WARNING: New Unpatched Microsoft Exchange Zero-Day Under Active Exploitation

Security researchers are warning of previously undisclosed flaws
in fully patched Microsoft Exchange servers being exploited by
malicious actors in real-world attacks to achieve remote code
execution on affected systems.

That’s according to Vietnamese cybersecurity company GTSC, which
discovered the shortcomings as part of its security monitoring and
incident response efforts in August 2022.

The two vulnerabilities, which are formally yet to be assigned
CVE identifiers, are being tracked^[1]
by the Zero Day Initiative as ZDI-CAN-18333 (CVSS
score: 8.8) and ZDI-CAN-18802 (CVSS score:
6.3).

GTSC said that successful exploitation of the flaws could be
abused to gain a foothold in the victim’s systems, enabling
adversaries to drop web shells and carry out lateral movements
across the compromised network.

“We detected webshells, mostly obfuscated, being dropped to
Exchange servers,” the company noted^[2]. “Using the user-agent,
we detected that the attacker uses Antsword, an active
Chinese-based open source cross-platform website administration
tool that supports web shell management.”

Exploitation requests in IIS logs are said to appear in the same
format as the https://thehackernews.com/2021/11/hackers-exploiting-proxylogon-and.html^[3]
Exchange Server vulnerabilities, with GTSC noting that the targeted
servers had already been patched against the flaws that came to
light in March 2021.

The cybersecurity company theorized that the attacks are likely
originating from a Chinese hacking group owing to the web shell’s
encoding in simplified Chinese (Windows Code page 936^[4]).

Also deployed in the attacks is the China Chopper web shell, a
lightweight backdoor that can grant persistent remote access and
allow attackers to reconnect at any time for further
exploitation.

It’s worth noting that the China Chopper web shell^[5]
was also deployed by Hafnium, a suspected state-sponsored group
operating out of China, when the ProxyShell vulnerabilities were
subjected to widespread exploitation^[6]
last year.

Further post-exploitation activities observed by GTSC involve
the injection of malicious DLLs into memory, drop and execute
additional payloads on the infected servers using the WMI
command-line (WMIC^[7]) utility.

The company said at least more than one organization has been
the victim of an attack campaign leveraging the zero-day flaws.
Additional details about the bugs have been withheld in light of
active exploitation.

We have reached out to Microsoft for further comment, and we
will update the story if we hear back.

In the interim, as temporary workarounds, it’s recommended to
add a rule to block requests with indicators of compromise using
the URL Rewrite Rule module^[8]
for IIS servers –

• In Autodiscover at FrontEnd, select tab URL Rewrite, and then
  select Request Blocking
• Add string “.*autodiscover\.json.*\@.*Powershell.*” to the URL
  Path, and
• Condition input: Choose {REQUEST_URI}

“I can confirm significant numbers of Exchange servers have been
backdoored – including a honeypot,” Security researcher Kevin
Beaumont said in a series of tweets, adding, “it looks like a
variant of proxying to the admin interface again.”

“If you don’t run Microsoft Exchange on premise, and don’t have
Outlook Web App facing the internet, you are not impacted,”
Beaumont said^[9].