CVE-2021-33766 [Exploited] | Microsoft EXCHANGE SERVER Information Disclosure

CVE-2021-33766 is rated Critical Active Threat (89.3/100): CVSS High severity, with high exploitation likelihood (EPSS 93.75%, 100th percentile). CISA KEV confirms active exploitation (added 2022-01-18) 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	2026-05-26	93.61%	93.75%	+0.14%
2	2026-03-29	93.75%	93.61%	-0.14%
3	2026-03-21	—	93.75%	—

Date

Old EPSS score

New EPSS score

Delta (New - Old)

2026-05-26

93.61%

93.75%

+0.14%

2026-03-29

93.75%

93.61%

-0.14%

2026-03-21

—

93.75%

—

Base score	Version	Severity	Vector	Exploitability	Impact	Score source
7.3	3.1	HIGH	`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L` 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: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:L) Some sensitive info could get out, but not a total data dump. Integrity (I:L) Attackers could change some data, but it’s limited—not everything goes. Availability (A:L) Might cause slowdowns, glitches, or partial disruption—not a full brick.	3.9	3.4	[email protected]
7.5	3.1	HIGH	`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/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: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:N) Data isn’t meaningfully altered or forged. Availability (A:N) Service keeps running; no real outage angle.	3.9	3.6	[email protected]
5.0	2.0	MEDIUM	`AV:N/AC:L/Au:N/C:P/I:N/A:N` Click to expand Access vector (AV:N) Can be exploited remotely over network reachability. Access complexity (AC:L) Exploitation conditions are straightforward and predictable. Authentication (AU:N) No authentication is required. Confidentiality impact (C:P) Partial confidentiality impact. Integrity impact (I:N) No integrity impact. Availability impact (A:N) No availability impact.	10.0	2.9	[email protected]

Base score

Version

Severity

Vector

Exploitability

Impact

Score source

7.3

3.1

HIGH

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L 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: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:L): Some sensitive info could get out, but not a total data dump.
Integrity (I:L): Attackers could change some data, but it’s limited—not everything goes.
Availability (A:L): Might cause slowdowns, glitches, or partial disruption—not a full brick.

3.9

3.4

[email protected]

7.5

3.1

HIGH

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/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: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:N): Data isn’t meaningfully altered or forged.
Availability (A:N): Service keeps running; no real outage angle.

3.9

3.6

[email protected]

5.0

2.0

MEDIUM

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

Access vector (AV:N): Can be exploited remotely over network reachability.
Access complexity (AC:L): Exploitation conditions are straightforward and predictable.
Authentication (AU:N): No authentication is required.
Confidentiality impact (C:P): Partial confidentiality impact.
Integrity impact (I:N): No integrity impact.
Availability impact (A:N): No availability impact.

10.0

2.9

[email protected]

Affected software / configurations for CVE-2021-33766

Vendor	Product	Version	Raw CPE
microsoft	exchange_server	2013	cpe:2.3:a:microsoft:exchange_server:2013:cumulative_update_23::::::
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_20::::::
microsoft	exchange_server	2019	cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_8::::::
microsoft	exchange_server	2019	cpe:2.3:a:microsoft:exchange_server:2019:cumulative_update_9::::::

References for CVE-2021-33766

URL	Tags
https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-33766	Patch Vendor Advisory
https://www.zerodayinitiative.com/advisories/ZDI-21-798/	Third Party Advisory VDB Entry
https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-33766	US Government Resource

URL

CVE-2021-33766 | Microsoft Exchange Server Information Disclosure Vulnerability

CISA KEV Record for CVE-2021-33766

Exploit prediction scoring system (EPSS) score for CVE-2021-33766

Common vulnerability scoring system (CVSS) metrics for CVE-2021-33766

Weakness enumeration for CVE-2021-33766

Affected software / configurations for CVE-2021-33766

References for CVE-2021-33766