How severe is CVE-2026-66307?

CVE-2026-66307 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.

Is CVE-2026-66307 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (67th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-66307?

CVE-2026-66307 primarily affects Microsoft Skype For Business Server. In total, 6 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.

How do I fix CVE-2026-66307?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.

When was CVE-2026-66307 published?

CVE-2026-66307 was published on 2026-09-08 and last updated on 2026-09-16.

CVE-2026-66307

CVE-2026-66307 is a high-severity vulnerability in Microsoft Skype For Business Server with a CVSS 3.x base score of 7.5. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-191.

Key facts

Severity: High (CVSS 3.x base score 7.5)
EPSS exploit prediction: 1% (67th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-191
Affected product: Microsoft Skype For Business Server
Published: 2026-09-08
Last modified: 2026-09-16

Description

Integer underflow (wrap or wraparound) in Skype for Business allows an unauthorized attacker to deny service over a network.

Frequently asked questions

What is CVE-2026-66307?: Integer underflow (wrap or wraparound) in Skype for Business allows an unauthorized attacker to deny service over a network.
How severe is CVE-2026-66307?: CVE-2026-66307 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.
Is CVE-2026-66307 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (67th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-66307?: CVE-2026-66307 primarily affects Microsoft Skype For Business Server. In total, 6 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.
How do I fix CVE-2026-66307?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
When was CVE-2026-66307 published?: CVE-2026-66307 was published on 2026-09-08 and last updated on 2026-09-16.

References

https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-66307

Affected products (6)

cpe:2.3:a:microsoft:skype_for_business_server:2015:cumulative_update_13:*:*:*:*:*:*
cpe:2.3:a:microsoft:skype_for_business_server:2015:cumulative_update_13_hotfix_2:*:*:*:*:*:*
cpe:2.3:a:microsoft:skype_for_business_server:2019:cumulative_update_8:*:*:*:*:*:*
cpe:2.3:a:microsoft:skype_for_business_server:2019:cumulative_update_8_hotfix1:*:*:*:*:*:*
cpe:2.3:a:microsoft:skype_for_business_server:2019:cumulative_update_8_hotfix2:*:*:*:*:*:*
cpe:2.3:a:microsoft:skype_for_business_server_subscription_edition:7.0.2046.849:*:*:*:*:*:*:*

More vulnerabilities in Microsoft Skype For Business Server

CVE-2026-66302 — Critical (CVSS 9.8): External control of file name or path in Skype for Business allows an unauthorized attacker to execute code over a…
CVE-2026-69646 — High (CVSS 8.3): Improper verification of cryptographic signature in Skype for Business allows an unauthorized attacker to perform…
CVE-2026-66304 — High (CVSS 7.5): Server-side request forgery (ssrf) in Skype for Business allows an unauthorized attacker to disclose information over a…
CVE-2023-36789 — High (CVSS 7.2): Skype for Business Remote Code Execution Vulnerability
CVE-2023-36786 — High (CVSS 7.2): Skype for Business Remote Code Execution Vulnerability
CVE-2023-36780 — High (CVSS 7.2): Skype for Business Remote Code Execution Vulnerability

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