What is the CVSS score of CVE-2021-26855?

CVE-2021-26855 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: 94.0%.

Which versions of Exchange Server are affected by CVE-2021-26855?

CVE-2021-26855 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-2021-26855?

Yes, a public proof-of-concept exploit is available for CVE-2021-26855. Prioritise patching immediately. EPSS: 94.0%.

How to fix or mitigate CVE-2021-26855?

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-2021-26855

CRITICAL

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

2021-03-03 secure@microsoft.com

Microsoft RCE 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

Dec 18, 2025 - 02:00 cisa

CISA KEV

PoC Detected

Dec 18, 2025 - 02:00 vuln.today

Public exploit code

Patch released

Dec 18, 2025 - 02:00 nvd

Patch available

CVE Published

Mar 03, 2021 - 00:15 nvd

CRITICAL 9.1

DescriptionCVE.org

Microsoft Exchange Server Remote Code Execution Vulnerability

AnalysisAI

Microsoft Exchange Server contains a server-side request forgery (SSRF) vulnerability known as 'ProxyLogon' that allows unauthenticated attackers to access Exchange backend services, chain with other vulnerabilities for full server compromise. The most impactful Exchange vulnerability in history.

Technical ContextAI

The CWE-918 SSRF in Exchange's autodiscover endpoint allows an unauthenticated attacker to send crafted requests that the Exchange server forwards to its backend services as SYSTEM. This bypass of authentication is chained with CVE-2021-26857 (deserialization), CVE-2021-26858, and CVE-2021-27065 (file write) for complete server compromise.

RemediationAI

Apply Microsoft security updates immediately. Run the MSERT tool to detect web shells. Check for indicators of compromise (aspx files in OWA/ECP directories, suspicious PowerShell execution). Assume compromise if running vulnerable versions during the exploitation window.