Microsoft Exchange Server Authentication Bypass

critical Nessus Plugin ID 147171
New! Plugin Severity Now Using CVSS v3

The calculated severity for Plugins has been updated to use CVSS v3 by default. Plugins that do not have a CVSS v3 score will fall back to CVSS v2 for calculating severity. Severity display preferences can be toggled in the settings dropdown.


The remote mail server is affected by an authentication bypass vulnerability.


The Microsoft Exchange running on the remote host is affected by an authentication bypass vulnerability. An unauthenticated remote attacker can exploit this to execute arbitrary code.


Microsoft has released the following security updates to address this issue:

See Also

Plugin Details

Severity: Critical

ID: 147171

File Name: exchange_cve-2021-26855.nbin

Version: 1.17

Type: remote

Agent: windows

Family: Windows

Published: 3/8/2021

Updated: 12/20/2021

Dependencies: exchange_detect.nbin

Risk Information

CVSS Score Source: CVE-2021-26855


Risk Factor: Critical

Score: 9.9


Risk Factor: High

Base Score: 7.5

Temporal Score: 6.5

Vector: AV:N/AC:L/Au:N/C:P/I:P/A:P

Temporal Vector: E:H/RL:OF/RC:C


Risk Factor: Critical

Base Score: 9.8

Temporal Score: 9.4

Vector: CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Temporal Vector: E:H/RL:O/RC:C

Vulnerability Information

CPE: cpe:/a:microsoft:exchange_server

Required KB Items: installed_sw/Exchange Server

Exploit Available: true

Exploit Ease: Exploits are available

Patch Publication Date: 3/2/2021

Vulnerability Publication Date: 3/2/2021

Exploitable With

Metasploit (Microsoft Exchange ProxyLogon RCE)

Reference Information

CVE: CVE-2021-26855

MSKB: 5000871

MSFT: MS21-5000871