CVE-2021-26421 [Medium] | Spoofing in Lync Server

CVE-2021-26421 is rated Moderate Risk (49.9/100): CVSS Medium severity, with medium exploitation likelihood (EPSS 1.41%). Review affected assets and schedule remediation.

#	Date	Old EPSS score	New EPSS score	Delta (New - Old)
1	2026-06-15	1.67%	1.41%	-0.26%
2	2026-02-08	0.78%	1.67%	+0.89%
3	2025-11-21	—	0.78%	—

Date

Old EPSS score

New EPSS score

Delta (New - Old)

2026-06-15

1.67%

1.41%

-0.26%

2026-02-08

0.78%

1.67%

+0.89%

2025-11-21

—

0.78%

—

Base score	Version	Severity	Vector	Exploitability	Impact	Score source
6.5	3.1	MEDIUM	`CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:C/C:H/I:L/A:N` Click to expand Attack vector (AV:N) Could be attacked over the internet or any normal routed network—not just someone sitting at the machine. Attack complexity (AC:H) Even with access, the exploit needs extra luck, timing, or a fussy environment to actually work. Privileges required (PR:L) A normal user session is enough; they don’t have to be admin. User interaction (UI:R) A real person has to do something—click, install, enable—otherwise it doesn’t land. Scope (S:C) Breaking this can reach past the original component and bite other resources—bigger blast radius. Confidentiality (C:H) Serious risk that confidential data gets exposed in a big way. Integrity (I:L) Attackers could change some data, but it’s limited—not everything goes. Availability (A:N) Service keeps running; no real outage angle.	1.3	4.7	[email protected]
7.1	3.1	HIGH	`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:L/A:N` Click to expand Attack vector (AV:N) Could be attacked over the internet or any normal routed network—not just someone sitting at the machine. Attack complexity (AC:L) Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup. Privileges required (PR:N) No account or special rights needed—anonymous or random user is enough. User interaction (UI:R) A real person has to do something—click, install, enable—otherwise it doesn’t land. Scope (S:U) Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems. Confidentiality (C:H) Serious risk that confidential data gets exposed in a big way. Integrity (I:L) Attackers could change some data, but it’s limited—not everything goes. Availability (A:N) Service keeps running; no real outage angle.	2.8	4.2	[email protected]
5.8	2.0	MEDIUM	`AV:N/AC:M/Au:N/C:P/I:P/A:N` Click to expand Access vector (AV:N) Can be exploited remotely over network reachability. Access complexity (AC:M) Exploitation needs some favorable conditions, but not exceptional ones. Authentication (AU:N) No authentication is required. Confidentiality impact (C:P) Partial confidentiality impact. Integrity impact (I:P) Partial integrity impact. Availability impact (A:N) No availability impact.	8.6	4.9	[email protected]

Base score

Version

Severity

Vector

Exploitability

Impact

Score source

6.5

3.1

MEDIUM

CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:C/C:H/I:L/A:N Click to expand

Attack vector (AV:N): Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:H): Even with access, the exploit needs extra luck, timing, or a fussy environment to actually work.
Privileges required (PR:L): A normal user session is enough; they don’t have to be admin.
User interaction (UI:R): A real person has to do something—click, install, enable—otherwise it doesn’t land.
Scope (S:C): Breaking this can reach past the original component and bite other resources—bigger blast radius.
Confidentiality (C:H): Serious risk that confidential data gets exposed in a big way.
Integrity (I:L): Attackers could change some data, but it’s limited—not everything goes.
Availability (A:N): Service keeps running; no real outage angle.

1.3

4.7

[email protected]

7.1

3.1

HIGH

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

Attack vector (AV:N): Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L): Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N): No account or special rights needed—anonymous or random user is enough.
User interaction (UI:R): A real person has to do something—click, install, enable—otherwise it doesn’t land.
Scope (S:U): Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:H): Serious risk that confidential data gets exposed in a big way.
Integrity (I:L): Attackers could change some data, but it’s limited—not everything goes.
Availability (A:N): Service keeps running; no real outage angle.

2.8

4.2

[email protected]

5.8

2.0

MEDIUM

AV:N/AC:M/Au:N/C:P/I:P/A:N Click to expand

Access vector (AV:N): Can be exploited remotely over network reachability.
Access complexity (AC:M): Exploitation needs some favorable conditions, but not exceptional ones.
Authentication (AU:N): No authentication is required.
Confidentiality impact (C:P): Partial confidentiality impact.
Integrity impact (I:P): Partial integrity impact.
Availability impact (A:N): No availability impact.

8.6

4.9

[email protected]

Affected software / configurations for CVE-2021-26421

Vendor	Product	Version	Raw CPE
microsoft	lync_server	2013	cpe:2.3:a:microsoft:lync_server:2013:cumulative_update_10::::::
microsoft	skype_for_business_server	2015	cpe:2.3:a:microsoft:skype_for_business_server:2015:cumulative_update_11::::::

References for CVE-2021-26421

URL	Tags
https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26421	Patch Vendor Advisory

URL

CVE-2021-26421 | Skype for Business and Lync Spoofing Vulnerability

Exploit prediction scoring system (EPSS) score for CVE-2021-26421

Common vulnerability scoring system (CVSS) metrics for CVE-2021-26421

Weakness enumeration for CVE-2021-26421

Affected software / configurations for CVE-2021-26421

References for CVE-2021-26421