CVE-2021-1647 [Exploited] | Microsoft WINDOWS DEFENDER RCE

CVE-2021-1647 is rated Critical Active Threat (90.5/100): CVSS High severity, with high exploitation likelihood (EPSS 77.39%, 99th percentile). CISA KEV confirms active exploitation (added 2021-11-03) affecting Microsoft / Defender. Unauthenticated remote administrative access may be possible. The CISA remediation deadline has passed—treat as an emergency patch priority.

#	Date	Old EPSS score	New EPSS score	Delta (New - Old)
1	2026-03-11	77.75%	77.39%	-0.36%
2	2026-03-06	77.89%	77.75%	-0.14%
3	2026-02-07	—	77.89%	—

Date

Old EPSS score

New EPSS score

Delta (New - Old)

2026-03-11

77.75%

77.39%

-0.36%

2026-03-06

77.89%

77.75%

-0.14%

2026-02-07

—

77.89%

—

Base score	Version	Severity	Vector	Exploitability	Impact	Score source
7.8	3.1	HIGH	`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H` Click to expand Attack vector (AV:L) They already need access on the box, or another person has to do something wrong; it’s not a remote drive-by. 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:L) A normal user session is enough; they don’t have to be admin. User interaction (UI:N) Nobody has to click “OK” or open a trap file; it can work without a victim helping. 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:H) They could widely tamper with or forge data—trust in the data is badly hurt. Availability (A:H) Could take the service down hard or make it unusable for people who depend on it.	1.8	5.9	[email protected]
7.8	3.1	HIGH	`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H` Click to expand Attack vector (AV:L) They already need access on the box, or another person has to do something wrong; it’s not a remote drive-by. 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:L) A normal user session is enough; they don’t have to be admin. User interaction (UI:N) Nobody has to click “OK” or open a trap file; it can work without a victim helping. 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:H) They could widely tamper with or forge data—trust in the data is badly hurt. Availability (A:H) Could take the service down hard or make it unusable for people who depend on it.	1.8	5.9	[email protected]
7.2	2.0	HIGH	`AV:L/AC:L/Au:N/C:C/I:C/A:C` Click to expand Access vector (AV:L) Requires local access to the target system. Access complexity (AC:L) Exploitation conditions are straightforward and predictable. Authentication (AU:N) No authentication is required. Confidentiality impact (C:C) Complete confidentiality impact. Integrity impact (I:C) Complete integrity impact. Availability impact (A:C) Complete availability impact.	3.9	10.0	[email protected]

Base score

Version

Severity

Vector

Exploitability

Impact

Score source

7.8

3.1

HIGH

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

Attack vector (AV:L): They already need access on the box, or another person has to do something wrong; it’s not a remote drive-by.
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:L): A normal user session is enough; they don’t have to be admin.
User interaction (UI:N): Nobody has to click “OK” or open a trap file; it can work without a victim helping.
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:H): They could widely tamper with or forge data—trust in the data is badly hurt.
Availability (A:H): Could take the service down hard or make it unusable for people who depend on it.

1.8

5.9

[email protected]

7.8

3.1

HIGH

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

Attack vector (AV:L): They already need access on the box, or another person has to do something wrong; it’s not a remote drive-by.
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:L): A normal user session is enough; they don’t have to be admin.
User interaction (UI:N): Nobody has to click “OK” or open a trap file; it can work without a victim helping.
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:H): They could widely tamper with or forge data—trust in the data is badly hurt.
Availability (A:H): Could take the service down hard or make it unusable for people who depend on it.

1.8

5.9

[email protected]

7.2

2.0

HIGH

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

Access vector (AV:L): Requires local access to the target system.
Access complexity (AC:L): Exploitation conditions are straightforward and predictable.
Authentication (AU:N): No authentication is required.
Confidentiality impact (C:C): Complete confidentiality impact.
Integrity impact (I:C): Complete integrity impact.
Availability impact (A:C): Complete availability impact.

3.9

10.0

[email protected]

Affected software / configurations for CVE-2021-1647

Vendor	Product	Version	Raw CPE
microsoft	windows_defender	—	cpe:2.3:a:microsoft:windows_defender:-:::::::*
microsoft	security_essentials	—	cpe:2.3:a:microsoft:security_essentials:-:::::::*
microsoft	system_center_endpoint_protection	—	cpe:2.3:a:microsoft:system_center_endpoint_protection:-:::::::*
microsoft	system_center_endpoint_protection	2012	cpe:2.3:a:microsoft:system_center_endpoint_protection:2012:-::::::
microsoft	system_center_endpoint_protection	2012	cpe:2.3:a:microsoft:system_center_endpoint_protection:2012:r2::::::

References for CVE-2021-1647

URL	Tags
https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-1647	Patch Vendor Advisory
https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-1647	Patch Vendor Advisory
https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-1647	US Government Resource

URL

CVE-2021-1647 | Microsoft Defender Remote Code Execution Vulnerability

CISA KEV Record for CVE-2021-1647

Exploit prediction scoring system (EPSS) score for CVE-2021-1647

Common vulnerability scoring system (CVSS) metrics for CVE-2021-1647

Weakness enumeration for CVE-2021-1647

Affected software / configurations for CVE-2021-1647

References for CVE-2021-1647