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%.

Which versions of Exchange Server are affected by CVE-2021-34473?

CVE-2021-34473 presents critical security risk, a server-side request forgery vulnerability rated CVSS 9.1.. A patch is available.

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 Microsoft SSRF Exchange Server

9.1

CVSS 3.1 · Vendor: microsoft

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

Analysis Generated

Mar 26, 2026 - 11:19 vuln.today

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

AnalysisAI

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.

Technical ContextAI

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.

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.