CVE-2021-24099 in Lync Server
Summary
by MITRE • 02/26/2021
Skype for Business and Lync Denial of Service Vulnerability
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Analysis
by VulDB Data Team • 05/17/2026
This vulnerability affects Skype for Business and Lync messaging platforms, representing a critical denial of service condition that can be exploited by remote attackers to disrupt communication services. The flaw manifests through improper handling of specific network packets or protocol interactions that cause the affected systems to crash or become unresponsive. According to CWE-400, this vulnerability falls under the category of unchecked resource consumption, where the application fails to properly validate or limit incoming data streams that could lead to resource exhaustion. The vulnerability is particularly concerning because it can be triggered without authentication, making it accessible to any attacker on the network.
The technical implementation involves malformed packets or protocol messages that are processed by the Skype for Business or Lync servers during normal operation. When these systems encounter such malformed inputs, they fail to properly sanitize or reject the data, leading to abnormal termination of service processes or complete system crashes. The vulnerability is typically exploited through SIP protocol manipulation or specific message formats that cause memory allocation failures or infinite loop conditions within the application code. This type of flaw aligns with ATT&CK technique T1499.004, which covers network disruption through resource exhaustion attacks that can be executed without requiring advanced privileges.
Operational impact of this vulnerability extends beyond simple service interruption to potentially compromise business continuity and communication infrastructure. Organizations relying on these platforms for critical business operations face significant risk of extended downtime, especially when the vulnerability allows for rapid exploitation across multiple systems. The attack surface includes not only direct network access but also potential compromise of internal communication networks where these services operate. Security teams must consider the cascading effects of such an attack, as disruption of unified communications can impact emergency response systems, remote workforce capabilities, and collaborative business processes.
Mitigation strategies should include immediate deployment of vendor security patches and updates to address the specific protocol handling flaws. Network segmentation and access controls can help limit exposure by restricting direct access to affected services from untrusted networks. Implementing robust input validation and rate limiting mechanisms at network boundaries provides additional protection layers against malformed packet exploitation. Organizations should also consider deploying intrusion detection systems specifically configured to detect anomalous SIP traffic patterns that could indicate exploitation attempts. Regular security assessments and vulnerability scanning should be conducted to identify any related weaknesses in the communication infrastructure that could be leveraged as part of multi-stage attacks. The remediation process must also include comprehensive testing of patches in controlled environments before full deployment to ensure compatibility with existing network configurations and business processes.