

CVE-2021-26857 HIGH Act Now

7.8 Mar 03

Microsoft Exchange Server Unified Messaging service contains a deserialization vulnerability that allows authenticated a

CVE-2021-31196 HIGH Act Now

7.2 Jul 14

Microsoft Exchange Server Remote Code Execution Vulnerability. Rated high severity (CVSS 7.2), this vulnerability is rem

CVE-2023-21707 HIGH Act Now

8.8 Feb 14

Microsoft Exchange Server Remote Code Execution Vulnerability. Rated high severity (CVSS 8.8), this vulnerability is rem

Same weakness CWE-918 – Server-Side Request Forgery (SSRF)

View all

CVE-2021-21985 CRITICAL POC

9.8

CVE-2022-41040 HIGH POC

8.8

CVE-2024-21893 HIGH POC

8.2

CVE-2021-40438 CRITICAL POC

9.0

CVE-2021-22054 HIGH POC

7.5

CVE-2021-21975 HIGH POC

7.5

CVE-2023-48022 CRITICAL POC

9.8

Same technique RCE

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CVE-2021-44228 CRITICAL POC

10.0

CVE-2017-7269 CRITICAL POC

9.8

CVE-2022-47966 CRITICAL POC

9.8

CVE-2012-4681 CRITICAL POC

9.8

CVE-2023-3519 CRITICAL POC

9.8

CVE-2015-5119 CRITICAL POC

9.8

CVE-2016-4117 CRITICAL POC

9.8

CVE-2015-3113 CRITICAL POC

9.8

Threat IntelligenceMISP

Threat Actors

Magic Hound

ATT&CK Techniques (observed)

T1190 - Exploit Public-Facing Application

Sources: MITRE ATT&CK, Metasploit/Nuclei

References

Advisories & patches

PATCH https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-34473 REF https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-34473 REF https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-34473 REF https://www.zerodayinitiative.com/advisories/ZDI-21-821/

Search Externally

GitHub PoC

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