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

> An integer underflow in Skype for Business Server allows an unauthorized attacker to cause a denial of service over the network.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H/E:U/RL:O/RC:C`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 8 Sep 2026
- **Updated:** 14 Oct 2026
- **Also known as:** GHSA-fmx2-865f-8c6j

## What is CVE-2026-66307?

CVE-2026-66307 is a high-severity vulnerability in Microsoft Skype for Business Server 2015 CU13, published on 8 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

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

Weakness: CWE-191 (Integer Underflow (Wrap or Wraparound))

## Which versions are affected by CVE-2026-66307?

It affects 2 products.


Products: Microsoft Skype for Business Server, Microsoft Skype for Business Server Subscription Edition.

## How do I fix CVE-2026-66307?

No fix has been published for CVE-2026-66307 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-66307 being exploited?

There is no known exploitation of CVE-2026-66307 in the wild and no public exploit code.

## Other vulnerabilities in Microsoft Skype for Business Server 2015 CU13

- [CVE-2023-41763](https://insights.sbomify.com/CVE-2023-41763/): Microsoft Skype for Business Privilege Escalation (medium 5.3, exploited in the wild)
- [CVE-2026-66302](https://insights.sbomify.com/CVE-2026-66302/): Skype for Business Remote Code Execution Vulnerability (critical 9.8)
- [CVE-2026-69646](https://insights.sbomify.com/CVE-2026-69646/): Skype for Business Spoofing Vulnerability (high 8.3)
- [CVE-2026-66304](https://insights.sbomify.com/CVE-2026-66304/): Skype for Business Information Disclosure Vulnerability (high 7.5)
- [CVE-2023-36780](https://insights.sbomify.com/CVE-2023-36780/): Skype for Business Remote Code Execution Vulnerability (high 7.2)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-66307 and fix it if it is affected.

What it is: CVE-2026-66307 is a high-severity vulnerability in Microsoft Skype for Business Server 2015 CU13, published on 8 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-66307 in the wild and no public exploit code.

Affected product: Microsoft Skype for Business Server 2015 CU13. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-66307/ (Markdown: https://insights.sbomify.com/CVE-2026-66307/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-66307
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-66307
- OSV: https://osv.dev/vulnerability/CVE-2026-66307

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-66307/
Summaries are generated and may contain mistakes; the official description above is the record.