Guard against Microsoft Exchange Server Remote Code Execution Vulnerability

Zyxel_Forum_Admin

CVE-2021-26855

Affected Package: Microsoft Exchange 2013, 2016 and 2019

This vulnerability is part of an attack chain. The initial attack requires the ability to make an untrusted connection to Exchange server port 443. This can be protected against by restricting untrusted connections, or by setting up a VPN to separate the Exchange OWA service from external access. Using this mitigation will only protect against the initial portion of the attack. Other portions of the chain can be triggered if an attacker already has access or can convince an administrator to open a malicious file.

Impact:

An unauthenticated, remote attacker could exploit this flaw by sending a specially crafted HTTP request to a vulnerable Exchange Server. The vulnerability exploits the Exchange Control Panel (ECP) via a Server-Side Request Forgery (SSRF). This would also allow the attacker to gain access to mailboxes and read sensitive information.

Mitigation (On Host Device):

1. The initial attack requires the ability to make an untrusted connection to Exchange server port 443. This can be protected against by restricting untrusted connections, or by setting up a VPN to separate the Exchange OWA service from external access.

2. Microsoft also recommend prioritizing installing updates on Exchange Servers that are externally facing. All affected Exchange Servers should ultimately be updated.

3. For more details, please refer to Released: March 2021 Exchange Server Security Updates

Mitigation (On Network):

1. Leveraging the VPN technology for remote access to Exchange OWA services, prevents unauthorized outside access.

2. For Zyxel ATP series firewall, you can use its IP Reputation security feature to block the malicious IPs.
   

Revision history

2021-03-10: Initial release 

gb5102

When will you add IDP signatures to block this?