CVE-2021-26421 in Lync Server
Summary
by MITRE • 05/12/2021
Skype for Business and Lync Spoofing Vulnerability
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Analysis
by VulDB Data Team • 05/13/2021
The CVE-2021-26421 vulnerability represents a significant spoofing flaw affecting Skype for Business and Lync communication platforms, which has profound implications for enterprise security and collaboration environments. This vulnerability stems from insufficient validation of user identities and session information within the communication protocols, creating opportunities for malicious actors to impersonate legitimate users within organizational networks. The flaw specifically impacts the authentication and authorization mechanisms that govern how users establish and maintain their communication sessions, potentially allowing unauthorized individuals to gain access to sensitive business communications and data.
The technical implementation of this vulnerability resides in the way Skype for Business and Lync handle session initiation and user verification processes, particularly when dealing with presence information and session management protocols. Attackers can exploit this weakness by crafting specially formatted messages or manipulating session data to appear as if they are legitimate users within the system. This type of vulnerability falls under the broader category of identity spoofing and session hijacking, which are commonly classified as CWE-287 (Improper Authentication) and CWE-305 (Authentication Bypass Using Alternative Input) within the Common Weakness Enumeration framework. The vulnerability's exploitation typically requires minimal privileges and can be executed through network-based attacks that manipulate the signaling protocols used by these communication platforms.
The operational impact of CVE-2021-26421 extends far beyond simple unauthorized access, as it can enable sophisticated attack chains including man-in-the-middle scenarios, session hijacking, and potential data exfiltration from corporate networks. Organizations utilizing these platforms face risks of unauthorized surveillance of business communications, disruption of critical collaboration workflows, and potential escalation to more severe security incidents. The vulnerability particularly affects enterprises that rely heavily on real-time communication systems for business operations, as attackers can leverage the spoofing capabilities to intercept sensitive business discussions, manipulate presence information to mislead colleagues, and potentially gain access to additional network resources through compromised communication sessions. This aligns with ATT&CK technique T1566 (Phishing for Information) and T1071.004 (Application Layer Protocol: DNS) when attackers use the compromised communication channels to further their objectives.
Mitigation strategies for CVE-2021-26421 should focus on strengthening authentication mechanisms and implementing additional validation layers for communication sessions. Organizations must ensure that all affected Skype for Business and Lync deployments receive immediate security updates from Microsoft, as the vendor has released patches addressing the underlying authentication validation issues. Network segmentation and monitoring solutions should be enhanced to detect anomalous session behavior and presence information changes that might indicate exploitation attempts. Security teams should also implement strict access controls for communication protocols, enable multi-factor authentication where possible, and establish robust network monitoring to detect unauthorized session establishment attempts. Additionally, regular security assessments should be conducted to identify and remediate similar vulnerabilities in other communication platforms within the enterprise environment, following best practices established in frameworks such as NIST SP 800-53 and ISO/IEC 27001 for secure communication system management.