CVE-2021-26421Network attack vector

Spoofing via undisclosed server flaw

Published May 11, 2021 · Updated Aug 3, 2024

Spoofing in Microsoft Skype for Business Server 2015 and Microsoft Lync Server 2013 allows remote authenticated users to impersonate trusted content. Microsoft has not published the affected component, crafted input, or faulty operation that enables the impersonation. Exploitation requires low privileges and user interaction; success exposes confidential information and permits limited content modification.

CVSS severity6.5
Medium
EPSS probability1.41%
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 and Microsoft Lync Server 2013 allows remote authenticated users to impersonate trusted content. Microsoft has not published the affected component, crafted input, or faulty operation that enables the impersonation. Exploitation requires low privileges and user interaction; success exposes confidential information and permits limited content modification.

The record

CVE
CVE-2021-26421
Published
May 11, 2021
Updated
Aug 3, 2024
Vendor
Microsoft
Product
Microsoft Skype for Business Server 2015
Classifications
Unavailable
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. May 11, 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

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

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

  1. Affected versionversion=2015 CU11 <2046.369

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Microsoft · Microsoft Skype for Business Server 2015version=2015 CU11 <2046.369
Microsoft · Microsoft Lync Server 2013version=8308.0 <8308.1144

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

No sourced classifications are available.

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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