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CVE-2026-66302 – Remote Code Execution – Microsoft Skype for Business Server (2015, 2019, Subscripti

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Summary

CVE-2026-66302 is a critical remote code execution vulnerability in Microsoft Skype for Business Server caused by external control of file name or path (CWE-73). An unauthenticated, remote attacker can exploit the flaw over the network without any user interaction, and the CVSS v3.1 base score of 9.8 reflects full compromise of confidentiality, integrity, and availability.

Technical details

  • Root cause: The server accepts attacker-influenced input that is used to construct a file name or path without adequate validation (CWE-73: External Control of File Name or Path).
  • Trigger conditions: An attacker sends specially crafted network traffic to a reachable Skype for Business Server instance; no prior authentication or user interaction is required.
  • Attack vector: Network (AV:N), with low attack complexity (AC:L) and no privileges (PR:N) or user interaction (UI:N) needed.
  • Impact: Successful exploitation allows arbitrary code execution on the server, resulting in high impact to confidentiality, integrity, and availability (C:H/I:H/A:H).

Affected software

  • Skype for Business Server 2015 CU13 (x64-based Systems): versions 6.0.9319.0 up to (but not including) 6.0.9319.885
  • Skype for Business Server 2019 CU8 (x64-based Systems): versions 7.0.2046.0 up to (but not including) 7.0.2046.569
  • Skype for Business Server Subscription Edition CU1 (x64-based Systems): versions 7.0.2046.0 up to (but not including) 7.0.2046.879

Severity

  • CVSS v3.1 Base Score: 9.8 (Critical)
  • Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C

Mitigation and recommended actions

  • Immediate: Apply the Microsoft security update to bring servers to the fixed builds — 6.0.9319.885 or later for Skype for Business Server 2015 CU13, 7.0.2046.569 or later for Skype for Business Server 2019 CU8, and 7.0.2046.879 or later for Skype for Business Server Subscription Edition CU1.
  • If patching cannot be completed immediately: Restrict network exposure of Skype for Business Server front-end and edge roles to trusted networks only, and monitor for anomalous inbound traffic until the update can be applied. Consult Microsoft’s official advisory for any interim guidance.

How IONIX identifies potentially affected assets

IONIX matches the following signals against data already collected when it crawled the asset; identifying the technology sends no request beyond that crawl.

  • Named response header x-ms-server-fqdn: infra.lync.com

References

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How IONIX’s External Exposure Management Platform Detects and Validates
Zero-Days to Shrink MTTR

1

Map your entire attack surface (continously)

IONIX uses multi-factor discovery methods, including DNS analysis, certificate mapping, metadata inspection, and more, to automatically map every internet-facing asset across your environment. This includes cloud instances, third-party platforms, shadow IT, and even forgotten infrastructure that traditional tools miss.

2

Monitor for new CVEs

Dozens of threat intel feeds using agentic technology are continuously analyzed to detect the appearance of proof-of-concept code, exploit kits, and indicators of active targeting. IONIX goes further by applying AI to proactively evaluate whether emerging vulnerabilities are likely to be exploited, even before PoCs go public.

3

Identify Potential External Exposures

Not all CVEs matter. IONIX filters vulnerabilities by asking attacker-centric questions: Can it be reached from the internet? Does it require authentication? Is it being exploited in the wild? This dramatically reduces noise and focuses teams on threats that can actually be weaponized.

4

Create Safe, Scalable Exploit Validations

IONIX transforms real-world PoCs into safe, non-intrusive test payloads that can be run in production environments without disruption. These simulations are precisely targeted to the systems that are vulnerable, ensuring rapid validation without unnecessary load.

5

Execute Exploit Validations

By combining context about software stack, versioning, exposure status, and reachability, IONIX ensures that only the right payloads are executed against the right assets, maximizing efficiency and minimizing risk.

6

Drive Fast and Actionable Remediation

Results are routed through integrations with ticketing, SOAR, and SIEM tools. Issues are written in plain language, bundled into remediation clusters, and prioritized based on asset criticality, exploitability, and blast radius. This shortens mean time to remediation (MTTR) and empowers teams to act with confidence.

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  • Confirmation of verified exploitable assets
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