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Vulnerable Application

CVE-2021-28855 is a pre-authentication SSRF (Server Side Request Forgery) which allows an attacker to bypass authentication by sending specially crafted HTTP requests. This vulnerability is part of an attack chain used to perform an RCE (Remote Code Execution).

This vulnerability affects (Exchange 2013 Versions < 15.00.1497.012, Exchange 2016 CU18 < 15.01.2106.013, Exchange 2016 CU19 < 15.01.2176.009, Exchange 2019 CU7 < 15.02.0721.013, Exchange 2019 CU8 < 15.02.0792.010).

Introduction

An issue was discovered in Microsoft Exchange Server that allows an attacker bypassing the authentication and impersonating as the admin (CVE-2021-26855). By chaining this bug with another post-auth arbitrary-file-write vulnerability to get code execution (CVE-2021-27065).

As a result, an unauthenticated attacker can execute arbitrary commands on Microsoft Exchange Server.

All components are vulnerable by default.

Verification Steps

1. Start msfconsole

2. Do: use auxiliary/scanner/http/exchange_proxylogon

3. Do: set RHOSTS [IP]

4. Do: run

Options

METHOD

HTTP Method to use for the check (only). Default: POST

Scenarios

msf6 auxiliary(scanner/http/exchange_proxylogon) > options 

Module options (auxiliary/scanner/http/exchange_proxylogon):

   Name     Current Setting      Required  Description
   ----     ---------------      --------  -----------
   METHOD   POST                 yes       HTTP Method to use for the check. (Accepted: GET, POST)
   Proxies                       no        A proxy chain of format type:host:port[,type:host:port][...]
   RHOSTS   172.20.2.110         yes       The target host(s), range CIDR identifier, or hosts file with syntax 'file:<path>'
   RPORT    443                  yes       The target port (TCP)
   SSL      true                 no        Negotiate SSL/TLS for outgoing connections
   THREADS  1                    yes       The number of concurrent threads (max one per host)
   VHOST                         no        HTTP server virtual host

msf6 auxiliary(scanner/http/exchange_proxylogon) > run

[+] https://172.20.2.110:443 - The target is vulnerable to CVE-2021-26855.
[*] Scanned 1 of 1 hosts (100% complete)
[*] Auxiliary module execution completed
msf6 auxiliary(scanner/http/exchange_proxylogon) > 

References

1. https://proxylogon.com/

2. https://raw.githubusercontent.com/microsoft/CSS-Exchange/main/Security/http-vuln-cve2021-26855.nse

3. https://aka.ms/exchangevulns