CVE-2026-66305: CWE-603: Use of Client-Side Authentication in Microsoft Skype for Business Server 2015 CU13
Description
Use of client-side authentication in Skype for Business allows an authorized attacker to perform spoofing over a network.
CVSS v3.1
Score 7.1high
Affected software
Microsoft
Skype for Business Server 2015 CU13
Microsoft
Skype for Business Server 2019 CU8
Microsoft
Skype for Business Server Subscription Edition CU1
Weaknesses
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
This vulnerability (CVE-2026-66305) in Microsoft Skype for Business Server 2015 CU13 is classified under CWE-603 (Use of Client-Side Authentication). It enables an authorized attacker to spoof identities over the network by exploiting the client-side authentication mechanism. The CVSS v3.1 base score is 7.1, indicating high severity, with network attack vector, low attack complexity, and no user interaction required. Microsoft has issued an official fix for this vulnerability.
Potential Impact
An authorized attacker can exploit this vulnerability to spoof identities within the Skype for Business Server environment, potentially leading to impersonation and unauthorized actions. The confidentiality impact is low, integrity impact is high, and availability is not affected.
Mitigation Recommendations
Microsoft has released an official fix for this vulnerability. It is recommended to apply the provided patches from Microsoft as soon as possible. Since the product is not a cloud service, remediation requires updating the affected Skype for Business Server 2015 CU13 installations to the fixed versions. Refer to the Microsoft advisory at https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-66305 for detailed patching instructions.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- microsoft
- Date Reserved
- 2026-07-24T18:06:51.294Z
- Cvss Version
- 3.1
- State
- PUBLISHED
- Remediation Level
- official-fix
- Vendor Advisory Urls
- [{"url":"https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-66305","vendor":"Microsoft"}]
Threat ID: 6aa059e1acd9273b4913b800
Added to database: 09/08/2026, 18:54:25 UTC
Last enriched: 10/14/2026, 21:29:34 UTC
Last updated: 10/23/2026, 01:15:25 UTC
Views: 79
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