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CVE-2021-24099

22
FAUCET Score

CVE-2021-24099 is a denial-of-service vulnerability affecting Microsoft Skype for Business and Lync servers. With a CVSS score of 6.5 (Medium), it can be exploited remotely with low complexity by an authenticated attacker, leading to a complete denial of service without impacting confidentiality or integrity. While it has a low EPSS score and is not on the KEV catalog, there is no public exploit code available, and it has received minimal community and media attention.

Impacted Technologies

VendorProductVersion(s)CPE
2013CPE matchmatch criteria
cpe:2.3:a:microsoft:lync_server:2013:*:*:*:*:*:*:*
2015CPE matchmatch criteria
cpe:2.3:a:microsoft:skype_for_business_server:2015:cu8:*:*:*:*:*:*
2019CPE matchmatch criteria
cpe:2.3:a:microsoft:skype_for_business_server:2019:cu2:*:*:*:*:*:*

CVSS Data

CVSS version used by this source: 3.1

6.5MEDIUM

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Attack Vector
NETWORK
Attack Complexity
LOW
Privileges Required
LOW
User Interaction
NONE
Scope
UNCHANGED
Confidentiality Impact
NONE
Integrity Impact
NONE
Availability Impact
HIGH
Exploitability Score
2.8
Impact Score
3.6
CvssVersion
3.1

Exploit Intelligence

EPSS Score
2.89%
Probability of exploitation in next 30 days
EPSS Percentile
85.4%
Percentile rank of EPSS score among Peer Group
As of 2026-07-27
Model: v2026.06.15
This CVE's current EPSS score of 0.0289 is in the 95th percentile among its peer group of 21,958 CVEs.

Social Chatter

The average CVE in this peer group has 0.0 Twitter, 0.0 Reddit, 0.1 Bluesky, 0.0 Mastodon, and 0.1 GitHub mentions.

Media Mentions

The average CVE in this peer group has 0.0 InfoSec Media, 0.0 Vendor Blog, and 0.0 Security Researcher mentions.

Remediation

Patch Available

Vendor Patches (4)

microsoftpatch availablevia nvd_reference
View patch
microsoftpatch availablevia msrc
Product: Skype for Business Server 2019 CU2
View patch
microsoftpatch availablevia msrc
Product: Skype for Business Server 2015 CU 8
View patch
microsoftpatch availablevia msrc
Product: Microsoft Lync Server 2013
View patch

Vendor Advisories (1)

microsoft2021-Feb/CVE-2021-24099Important

Skype for Business and Lync Denial of Service Vulnerability

Feb 9, 2021

References

portal.msrc.microsoft.com / en-US/security-guidance/advisory/CVE-2021-24099
PatchVendor Advisory