# Skype for Business and Lync Denial of Service Vulnerability

CVE record · `CVE-2021-24099`

- **Identifiers:** `CVE-2021-24099`
- **State:** Confirmed
- **Severity:** CVSS 6.5 Medium
- **Critical:** No
- **CNA:** microsoft
- **Technologies:** [Microsoft Skype for Business Server 2019 CU2](https://agentprime.io/advisories/technology/microsoft-skype-for-business-server-2019-cu2.md) · [Microsoft Skype for Business Server 2015 CU 8](https://agentprime.io/advisories/technology/microsoft-skype-for-business-server-2015-cu-8.md) · [Microsoft Lync Server 2013](https://agentprime.io/advisories/technology/microsoft-lync-server-2013.md)
- **Published:** 25 Feb 2021
- **Updated:** 3 Aug 2024
- **Reviewed by a person:** No

Published by the CVE Program (CNA: microsoft) at 25 Feb 2021, 23:01 UTC. agentprime has not analysed this yet, so whether it affects any app is not known. This CVE names a vendor and product rather than a package, so the lockfile check doesn’t match it.

## Affected products, as the CNA wrote them

- Vendor: Microsoft. Product: Skype for Business Server 2019 CU2.
  - affected: `7.0.0 to before publication` (custom)
- Vendor: Microsoft. Product: Skype for Business Server 2015 CU 8.
  - affected: `2015 CU 8 to before publication` (custom)
- Vendor: Microsoft. Product: Microsoft Lync Server 2013.
  - affected: `before publication` (custom)

## Not analysed yet

We haven’t published our own analysis of this CVE. If a person reviews it, this page will say what’s affected and what has to be true for it to matter.

## Severity and exploitation

- 6.5 Medium, CVSS 3.1: `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`, from CNA (microsoft)

- **CISA KEV:** Not listed.
- **EPSS:** 0.032, the 88th percentile, on 5 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

## Timeline

This record came from the full history of the CVE Program, published 25 Feb 2021. New records show when each source confirmed them.

## Sources

- [CVE List: CVE-2021-24099](https://www.cve.org/CVERecord?id=CVE-2021-24099)
- [NVD: CVE-2021-24099](https://nvd.nist.gov/vuln/detail/CVE-2021-24099)

### References from the CNA

- <https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-24099>

Facts on this page come from the CVE List as the CNA published them, with CVSS from the sources named above, CISA KEV and FIRST EPSS. Vendors, products and versions are shown as the CNA wrote them. We don’t reproduce the CVE’s description.

## Check whether this affects you

This CVE names a vendor and product rather than a package, so the lockfile check doesn’t match it. Compare the affected versions above with what you run.

- With the agentprime MCP server (<https://api.agentprime.io/mcp>), `get_advisory` returns this record and `search_advisories` finds related ones. Setup: <https://agentprime.io/mcp.md>
- As JSON: <https://api.agentprime.io/v1/advisories/CVE-2021-24099>
- Everything that needs attention now: <https://agentprime.io/advisories/critical.md>

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The web page: <https://agentprime.io/advisories/CVE-2021-24099/>. Everything for agents: <https://agentprime.io/llms.txt>.