Cyber Resilience

CVE-2026-66305

Published
08 September 2026
Modified
09 September 2026
CVSS Score v3.1 7.1
Click a component to see what it means
Raw vectorCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:H/A:N
EPSS Score 0.0030 23th percentile
Risk Priority 53 floored blend · peak EPSS

CVSS and EPSS are reproduced from their sources (NVD, FIRST EPSS). Risk Priority is our own derived reading, not an NVD score.

Summary

CVE-2026-66305 is a high-severity Use of Client-Side Authentication (CWE-603) vulnerability. Its CVSS base score is 7.1 (High).

Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked at the 23th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.

The strongest mitigations our analysis identified map to AC-3 (Access Enforcement) and IA-2 (Identification and Authentication (Organizational Users)) — see the control section below for these in your framework.

EU & UK References

Vulnerability Data

Use of client-side authentication in Skype for Business allows an authorized attacker to perform spoofing over a network.

CWE(s)
CWE-603

Related Threats

MITRE ATT&CK Enterprise Techniques

T1190 Exploit Public-Facing Application Initial Access
Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.
attack.mitre.org →
Derived from this CVE’s CWE(s) via the direct CWE→ATT&CK cross-walk.

CVEs Like This One

CVE-2024-39375Shared CWE-603
CVE-2025-30042Shared CWE-603
CVE-2026-1363Shared CWE-603
CVE-2026-42098Shared CWE-603
CVE-2024-45785Shared CWE-603
CVE-2026-71187Shared CWE-603
CVE-2026-76945Shared CWE-603
CVE-2025-62649Shared CWE-603
CVE-2025-61940Shared CWE-603
CVE-2025-64119Shared CWE-603

Affected Assets

Mitigating Controls

Control response

Prevent
Stop it (NIST 800-53)

—

Detect
Catch it (NIST detect / respond)

—

Harden
Shrink the surface (DISA STIG)

—

Validate
Prove the fix (OWASP ASVS)
  • V10.4.10
  • V10.4.15
  • V10.4.16
  • V10.7.1

Mitigating Controls (NIST 800-53 r5) AI

AC-3 Access Enforcement good match

Enforces access authorizations on the server, making client-side bypass ineffective.

IA-2 Identification and Authentication (Organizational Users) good match

Requires server-side identification and authentication of users before granting access.

Mitigating Controls (NIST CSF 2.0) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.

PR.AA-03 full match
prevents

Server-enforced authentication directly prevents client-only checks from being bypassed.

Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.

A.8.5 Secure authentication full match
degrades

Requires server-side authentication mechanisms that directly prevent client-only checks.

A.8.27 Secure system architecture and engineering principles mostly match
prevents

Promotes secure architecture principles that reject client-only authentication designs.

A.8.29 Security testing in development and acceptance mostly match
finds

Security testing can detect missing server-side authentication checks.

A.8.26 Application security requirements partial match
prevents

Defines application security requirements that include server-side authentication enforcement.

A.8.28 Secure coding partial match
prevents

Secure coding standards discourage reliance on client-side authentication logic.

A.8.3 Information access restriction partial match
mitigates

Enforces access restrictions that must be validated on the server, mitigating client-side bypass.

References