CVE-2026-66307: Skype for Business and Lync Denial of Service Vulnerability

19

Low

Who exploits it and how urgent it is

A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.

No exploitation signal in the sources consulted so far.

How the score was calculated

EPSS 1.2% (30-day exploitation probability)+10
Technical severity (high)+6
Automatable exploitation (SSVC)+3

The score orders the patch queue; it does not replace your asset context (internet exposure, criticality, compensating controls).

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the sources call it.

Exploitation

Not confirmed

Not in CISA KEV — an absence of record, not proof it is unexploited.

EPSS FIRST

1.17%

ahead of 67% of scored CVEs · Oct 14, 2026

Affects

Microsoft

Skype For Business Server and 4 more

CVSS base

7.5

High · 2 source(s)

CISA SSVC assessment CISA

Three decision points CISA publishes for the CVEs it assesses (SSVC 2.0.3). A stakeholder decision, not a severity score.

Exploitation

None

none · proof-of-concept · active

Automatable

Yes

can an attacker script the kill-chain steps

Technical impact

Partial

partial · total control of the vulnerable component

Remediation

Fixes and mitigations documented by the sources — linked to the original advisory.

Documented patch — msrc.microsoft.com (NIST National Vulnerability Database)

First 24 hours

Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.

Identify exposed assets running the affected vendor/product/version combinations (Microsoft Skype For Business Server, Microsoft Skype for Business Server 2015 CU13, Microsoft Skype for Business Server 2019 CU8…).
The attack is network-based and needs no privileges: restrict exposure (ACLs, VPN, WAF) until patched.
Apply the fixed version or the patch documented under Remediation.
Search available logs for exploit probes matching the vulnerability class (memory corruption), errors and authentication anomalies.

Affected scope

Vendor, product and versions as the sources word them.

Vendor	Product	Versions	Status	Source
Microsoft	Skype For Business Server	version 2015-cumulative_update_13version 2015-cumulative_update_13_hotfix_2version 2019-cumulative_update_8version 2019-cumulative_update_8_hotfix1version 2019-cumulative_update_8_hotfix2	Vulnerable	NIST National Vulnerability Database
Microsoft	Skype for Business Server 2015 CU13	≥ 9319.0 and < 6.0.9319.885	Vulnerable	CVE Program (cve.org) e CISA ADP
Microsoft	Skype for Business Server 2019 CU8	≥ 2046.0 and < 7.0.2046.569	Vulnerable	CVE Program (cve.org) e CISA ADP
Microsoft	Skype For Business Server Subscription Edition	version 7.0.2046.849	Vulnerable	NIST National Vulnerability Database
Microsoft	Skype for Business Server Subscription Edition CU1	≥ 2046.0 and < 7.0.2046.879	Vulnerable	CVE Program (cve.org) e CISA ADP

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Attack vector

Network

Attack complexity

Low

Privileges required

None

User interaction

None

Scope

Unchanged

Confidentiality

None

Integrity

None

Availability

High

Every base score collected

Sources score independently and may disagree; each row says who scored it and under which version.

Score	Version	Severity	Expl.	Impact	Scored by
7.5	CVSS 3.1	High	3.9	3.6	secure@microsoft.com via NIST National Vulnerability Database
7.5	CVSS 3.1	High	—	—	microsoft via CVE Program (cve.org) e CISA ADP

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H (score shown at the top: secure@microsoft.com)

Weakness MITRE

Weakness class according to MITRE’s CWE catalog.

CWE-191 · Integer Underflow

Other CVEs with this weakness →

Detection

Read off the CVSS vector and the weakness class. Starting points, not tested rules.

Prioritize edge telemetry for the network-reachable component (proxy, WAF, IDS/IPS).
Correlate service crashes/restarts with inbound traffic; repeated crashes may indicate attempts.

web/proxy/WAF logsnetwork IDS/IPSendpoint telemetry (EDR)crash dumps

Timeline

What happened to this CVE, newest first.

2026
Fix documented
Oct 15, 2026 · NIST National Vulnerability Database
Affected products added · microsoft/skype for business server, microsoft/skype for business server subscription edition
Oct 15, 2026 · NIST National Vulnerability Database
Source status changed · Published → Analyzed
Oct 15, 2026 · NIST National Vulnerability Database
Record published
Oct 14, 2026
CVSS changed · 7.5
Oct 14, 2026 · CVE Program (cve.org) e CISA ADP
Source status changed · → Published
Oct 14, 2026 · CVE Program (cve.org) e CISA ADP
SSVC assessment updated
Oct 14, 2026 · CVE Program (cve.org) e CISA ADP
Record created in this database
Oct 14, 2026 · CVE Program (cve.org) e CISA ADP
CVE published
Sep 8, 2026 · microsoft

Source activity

Repeated readings are counted rather than listed.

1× Official description updated · Oct 14, 2026 – Oct 14, 2026

References

1 on the record

msrc.microsoft.com/update-guide/vulnerability/CVE-2026-66307
vendor-advisory, patch, Patch, Vendor Advisory, CVE Program (cve.org) e CISA ADP

Elsewhere on this site

Microsoft
every CVE for this vendor

Memory corruption
same class

CWE-191
other pages naming this weakness

Related CVEs (same products)

Medium CVE-2026-85873 Skype for Business Elevation of Privilege Vulnerability 2d
High CVE-2026-83746 Skype for Business Elevation of Privilege Vulnerability 2d
High CVE-2026-70297 Skype for Business Remote Code Execution Vulnerability 2d
High CVE-2026-69646 Skype for Business Spoofing Vulnerability Sep 8, 2026
Medium CVE-2026-69642 Skype for Business Spoofing Vulnerability Sep 8, 2026
High CVE-2026-66305 Skype for Business Spoofing Vulnerability Sep 8, 2026

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record

Text assembled only from the fields above — no additional claims.

What should defenders know first?

CVE-2026-66307 is "Skype for Business and Lync Denial of Service Vulnerability", rated High. It affects Skype For Business Server (Microsoft); versions are in the scope table. It is not in CISA KEV, which does not rule out exploitation. CVSS 7.5 describes technical severity. EPSS 1.17% estimates the probability of exploitation in 30 days; neither replaces your asset context. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.

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