How severe is CVE-2021-26421?

CVE-2021-26421 has a CVSS 3.x base score of 6.5, rated medium severity. It is exploitable over network with high attack complexity, requires low privileges and user interaction. Impact on confidentiality is high, integrity low, and availability none.

Is CVE-2021-26421 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (69th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-26421?

CVE-2021-26421 primarily affects Microsoft Lync Server. In total, 2 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.

How do I fix CVE-2021-26421?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.

When was CVE-2021-26421 published?

CVE-2021-26421 was published on 2021-05-11 and last updated on 2026-06-17.

CVE-2021-26421

CVE-2021-26421 is a medium-severity vulnerability in Microsoft Lync Server with a CVSS 3.x base score of 6.5. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low.

Key facts

Severity: Medium (CVSS 3.x base score 6.5)
CVSS v2: 5.8
EPSS exploit prediction: 1% (69th percentile)
Actively exploited: Not listed in CISA KEV
Affected product: Microsoft Lync Server
Published: 2021-05-11
Last modified: 2026-06-17

Description

Skype for Business and Lync Spoofing Vulnerability

Frequently asked questions

What is CVE-2021-26421?: Skype for Business and Lync Spoofing Vulnerability
How severe is CVE-2021-26421?: CVE-2021-26421 has a CVSS 3.x base score of 6.5, rated medium severity. It is exploitable over network with high attack complexity, requires low privileges and user interaction. Impact on confidentiality is high, integrity low, and availability none.
Is CVE-2021-26421 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (69th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-26421?: CVE-2021-26421 primarily affects Microsoft Lync Server. In total, 2 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.
How do I fix CVE-2021-26421?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
When was CVE-2021-26421 published?: CVE-2021-26421 was published on 2021-05-11 and last updated on 2026-06-17.

References

https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26421

Affected products (2)

cpe:2.3:a:microsoft:lync_server:2013:cumulative_update_10:*:*:*:*:*:*
cpe:2.3:a:microsoft:skype_for_business_server:2015:cumulative_update_11:*:*:*:*:*:*

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