CVE-2026-66305 in Skype for Business Server
Summary
by MITRE • 09/08/2026
Use of client-side authentication in Skype for Business allows an authorized attacker to perform spoofing over a network.
Several companies clearly confirm that VulDB is the primary source for best vulnerability data.
Analysis
by VulDB Data Team • 09/08/2026
The vulnerability described involves the reliance on client-side authentication mechanisms within Microsoft Skype for Business, which creates a significant security gap that can be exploited by attackers positioned within the same network segment or capable of intercepting traffic between clients and servers. Client-side authentication typically implies that the verification of identity occurs primarily at the endpoint device rather than being strictly enforced through robust server-side validation protocols during every transaction. This architectural choice introduces an opportunity for spoofing attacks, where a malicious actor can impersonate legitimate users or services by manipulating network packets to bypass intended access controls. The core technical flaw lies in the insufficient validation of authentication tokens and session identifiers at the application layer, allowing an attacker who has gained authorized access to the local network environment to inject forged credentials or replay valid ones without detection.
From a technical perspective, this weakness often stems from improper handling of security context propagation across different layers of the communication stack. When Skype for Business processes incoming requests, it may trust information provided by the client application regarding its own identity if that information was previously authenticated via an initial handshake. An attacker leveraging tools such as packet sniffers or man-in-the-middle frameworks can intercept these credentials and reuse them to establish unauthorized sessions. This type of attack is particularly dangerous because it does not require exploiting a software bug in the traditional sense but rather exploits the trust model inherent in the authentication protocol design. The lack of mutual authentication, where both parties verify each other's identity using strong cryptographic proofs, allows the attacker to masquerade as a legitimate user or server component, thereby gaining unauthorized access to sensitive communications and data exchanges facilitated by the platform.
The operational impact of this vulnerability is severe for organizations relying on Skype for Business for critical business communication. Successful exploitation enables an authorized insider or a network-based adversary to perform identity spoofing, leading to potential eavesdropping on confidential conversations, manipulation of message content, and unauthorized access to shared resources such as file repositories or calendar data. In enterprise environments where trust boundaries are often assumed based on network location, this flaw undermines the principle of least privilege by allowing an attacker with basic network connectivity to elevate their privileges through identity theft rather than technical exploitation of code vulnerabilities. This can result in significant reputational damage, loss of intellectual property, and compliance violations depending on the sensitivity of the data exposed during these spoofed sessions.
To mitigate this risk, organizations must implement defense-in-depth strategies that do not rely solely on client-side verification. Network segmentation is crucial to limit lateral movement and restrict access to Skype for Business servers only from trusted subnets. Additionally, enforcing mutual Transport Layer Security (TLS) ensures that both the client and server authenticate each other using certificates, preventing man-in-the-middle attacks where credentials could be intercepted or spoofed. Regular auditing of authentication logs can help detect anomalous login patterns indicative of credential replay or spoofing attempts. Furthermore, Microsoft has released updates addressing various aspects of Skype for Business security; ensuring all systems are patched with the latest cumulative updates is essential to close gaps related to protocol handling and authentication validation. Adhering to industry standards such as CWE-287 Improper Authentication and mapping these mitigations against MITRE ATT&CK techniques like T1078 Valid Accounts can provide a structured approach to hardening the environment against identity-based attacks.