CVE-2021-24073 in Lync Server
Summary
by MITRE • 02/26/2021
Skype for Business and Lync Spoofing Vulnerability
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Analysis
by VulDB Data Team • 02/26/2021
The Skype for Business and Lync spoofing vulnerability represents a significant security flaw that allows attackers to manipulate communication sessions and potentially gain unauthorized access to sensitive corporate data. This vulnerability affects Microsoft's unified communications platforms, specifically targeting the authentication and session management mechanisms used in enterprise environments. The issue stems from improper validation of session identifiers and insufficient verification of caller credentials during communication establishment processes.
The technical implementation of this vulnerability resides in the way Skype for Business and Lync handle session initiation and authentication tokens. When users establish communication sessions through these platforms, the systems should validate that incoming requests originate from legitimate sources and possess valid credentials. However, the flaw allows malicious actors to craft spoofed session requests that appear authentic to the system's validation mechanisms. This occurs due to weak cryptographic token handling and inadequate session state verification protocols within the platform's core communication modules.
From an operational perspective, this vulnerability creates substantial risks for enterprise environments that rely heavily on unified communications systems for business operations. Attackers can exploit this weakness to intercept and manipulate voice and video conferences, potentially gaining access to sensitive discussions containing proprietary information, strategic plans, or confidential business data. The impact extends beyond simple eavesdropping as attackers may also be able to inject malicious content into ongoing sessions or redirect communication flows to compromise additional network resources.
The vulnerability aligns with CWE-306, which addresses missing authentication for critical functions, and can be mapped to ATT&CK technique T1566 related to spearphishing with malicious attachments. Organizations using these platforms face increased risk of advanced persistent threats where attackers leverage this weakness to establish long-term access within corporate networks. The spoofing capability also enables man-in-the-middle attacks that can persist across multiple communication sessions, making detection and remediation more challenging.
Mitigation strategies should focus on implementing robust authentication mechanisms, including multi-factor authentication for all communication endpoints, and regular security updates from Microsoft that address the underlying session validation flaws. Network segmentation and monitoring systems should be deployed to detect anomalous communication patterns that may indicate spoofing attempts. Security teams should also conduct regular vulnerability assessments targeting unified communications platforms and ensure proper configuration of authentication policies within the Skype for Business environment.
The broader implications of this vulnerability highlight the critical importance of secure session management in enterprise communication systems, particularly as organizations increasingly rely on cloud-based collaboration tools. Organizations must maintain comprehensive security postures that include continuous monitoring of communication protocols, regular patch management schedules, and employee training on recognizing potential social engineering attacks that may accompany such technical exploits. Proper incident response procedures should be established to quickly identify and contain any exploitation attempts targeting these specific platforms.
Additional protective measures include implementing network access controls that restrict communication flows between trusted and untrusted networks, deploying intrusion detection systems specifically configured to monitor for suspicious session establishment patterns, and establishing secure communication protocols that utilize end-to-end encryption. Regular security audits of unified communications infrastructure should verify proper implementation of authentication requirements and identify potential weaknesses in session management processes that could be exploited by adversaries. Organizations must also consider the regulatory implications of such vulnerabilities, particularly in industries subject to compliance requirements for data protection and privacy.