Vulnerability identifier: #VU51171

Vulnerability risk: Critical

CVSSv3.1: 9.5 [CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H/E:H/RL:O/RC:C]

CVE-ID: CVE-2021-26855

CWE-ID: CWE-918

Exploitation vector: Network

Exploit availability: Yes

Vulnerable software:
Microsoft Exchange Server
Server applications / Mail servers

Vendor: Microsoft

Description
The vulnerability allows a remote attacker to execute arbitrary code on the system.

The vulnerability exists due to insufficient validation of user-supplied input. A remote attacker can send specially crafted HTTP request to the Microsoft Exchange OWA interface, upload arbitrary file on the server and execute it.

Note, this vulnerability is being actively exploited in the wild.

Mitigation
Install updates from vendor's website.

Vulnerable software versions

Microsoft Exchange Server: 2013 RTM 15.00.0516.032 - 2013 Cumulative Update 23 15.00.1497.002, 2019 RTM 15.02.0221.012 - 2019 Cumulative Update 8 15.02.0792.003, 2016 RTM 15.01.0225.042 - 2016 Cumulative Update 19 15.01.2176.002

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External links
http://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26855
http://msrc-blog.microsoft.com/2021/03/02/multiple-security-updates-released-for-exchange-server
http://github.com/microsoft/CSS-Exchange/tree/main/Security

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Q & A

Can this vulnerability be exploited remotely?

Yes. This vulnerability can be exploited by a remote non-authenticated attacker via the Internet.

Is there known malware, which exploits this vulnerability?

Yes. This vulnerability is being exploited in the wild.