On December 20, 2022, cybersecurity researchers uncovered a novel exploit method dubbed OWASSRF that involves chaining CVE-2022-41080 and CVE-2022-41082 vulnerabilities to gain RCE through privilege escalation via Outlook Web Access (OWA). OWASSRF is capable of bypassing ProxyNotShell mitigations. Cyber defenders highlight that these ongoing attacks pose a threat to an increasing number of Microsoft Exchange servers.
Microsoft Exchange zero-day vulnerabilities known as ProxyNotShell have been actively exploited in the wild since September 2022 making cyber defenders from across the globe stay alert to their potential impact. With the discovery of a new exploit method called OWASSRF that takes advantage of chaining CVE-2022-41080 and CVE-2022-41082 vulnerabilities and bypasses Microsoft mitigations for ProxyNotShell, defenders have to brace themselves for a new threat.
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A major headache for security practitioners during the holiday season has been uncovered by CrowdStrike researchers. The recent inquiry details a novel exploitation method enabling adversaries to compromise Microsoft Exchange servers for RCE. Dubbed OWASSRF, the malicious technique allows attackers to bypass URL rewrite mitigations introduced by Microsoft for ProxyNotShell and achieve RCE via privilege escalation via Outlook Web Access (OWA).
Initially, OWASSRF has been observed during the research of Play ransomware campaigns. The adversaries relied on affected Exchange servers to penetrate the targeted network. Researchers suspected attackers leveraged a typical Microsoft Exchange ProxyNotShell (CVE-2022-41040, CVE-2022-41082). However, the log data showed no signs of CVE-2022-41040 being exploited for the initial access. Instead, the requests were spotted directly through the OWA endpoint, revealing an unknown exploit chain for Exchange.
The investigation revealed that attackers relied on another vulnerability while leveraging the OWASSRF method. Particularly, hackers exploited the CVE-2022-41080 enabling remote privilege escalation on Exchange servers. The flaw has been reported to Microsoft and fixed in November 2022. Interestingly, this security gap has been considered critical but not exploited in the wild at that time.
On December 14, 2022, the proof-of-concept (PoC) exploit has been posted on the web by Dray Agha researcher along with another offensive toolkit. According to CrowdStrike, this PoC matched the exploit used in Play ransomware attacks being utilized to deliver remote access tools such as Plink and AnyDesk.
According to the latest report by Rapid7, security experts are observing a growing rate of Microsoft Exchange Servers compromised via OWASSRF exploit chain, including software versions 2013, 2016, and 2018. Rapid7 researchers note that Exchange servers leveraging Microsoft mitigations can be affected, while the patched servers seem to remain not vulnerable. To timely protect their infrastructure, exposed organizations are urged to leverage November 8, 2022 Patch Tuesday fix by Microsoft addressing CVE-2022-41082. In case immediate patching is not an option, vendors are recommended to disable OWA completely.
As additional mitigation measures, vendors should follow Microsoft recommendations to disable PowerShell for users without admin privileges, constantly monitor their Exchange servers for any signs of compromise, apply web application firewalls, and adopt best security practices to keep the cyber hygiene.
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