CVE-2018-0940 in Exchange Server
Summary
by MITRE
Microsoft Exchange Outlook Web Access (OWA) in Microsoft Exchange Server 2010 Service Pack 3 Update Rollup 20, Microsoft Exchange Server 2013 Cumulative Update 18, Microsoft Exchange Server 2013 Cumulative Update 19, Microsoft Exchange Server 2013 Service Pack 1, Microsoft Exchange Server 2016 Cumulative Update 7, and Microsoft Exchange Server 2016 Cumulative Update 8 allows an elevation of privilege vulnerability due to how links in the body of an email message are rewritten, aka "Microsoft Exchange Elevation of Privilege Vulnerability".
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Analysis
by VulDB Data Team • 02/05/2021
The Microsoft Exchange Server vulnerability CVE-2018-0940 represents a critical elevation of privilege flaw within the Outlook Web Access component that affects multiple server versions including Exchange 2010, 2013, and 2016. This vulnerability specifically exploits how email messages are processed when they contain hyperlinks, creating a pathway for unauthorized privilege escalation. The flaw resides in the manner in which OWA rewrites links found within the body of email messages, allowing malicious actors to manipulate the system's privilege levels through crafted email content. This issue falls under the CWE-264 category of Permissions, Privileges, and Access Controls, specifically addressing improper privilege management within email processing systems. The vulnerability operates at the application layer and can be leveraged by attackers who have already gained access to a legitimate user account within the Exchange environment.
The technical exploitation of this vulnerability occurs when an attacker sends a specially crafted email containing malicious links to a victim user. When the victim accesses their email through OWA, the system processes these links in a manner that allows the attacker to escalate their privileges from a standard user level to a higher administrative level. The flaw stems from improper input validation and link rewriting mechanisms within the OWA component, where the system fails to properly sanitize or validate the links before processing them with elevated privileges. This creates a scenario where a regular user can potentially execute commands or access resources that should be restricted to administrators or privileged system accounts. The vulnerability demonstrates characteristics consistent with the ATT&CK technique T1068 which involves exploiting a legitimate system process to gain elevated privileges, and T1548.001 which covers abuse of Windows privileges through legitimate system tools.
The operational impact of CVE-2018-0940 extends beyond simple privilege escalation as it provides attackers with a potential foothold for further compromise within the Exchange environment. Once elevated privileges are obtained, attackers can access sensitive email data, modify user accounts, read confidential messages, and potentially establish persistence within the organization's email infrastructure. The vulnerability affects organizations that rely heavily on Exchange Server for email services and can lead to significant data breaches or unauthorized access to corporate communications. The attack vector requires minimal user interaction beyond opening the malicious email, making it particularly dangerous in environments where users frequently access email through web browsers. Organizations may experience cascading effects as attackers use the elevated privileges to move laterally within their network or target other systems that share credentials or access patterns with the Exchange server.
Mitigation strategies for CVE-2018-0940 primarily involve applying the official Microsoft security patches released for each affected Exchange Server version. Organizations should immediately deploy the cumulative updates for Exchange Server 2010 SP3, Exchange Server 2013 CU18 and CU19, and Exchange Server 2016 CU7 and CU8 to address the privilege escalation vulnerability. Additional defensive measures include implementing email filtering solutions that can detect and block suspicious links, enabling strict URL validation within OWA settings, and conducting regular security assessments of Exchange server configurations. Network segmentation and access controls should be reviewed to limit the potential impact of privilege escalation, while monitoring systems should be enhanced to detect unusual access patterns or privilege changes within the Exchange environment. The vulnerability highlights the importance of maintaining up-to-date security patches and demonstrates how seemingly minor processing flaws in email systems can create significant security risks. Organizations should also consider implementing zero-trust network principles and multi-factor authentication for Exchange access to reduce the overall attack surface and limit the potential damage from such privilege escalation vulnerabilities.