Microsoft Exchange server being hacked through ProxyShell exploit –

An attacker has exploited a vulnerability in ProxyShell to aggressively exploit a Microsoft Exchange server and install a backdoor for later access.

ProxyShell is the name of an attack that uses three chained Microsoft Exchange vulnerabilities to perform unauthenticated remote code execution.

The three vulnerabilities listed below were discovered by the Devcore Principal Security Researcher. Orange TsaiChained them to take over the Microsoft Exchange server in April Pwn2Own2021 Hacking Contest..

Last week, Orange Tsai Black hat talk A recent Microsoft Exchange vulnerability he discovered when he targeted the attack surface of the Microsoft Exchange Client Access Service (CAS).

Tsai revealed that the ProxyShell exploit is using Microsoft Exchange’s AutoDiscover feature to perform SSRF attacks as part of its talk.

After watching the talk, security researchers Peter Json and Nguyen Jang Release More detailed technical information to successfully reproduce the ProxyShell exploit.

Shortly thereafter, a security researcher Kevin Beaumont Started watching threat actors Scan for vulnerable Microsoft Exchange servers against Proxy Shell..

Proxy Shell is aggressively abused to drop web shell

Today, Vulnerability Researchers in Beaumont and NCC Group Rich Warren It was revealed that an attacker used a vulnerability in ProxyShell to exploit a Microsoft Exchange honeypot.

Tweet from Rich Warren

Tweets from Kevin Beaumont

When exploiting Microsoft Exchange, an attacker uses an initial URL similar to the following:

https://Exchange-server/autodiscover/[email protected]/mapi/nspi/?&Email=autodiscover/autodiscover.json%[email protected]

Note: The email addresses listed in the URL do not need to exist and have not changed between attackers.

This exploit is currently dropping a 265KB size webshell into the “c: inetpub wwwroot aspnet_client ” folder.

Last week, Jang told Bleeping Computer that the minimum file size that could be created by exploiting the ProxyShell exploit was 265KB. Exchange mailbox export feature Powershell Create a PST file.

From the sample Warren shares with Bleeping Computer, the web shell consists of a simple, authenticated script that an attacker can use to upload files to a compromised Microsoft Exchange server.

According to Warren, the attacker used the first web shell to upload an additional web shell to a remotely accessible folder and then upload the following two executables to the C: Windows System32 folder: To do.

C:WindowsSystem32createhidetask.exe C:WindowsSystem32ApplicationUpdate.exe

If the two executables are not found, another web shell is created as a randomly named ASPX file in the following folder:

C:Program FilesMicrosoftExchange ServerV15FrontEndHttpProxyowaauth

The attacker uses a second web shell to launch “createhidetask.exe”. This creates a scheduled task named “PowerManager” and launches the “ApplicationUpdate.exe” executable file every day at 1am.

Warren told Bleeping Computer that the ApplicationUpdate.exe executable is a custom .NET loader used as a backdoor.

“ApplicationUpdate.exe is a .NET loader that fetches another .NET binary from a remote server (which currently provides a harmless payload),” Warren explains.

The current payload is harmless, but when a sufficient number of servers are compromised, it is expected to be replaced with a malicious payload.

Cyber ​​Security Intelligence Company Bad packet He told Bleeping Computer that he is currently seeing attackers scanning vulnerable ProxyShell devices from IP addresses in the United States, Iran, and the Netherlands.

The known addresses are:

  • / 24

BadPackets also stated that the email domains used in the scan were from @ and @, as shown below.

Bad packets that detect ProxyShell scans
Bad packets that detect ProxyShell scans

Because an attacker is aggressively exploiting a vulnerable Microsoft Exchange server, Beaumont advises administrators to run Azure Sentinel queries to see if the device is being scanned.

W3CIISLog | where csUriStem == "/autodiscover/autodiscover.json" | where csUriQuery has "PowerShell" | where csMethod == "POST"

If you haven’t updated your Microsoft Exchange server recently, we strongly recommend that you update it immediately.

As before ProxyLogon attack Led to Ransomware, Malware, and data theft on exposed servers can lead to similar attacks using ProxyShell.

Source of this news:

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