SecOps SignalSecurity Intelligence Workspace
Collection Degraded31 / 46 Sources HealthyPublic Intel Only
Vulnerabilities/CVE-2026-66303Public Intelligence Dossier
Source: Microsoft · Limited verification•Last verified: •Collection Degraded
CVE-2026-66303Unknown / EvaluatingImportant (raw)CVSS 6.5

Skype for Business and Lync Denial of Service Vulnerability

Published: 08 Sept 2026•Last updated: 08 Sept 2026, 12:30 PM IST•Source identifier: Unknown
Dossier completeness: source-provided sections 3 / 3 · rule guidance not available. Missing source facts count as incomplete.
Official source
CISA KEV catalog
Unknown
KEV status was not determined.
CVSS score
6.5Important (raw)
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H/E:U/RL:O/RC:C
Metric sourceMicrosoft MSRC CVRF
EPSS probability
1.1%0.011
Percentile66th
Weakness type
—
No further CWE recorded
Remediation
Patch status unknown
KB5123301, KB5123300, KB5123287

Why It Matters

Analysis dossier

Why It Matters

Null pointer dereference in Skype for Business allows an authorized attacker to deny service over a network.

SOC priority

Medium

Actively exploited

No

Publicly disclosed

Unknown

Risk basis: No confirmed exploitation signal in current enrichment

Affected Products & Remediation

3 source-backed products
Affected products and versions (source)
  • Skype for Business Server 2015 CU13 · version range not provided by source · Microsoft MSRC · Source
  • Skype for Business Server 2019 CU8 · version range not provided by source · Microsoft MSRC · Source
  • Skype for Business Server Subscription Edition CU1 · version range not provided by source · Microsoft MSRC · Source
Recorded patch / KBKB5123301, KB5123300, KB5123287

Vendor or catalog guidance (source)

  • Microsoft MSRC: Install the security update KB5123301 listed by MSRC
  • Microsoft MSRC: Install the security update KB5123300 listed by MSRC
  • Microsoft MSRC: Install the security update KB5123287 listed by MSRC

Known exploitation (source facts)

Only what a source states. Unknown means no source has said either way; it is not "not exploited".

CISA KEV
Unknown
Exploited (source statement)
No
Publicly disclosed (source)
Unknown

Suggested hunting (rule-based inference)

SecOps Signal recommendation (rule-based inference). These steps are hunting hypotheses from the vulnerability class; they do not state or imply that exploitation occurred here or anywhere.

No rule-based guidance for this vulnerability class

CISA KEV Record

CISA KEV status was not determined for this record.

Change History & Source Data

— No recorded material change

CNA / source
Unknown
Record status · verification
UPDATED · Unknown
CPE
Unknown

Evidence & Provenance

1 public source
api.msrc.microsoft.comSource record
Detection Hunting Query

— No published hunting query names this record. Browse community and library queries instead.

Open KQL Library

Correlated briefings

Microsoft Security Update Guide · Limited verificationSeptember 2026 Security UpdatesPublished 08 Sept 2026
SecOps Signal surfaces public advisory facts only. A missing value means the source has not published it, not that your environment is unaffected. No tenant inventory is connected.