Microsoft Exchange Server Arbitrary File Write and Remote Code Execution Vulnerability (CVE-2021-27065, ProxyLogon)
Executive & SecOps Summary
SECOPS ASSESSMENTOn-premises Microsoft Exchange Server 2013, 2016, and 2019 without the latest Cumulative Updates (CU) and Security Updates (SU)
Unauthenticated remote attackers can bypass authentication, abuse Exchange Admin Center (EAC) APIs to write arbitrary files, and execute malicious code with SYSTEM privileges
Immediately apply the official Microsoft Cumulative Updates (CU) and Security Updates (SU), and perform a thorough compromise assessment
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
To remediate this vulnerability, run the Exchange Management Shell (EMS) or PowerShell as Administrator to verify the current version and apply the latest Cumulative Update (CU) and Security Update (SU).
# Check current Exchange Server version and build number
Get-ExchangeServer | Format-List Name, Edition, AdminDisplayVersion
# Restart IIS services after applying the security patch
iisreset /noforce2. Temporary Workaround & Hardening Configuration
If immediate patching is not feasible, restrict external access to the EAC (/ecp) and OAB (/OAB) virtual directories using IIS URL Rewrite or IP address restrictions.
# Example: Restrict access to the IIS default web site using IP security configurations
Import-Module WebAdministration
Set-WebConfigurationProperty -pspath 'MACHINE/WEBROOT/APPHOST' -filter 'system.webServer/security/ipSecurity' -name 'allowUnlisted' -value $false3. Log Audit & IoC Verification Commands
Audit the Exchange and IIS logs to detect potential exploitation attempts and unauthorized virtual directory resets.
# Search Exchange ECP logs for ResetVirtualDirectory activities containing potential webshell payloads
Select-String -Path 'C:\Program Files\Microsoft\Exchange Server\V15\Logging\ECP\Server\*.log' -Pattern 'ResetVirtualDirectory' | Select-Object -First 50
# Scan the IIS web directories for newly created or suspicious ASPX files
Get-ChildItem -Path 'C:\inetpub\wwwroot\aspnet_client\' -Recurse -Filter '*.aspx'Block Exploits Instantly with Managed Virtual Patching Rules
Can't patch production immediately? Deploy managed WAF rules at the edge to intercept RCE, SQLi, and deserialization payloads before they hit your origin server.
Technical Root Cause & Exploitation Mechanics
1. Vulnerability Overview & Root Cause
CVE-2021-27065 is an arbitrary file write vulnerability in the Exchange Admin Center (EAC) virtual directory configuration mechanism of Microsoft Exchange Server. The root cause lies in insufficient input validation and path traversal (CWE-22) flaws, allowing authenticated (or post-authentication bypass) attackers to write files to arbitrary paths on the server. This vulnerability is famously chained with the pre-authentication bypass vulnerability (CVE-2021-26855) to form the 'ProxyLogon' exploit chain.
2. Exploitation Mechanics & Attack Scenario
An attacker first bypasses frontend authentication to access the backend EAC API with administrative privileges. The attacker then modifies the 'ExternalUrl' parameter of an Offline Address Book (OAB) virtual directory to contain a malicious payload (such as an ASPX webshell). Finally, the attacker triggers the 'Reset Virtual Directory' function to export the configuration to a specified path within the IIS web root directory. By leveraging path traversal, the attacker forces the server to write the configuration file with a '.aspx' extension, allowing the web server to execute the embedded payload as code.
3. Infrastructure & Business Impact
This vulnerability is highly critical and is actively exploited in the wild, as documented in the CISA KEV catalog. Successful exploitation grants the attacker 'NT AUTHORITY\SYSTEM' privileges on the Exchange Server. This allows threat actors to compromise the entire Active Directory (AD) domain, exfiltrate sensitive email communications, deploy ransomware, and establish persistent backdoors within the enterprise network.
Official Vendor Advisories & References
Microsoft Exchange Server Remote Code Execution Vulnerability
Frequently Asked SecOps Questions (CVE-2021-27065)
FAQQ1. How critical is CVE-2021-27065 and is it actively exploited in the wild?
CVE-2021-27065 is rated CVSS 7.8 (HIGH) with a 30-day EPSS exploitation probability of 99.9%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2021-27065?
Primary impacted vendors include microsoft across exchange_server. Scope summary: On-premises Microsoft Exchange Server 2013, 2016, and 2019 (Exchange Online is not affected)
Q3. What temporary mitigations or workarounds exist for CVE-2021-27065 before patching?
[Mitigation] Immediately apply the official Microsoft Cumulative Updates (CU) and Security Updates (SU), and perform a thorough compromise assessment Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.
Generate an Internal Engineering Notice or CISO Report for CVE-2021-27065
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