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CVE-2021-26855: Microsoft Exchange Server Remote Code Execution Vulnerability

CVE-2021-26855 is rated critical with a CVSS v3.1 base score of 9.8 and affects Microsoft Exchange Server. Per its scoring, it can be exploited over the network, without authentication, without any user interaction. The scored impact on confidentiality, integrity and availability is the highest available. CISA added it to its Known Exploited Vulnerabilities catalog on 2021-11-03, which means exploitation in the wild has been observed.

Vulnerability description

Microsoft Exchange Server Remote Code Execution Vulnerability

Severity breakdown

CVSS v3.1 9.8: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Attack vector: Network. Exploitable remotely over the network, with no need for local or physical access.
Attack complexity: Low. No special conditions; the attack can be repeated reliably.
Privileges required: None. No account or prior access is needed.
User interaction: None. No action from any user is needed.
Scope: Unchanged. Impact stays within the vulnerable component.
Confidentiality impact: High. Total loss of confidentiality; sensitive data can be fully disclosed.
Integrity impact: High. Total loss of integrity; data or system state can be fully modified.
Availability impact: High. Total loss of availability; the component can be shut down.

Underlying weakness

CWE-918: Server-Side Request Forgery

The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination.

Affected products

Microsoft Exchange Server: 2013 cumulative_update_21; 2013 cumulative_update_22; 2013 cumulative_update_23; 2016 cumulative_update_10; 2016 cumulative_update_11; 2016 cumulative_update_12; 2016 cumulative_update_13; 2016 cumulative_update_14; 2016 cumulative_update_15; 2016 cumulative_update_16; 2016 cumulative_update_17; 2016 cumulative_update_18