7.5High
Microsoft Skype For Business Server
CVE-2026-66307
Integer underflow (wrap or wraparound) in Skype for Business allows an unauthorized attacker to deny service over a network.
What this means for your business
- It affects Microsoft Skype For Business Server. It matters if your company, or a supplier that handles your data, runs it.
- An attacker can use it remotely, over a network, without a login, and without anyone at your company clicking anything.
- FIRST's prediction model gives it a 0.6% chance of attack attempts being seen in the next 30 days, ranking above 49% of all known flaws.
What to do
- 1Check whether your company or your suppliers run Microsoft Skype For Business Server, and which version. The affected versions are listed further down this page.
- 2If you do, apply the vendor's fix. A patch or vendor advisory has been published.
Fastnexa security experts
Not sure if your company is exposed to CVE-2026-66307?
Tell us where you run Microsoft Skype For Business Server and a Fastnexa penetration tester will check whether this flaw, or others like it, can be used against your websites, apps and network.
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Scoring
- CVSS
- 7.5 (v3.1)
- Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H- Weakness
- CWE-191
- Assigned by
- secure@microsoft.com
Dates
- Published
- 2026-09-08
- Last modified
- 2026-09-16
- Sources
- NVD
Affected products
- Microsoft Skype For Business Server2015, 2019
- Microsoft Skype For Business Server Subscription Edition7.0.2046.849
As listed in the NVD configuration data. Not a statement about your estate.