Microsoft releases On-premises Mitigation Tool (EOMT) tool to fix ProxyLogon issues
Vulnerabilities mentionedAll →
| CVE | Vulnerability | CVSS | EPSS | Flags | Affected | Exposure | Published |
|---|---|---|---|---|---|---|---|
| CVE-2021-26855 | Unauthenticated SSRF/RCE in Microsoft Exchange Server (ProxyLogon) CVE-2021-26855 is a server-side request forgery flaw (CWE-918) in Microsoft Exchange Server that allows an unauthenticated remote attacker to send specially crafted HTTP requests and have the Exchange server process them as itself, disclosing sensitive session information. When chained with sibling Exchange flaws (the 'ProxyLogon' chain), it yields authentication bypass and arbitrary file write, escalating to full remote code execution with SYSTEM-level privileges on the on-premises Exchange server. Any organization running an affected on-premises Exchange server reachable over HTTP/HTTPS (typically outbound webmail) is exposed; Exchange Online was not affected. Exploitation is confirmed in the wild at large scale: the flaw was mass-exploited beginning in early 2021 (notably by the HAFNIUM group), is on the CISA KEV with documented ransomware use, and has a maximum EPSS score of 100% (100th percentile), despite no public PoC listing. Do: Apply the vendor's March 2021 Exchange security updates (or later cumulative updates) immediately, per the CISA required action; until patched, limit Exchange (ECP/OWA) exposure to the internet via firewall/VPN rules. Hunt for compromise: review IIS logs for unrecognized authenticated activity against FrontEnd HttpProxy endpoints, and check for malicious files or webshells under inetpub\wwwroot\aspnet_client, given the known ransomware use. | 9.1 group max | 100% | KEV ransomware PoC ×4 |
| masshundreds of thousands of on-premises deployments; tens of thousands of internet-exposed Exchange servers |
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Microsoft released an Exchange On-premises Mitigation Tool (EOMT) tool to small businesses for the fix of ProxyLogon vulnerabilities.
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:
- Mitigate against current known attacks using CVE-2021-26855 via a URL Rewrite configuration
- Scan the Exchange Server using the Microsoft Safety Scanner
- Attempt to remediate compromises detected by the Microsoft Safety Scanner.
Microsoft provides the following system requirements:
- PowerShell 3 or later
- IIS 7.5 and later
- Exchange 2013, 2016, or 2019
- Windows Server 2008 R2, Server 2012, Server 2012 R2, Server 2016, Server 2019
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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Text extracted automatically; images, tables and formatting may be missing. Original: https://securityaffairs.com/115648/security/eomt-tool-microsoft-exchange.html