CVE-2021-26858 [Exploited] | Microsoft EXCHANGE SERVER RCE

CVE-2021-26858 is rated Critical Active Threat (85.9/100): CVSS High severity, with high exploitation likelihood (EPSS 73.24%, 99th percentile). CISA KEV confirms active exploitation (added 2021-11-03) affecting Microsoft / Exchange Server. 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	2026-05-29	78.11%	73.24%	-4.86%
2	2026-05-26	80.17%	78.11%	-2.06%
3	2026-05-25	—	80.17%	—

Date

Old EPSS score

New EPSS score

Delta (New - Old)

2026-05-29

78.11%

73.24%

-4.86%

2026-05-26

80.17%

78.11%

-2.06%

2026-05-25

—

80.17%

—

Base score	Version	Severity	Vector	Exploitability	Impact	Score source
7.8	3.1	HIGH	`CVSS:3.1/AV:L/AC:L/PR:N/UI:R/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: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: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:N/UI:R/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: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: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]
6.8	2.0	MEDIUM	`AV:N/AC:M/Au:N/C:P/I:P/A:P` 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:P) Partial availability impact.	8.6	6.4	[email protected]

Base score

Version

Severity

Vector

Exploitability

Impact

Score source

7.8

3.1

HIGH

CVSS:3.1/AV:L/AC:L/PR:N/UI:R/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: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: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:N/UI:R/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: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: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]

6.8

2.0

MEDIUM

AV:N/AC:M/Au:N/C:P/I:P/A:P 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:P): Partial availability impact.

8.6

6.4

[email protected]

Affected software / configurations for CVE-2021-26858

Vendor	Product	Version	Raw CPE
microsoft	exchange_server	2010	cpe:2.3:a:microsoft:exchange_server:2010:sp3::::::
microsoft	exchange_server	2013	cpe:2.3:a:microsoft:exchange_server:2013:cumulative_update_22::::::
microsoft	exchange_server	2013	cpe:2.3:a:microsoft:exchange_server:2013:cumulative_update_23::::::
microsoft	exchange_server	2013	cpe:2.3:a:microsoft:exchange_server:2013:sp1::::::
microsoft	exchange_server	2016	cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_10::::::
microsoft	exchange_server	2016	cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_11::::::
microsoft	exchange_server	2016	cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_12::::::
microsoft	exchange_server	2016	cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_13::::::
microsoft	exchange_server	2016	cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_14::::::
microsoft	exchange_server	2016	cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_15::::::
microsoft	exchange_server	2016	cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_16::::::
microsoft	exchange_server	2016	cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_17::::::
microsoft	exchange_server	2016	cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_18::::::
microsoft	exchange_server	2016	cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_19::::::
microsoft	exchange_server	2016	cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_8::::::
microsoft	exchange_server	2016	cpe:2.3:a:microsoft:exchange_server:2016:cumulative_update_9::::::
microsoft	exchange_server	2019	cpe:2.3:a:microsoft:exchange_server:2019:-::::::
microsoft	exchange_server	2019	cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_1::::::
microsoft	exchange_server	2019	cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_2::::::
microsoft	exchange_server	2019	cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_3::::::
microsoft	exchange_server	2019	cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_4::::::
microsoft	exchange_server	2019	cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_5::::::
microsoft	exchange_server	2019	cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_6::::::
microsoft	exchange_server	2019	cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_7::::::
microsoft	exchange_server	2019	cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_8::::::

References for CVE-2021-26858

URL	Tags
https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26858	Patch Vendor Advisory
https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-26858	US Government Resource

URL

CVE-2021-26858 | Microsoft Exchange Server Remote Code Execution Vulnerability

CISA KEV Record for CVE-2021-26858

Exploit prediction scoring system (EPSS) score for CVE-2021-26858

Common vulnerability scoring system (CVSS) metrics for CVE-2021-26858

Weakness enumeration for CVE-2021-26858

Affected software / configurations for CVE-2021-26858

References for CVE-2021-26858