CVE-2021-24073Network attack vector

Authenticated session identity spoofing

Published Feb 25, 2021 · Updated Aug 3, 2024

Spoofing in Microsoft Skype for Business Server 2015 CU 8 and Lync Server 2013 allows remote authenticated users to present a false session identity. Microsoft has not published the affected component or the operation that mishandles attacker-controlled input. Exploitation requires existing low-privilege access and user interaction; successful spoofing can expose information and alter user-attributed actions.

CVSS severity6.5
Medium
EPSS probability1.80%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Spoofing in Microsoft Skype for Business Server 2015 CU 8 and Lync Server 2013 allows remote authenticated users to present a false session identity. Microsoft has not published the affected component or the operation that mishandles attacker-controlled input. Exploitation requires existing low-privilege access and user interaction; successful spoofing can expose information and alter user-attributed actions.

The record

CVE
CVE-2021-24073
Published
Feb 25, 2021
Updated
Aug 3, 2024
Vendor
Microsoft
Product
Microsoft Skype for Business Server 2015
Classifications
T1656
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Feb 25, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=2015 CU 8 <publication

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Microsoft · Microsoft Skype for Business Server 2015version=2015 CU 8 <publication
Microsoft · Microsoft Lync Server 2013version=0 <publication

Published CVSS scores

5.8NVDAV:N/AC:M/Au:N/C:P/I:P/A:N
6.5MicrosoftCVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:C/C:H/I:L/A:N
7.1NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:L/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

T1656Impersonation

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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