Pierluigi Paganini
March 16, 2021
On March 2nd, Microsoft released emergency out-of-band security updates that address four zero-day issues (CVE-2021-26855, CVE-2021-26857, CVE-2021-26858, and CVE-2021-27065) in all supported MS Exchange versions that are actively exploited in the wild.
The IT giant reported that at least one China linked APT group, tracked as HAFNIUM, chained these vulnerabilities to access on-premises Exchange servers to access email accounts, and install backdoors to maintain access to victim environments.
Immediately after the disclosure of the flaws, some PoC exploits were shared online causing a spike in the number of the attacks. Check Point Research team reported that that in a time span of 24 hours the exploitation attempts are doubling every two hours.
Most of the exploit attempts targeted organizations in Turkey (19%), followed by United States (18%) and Italy (10%). Most targeted sectors have been Government/Military (17% of all exploit attempts), followed by Manufacturing (14%), and then Banking (11%).
Microsoft researchers also spotted a ransomware gangs that is exploiting ProxyLogon flaws to spread a piece of malware tracked as DearCry.
Now Microsoft has released an Exchange On-premises Mitigation Tool (EOMT) tool to allow small businesses to quickly address the vulnerabilities exploited in the recent attacks.
The EOMT tool is a one-click PowerShell script that allows organizations that do not have dedicated security or IT teams to apply the security updates to address the CVE-2021-26855 flaws.
“Microsoft has released a new, one-click mitigation tool, Microsoft Exchange On-Premises Mitigation Tool to help customers who do not have dedicated security or IT teams to apply these security updates. We have tested this tool across Exchange Server 2013, 2016, and 2019 deployments.” reads the post published by Microsoft. “This new tool is designed as an interim mitigation for customers who are unfamiliar with the patch/update process or who have not yet applied the on-premises Exchange security update.”
Microsoft released the tool to provide an easy to use, automated solution to fix the flaws on both current and out-of-support versions of on-premises Exchange Server, anyway the company pointed out that the EOMT tool is not a replacement for the Exchange security update.
Microsoft released the ‘EOMT.ps1‘ script on its GitHub repository, upon executing it, the tool will perform the following operations:
Microsoft provides the following system requirements:
and suggests running the Exchange On-premises mitigation tool in the following situations:
| Situation | Guidance |
|---|---|
| If you have done nothing to date to patch or mitigate this issue… | Run EOMT.PS1 as soon as possible.This will both attempt to remediate as well as mitigate your servers against further attacks. Once complete, follow patching guidance to update your servers on http://aka.ms/exchangevulns |
| If you have mitigated using any/all of the mitigation guidance Microsoft has given (Exchangemitigations.Ps1, Blog post, etc..) | Run EOMT.PS1 as soon as possible. This will both attempt to remediate as well as mitigate your servers against further attacks. Once complete, follow patching guidance to update your servers on http://aka.ms/exchangevulns |
| If you have already patched your systems and are protected, but did NOT investigate for any adversary activity, indicators of compromise, etc…. | Run EOMT.PS1 as soon as possible. This will attempt to remediate any existing compromise that may not have been full remediated before patching. |
| If you have already patched and investigated your systems for any indicators of compromise, etc…. | No action is required |
The experts pointed out that the Exchange On-premises Mitigation Tool runs the Microsoft Safety Scanner in a quick scan mode. If the administrators suspect that their installs have been compromised, the company recommends them to run the EOMT in the FULL SCAN mode.
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(SecurityAffairs – hacking, Microsoft Exchange)
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