CVE-2026-66307 in Skype for Business Server
Summary
by MITRE • 09/08/2026
Integer underflow (wrap or wraparound) in Skype for Business allows an unauthorized attacker to deny service over a network.
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Analysis
by VulDB Data Team • 09/08/2026
The vulnerability identified as an integer underflow within the Microsoft Skype for Business application represents a critical memory management flaw that can be exploited by remote, unauthenticated attackers to cause a denial of service condition. This specific type of error occurs when a calculation results in a value lower than the minimum representable unsigned integer value, causing the result to wrap around to a very large positive number due to how binary arithmetic handles underflows in fixed-width data types. In the context of Skype for Business, this flaw likely resides within the processing logic that handles network packets or internal message structures where size calculations are performed without adequate bounds checking. When an attacker crafts and sends specially malformed input containing specific numeric values designed to trigger this underflow condition, the application misinterprets the resulting large positive integer as a valid buffer length or allocation size.
The operational impact of exploiting this vulnerability is primarily centered on service availability rather than data confidentiality or integrity. Because the miscalculated value leads to an attempt to allocate memory that far exceeds available system resources or violates internal array bounds, the application process typically encounters a fatal exception and crashes immediately upon processing the malicious payload. This results in the immediate termination of the Skype for Business client instance on the victim's machine. For end-users, this manifests as unexpected application closure requiring manual restart, disrupting communication workflows. In enterprise environments where Skype for Business is integrated with broader collaboration tools or used for critical business communications, repeated exploitation can lead to significant productivity losses and operational downtime across multiple endpoints simultaneously if automated attack vectors are employed against shared infrastructure components such as edge servers or conferencing services that process client inputs.
From a technical classification perspective, this vulnerability aligns closely with CWE-192, which defines integer coercion errors leading to underflow wraparound, often resulting in out-of-bounds memory access or buffer overflows depending on subsequent usage of the corrupted value. The exploitation technique leverages the application's failure to validate input ranges before performing arithmetic operations, a common pattern in legacy software components that may not have been updated with modern secure coding practices emphasizing integer overflow protection mechanisms such as checked arithmetic functions provided by contemporary compilers and runtime environments. While primarily categorized under denial-of-service impacts, it is important to note that certain integer underflows can sometimes be chained with other vulnerabilities like buffer over-reads or use-after-free conditions to escalate privileges or execute arbitrary code, although the current description emphasizes the service disruption aspect.
Mitigation strategies for this vulnerability require a multi-layered approach focusing on patch management and network segmentation. The primary defense is the immediate application of vendor-provided security updates that address the specific integer handling logic within the affected versions of Skype for Business. Organizations should prioritize deploying these patches to all client endpoints and server infrastructure components exposed to untrusted networks. Additionally, implementing strict input validation at the perimeter using intrusion prevention systems can help detect and block malformed packets exhibiting patterns consistent with this exploitation attempt before they reach the vulnerable application layer. Network administrators should also enforce least-privilege principles for user accounts running Skype for Business to limit potential collateral damage if a crash occurs, ensuring that critical system resources remain protected even during transient service disruptions caused by such attacks.