OWASSRF – New Exploit Let Attacker Execute Remote Code on Microsoft Exchange Server

There is a new exploit chain dubbed, OWASSRF that threat actors are actively exploiting to gain arbitrary code execution through Outlook Web Access (OWA) on vulnerable servers that bypasses ProxyNotShell URL rewrite mitigations.

A recent investigation by CrowdStrike Services found that Microsoft Exchange ProxyNotShell vulnerabilities are probably enabled the common entry vector for several Play ransomware intrusions:-

• CVE-2022-41040
• CVE-2022-41082

The relevant logs were reviewed by CrowdStrike and no evidence of initial access exploiting CVE-2022-41040 was found. 

ProxyNotShell and Exchange Architecture Primer

There are two major components that make up a Microsoft Exchange server:- 

• The frontend
• The backend

All client connections are handled by the frontend, which proxies any given request to the backend according to the request. Here in this scenario the specific requests made to the frontend, like URLs, are dealt with by backend services.

A ProxyNotShell attack targets the Remote PowerShell service, which in this case is the backend service that is targeted. It is known this kind of vulnerability is referred to as an SSRF (Server-Side Request Forgery) vulnerability.

The CVE-2022-41082 vulnerability has been exploited by ransomware operators in order to execute arbitrary commands on compromised servers using Remote PowerShell.

OWASSRF PoC exploit leak

POC code for an exploit method based on Play ransomware logging was currently under development by CrowdStrike security researchers.

A recent discovery has shown that an attacker has downloaded all of the tools from an open repository, uploaded them in a MegaUpload link, and made them accessible to the public via the Twitter site.

CrowdStrike researchers replicated the log files generated in recent Play ransomware attacks using a Python script from the leaked toolkit, named poc.py, that was included in the leaked toolkit.

Recommendations from CrowdStrike

Here below we have mentioned all the recommendations offered by CrowdStrike:-

• Since the URL rewrite mitigations for ProxyNotShell do not function against this exploit method, organizations should apply the Exchange patches of November 8, 2022.
• The KB5019758 patch should be applied as soon as possible, but if this cannot be done, then you should disable OWA until you are able to apply it.
• Make sure to disable remote PowerShell for non-administrative users in accordance with Microsoft’s recommendations.
• Implement the use of advanced endpoint detection and response (EDR) solutions on each and every endpoint.
• Utilize the script developed by CrowdStrike Services to check for signs of exploitation on Exchange servers visible in IIS logs and Remote PowerShell logs.
• Take into account application-level controls, such as firewalls for web applications, as well as system-level controls.
• Ensure that the X-Forwarded-For header in the HTTP request has been configured to log the true IP address of the external proxy server.

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