CVE-2026-66307: Integer underflow (wrap or wraparound) in Skype for Business allows an unauthorized attacker to deny service o

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HIGH 7.5REMOTEWATCHLISTDOWNGRADEDOWNGRADE

CVE-2026-66307 is a high-severity flaw (CVSS 7.5) 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.

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

Why this priority

High severity
Remotely exploitable, no login, no user action
On your watchlist

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

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	1.2% (top 33%)
CISA exploited list	No
Weakness	CWE-191
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-2026-66307?

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-2026-66307 being exploited?

CVE-2026-66307 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-2026-66307 affect?

CVE-2026-66307 affects skype for business server, skype for business server subscription edition. Severity is high (CVSS 7.5). Integer underflow (wrap or wraparound) in Skype for Business allows an unauthorized attacker to deny service over a network.

How urgent is CVE-2026-66307?

Priority 53/100. High severity; Remotely exploitable, no login, no user action; On your watchlist

Related CVEs

CVE-2026-69646 HIGH 8.3 DOWNGRADE DOWNGRADE Improper verification of cryptographic signature in Skype for Business allows an unauthorized attacker to perf
CVE-2026-69642 MEDIUM 6.5 DOWNGRADE DOWNGRADE Improper neutralization of input during web page generation ('cross-site scripting') in Skype for Business all
CVE-2026-66308 MEDIUM 6.5 DOWNGRADE DOWNGRADE Out-of-bounds read in Skype for Business allows an authorized attacker to deny service over a network.
CVE-2026-66306 MEDIUM 6.5 DOWNGRADE DOWNGRADE Generation of error message containing sensitive information in Skype for Business allows an unauthorized atta
CVE-2026-66305 HIGH 7.1 DOWNGRADE DOWNGRADE Use of client-side authentication in Skype for Business allows an authorized attacker to perform spoofing over

References

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