CVE-2026-66305: Use of client-side authentication in Skype for Business allows an authorized attacker to perform spoofing over

This month

HIGH 7.1WATCHLISTDOWNGRADEDOWNGRADE

CVE-2026-66305 is a high-severity flaw (CVSS 7.1) affecting skype for business server, skype for business server subscription edition. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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

Why this priority

High severity
On your watchlist

No fix is published yet: follow the downgrade or mitigation steps.

Risk facts

CVSS score	7.1 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	Low
User interaction	None
EPSS exploit chance	0.5% (top 56%)
CISA exploited list	No
Weakness	CWE-603
Published	2026-09-08
Last modified	2026-09-16

Publisher (CNA): Microsoft Corporation · Vendor: microsoft

Last updated 2026-09-16 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-09-08 19:18 UTCPublished CVE record published
2026-09-16 19:24 UTCUpdated Record last modified

microsoft skype for business server DOWNGRADE

Affected: 2015 cumulative_update_13 · 2015 cumulative_update_13_hotfix_2 · 2019 cumulative_update_8 · 2019 cumulative_update_8_hotfix1 · 2019 cumulative_update_8_hotfix2

Fixed in: no fixed release known

Safe to downgrade to: a release before 2015

1. Check installed version

apt-cache policy skype-for-business-server || rpm -q skype-for-business-server || skype-for-business-server --version

2. No fixed release: DOWNGRADE to a release before 2015 (Debian / Ubuntu)

apt-cache madison skype-for-business-server sudo apt install skype-for-business-server=<VERSION_BEFORE_2015> sudo apt-mark hold skype-for-business-server

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list skype-for-business-server sudo dnf downgrade skype-for-business-server-<VERSION_BEFORE_2015> sudo dnf versionlock add skype-for-business-server

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

microsoft skype for business server subscription edition DOWNGRADE

Affected: 7.0.2046.849

Fixed in: no fixed release known

Safe to downgrade to: a release before 7.0.2046.849

1. Check installed version

apt-cache policy skype-for-business-server-subscription-edition || rpm -q skype-for-business-server-subscription-edition || skype-for-business-server-subscription-edition --version

2. No fixed release: DOWNGRADE to a release before 7.0.2046.849 (Debian / Ubuntu)

apt-cache madison skype-for-business-server-subscription-edition sudo apt install skype-for-business-server-subscription-edition=<VERSION_BEFORE_7.0.2046.849> sudo apt-mark hold skype-for-business-server-subscription-edition

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list skype-for-business-server-subscription-edition sudo dnf downgrade skype-for-business-server-subscription-edition-<VERSION_BEFORE_7.0.2046.849> sudo dnf versionlock add skype-for-business-server-subscription-edition

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-66305?

skype for business server: downgrade or apply the listed mitigation; skype for business server subscription edition: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2026-66305 being exploited?

CVE-2026-66305 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-66305 affect?

CVE-2026-66305 affects skype for business server, skype for business server subscription edition. Severity is high (CVSS 7.1). Use of client-side authentication in Skype for Business allows an authorized attacker to perform spoofing over a network.

How urgent is CVE-2026-66305?

Priority 41/100. High severity; On your watchlist

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References

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