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In TRU Positives, eSentire’s Threat Response Unit (TRU) provides a summary of a recent threat investigation. We outline how we responded to the confirmed threat and what recommendations we have going forward.
CVE-2023–27350 (impacts version PaperCut MF/NG 8.0 and below) allows an unauthenticated attacker to get Remote Code Execution (RCE) on a PaperCut Application Server.
CVE-2023–27351 (impacts PaperCut NG MF/NG 15.0 and below) allows an unauthenticated attacker to potentially pull user information, including usernames, full names, email addresses, office/department info, and any proximity card numbers assigned to the user.
Although both vulnerabilities were originally reported to PaperCut in January 2023 and patched in March 2023, they have been widely exploited in April 2023.
PaperCut NG/MF allows network administrators to manage print jobs, specifically who can print and whether the user has enough allocated credits. This requirement is typically found in educational institutions where students are allotted a set number of credits for print jobs.
PaperCut NG/MF uses an application server to intercept and process print jobs. It also provides an Admin web interface for administrators to remotely configure and manage the system.
In a typical scenario, an attacker could exploit CVE-2023–27350 by visiting the “SetupCompleted” page on the PaperCut MF/NG web interface (described here) to bypass authentication and log in as an administrator. Once authenticated, command execution can be achieved using print scripts via the web interface.
Exploitation of CVE-2023–27350 by Cryptocurrency Miner Botnet
On April 22, eSentire MDR for Endpoint alerted our SOC of unusual PowerShell activity on the server of a customer in the education sector.
A subsequent investigation confirmed this activity was tied to exploitation of CVE-2023–27350 against the customer’s public-facing PaperCut web interface.
Figure 2 Endpoint process-view of PaperCut exploitation
The coinminer deployment followed standard tradecraft for this class of threat. The first stage batch file me1.bat (MD5: a3ffb336aee9f01275c92ac529c8f70e) kills existing mining processes and persistence mechanisms and attempts to disable Windows Defender.
Figure 3 Snippet of batch file me1.bat.
The batch file then retrieves and executes a second batch file me2.bat (MD5: 5d32f0eee7adf20e0766d5481a1953a5).
Figure 4 Final command in me1.bat retrieves second stage.
The second batch file handles installing XMRIG miner on the system. Me2.bat is a slightly modified version of https://github.com/MoneroOcean/xmrig_setup/blob/master/setup_moneroocean_miner.bat, where xmrig.exe is renamed to dom.exe and written to %USERPROFILE%\dom. The miner uses key wallet id 43DTEF92be6XcPj5Z7U96g4oGeebUxkFq9wyHcNTe1otM2hUrfvdswGdLHxabCSTio7apowzJJVwBZw6vVTu7NoNCNAMoZ4 and retrieves a zip package from http://50.19.48[.]59. This package contains the necessary binaries and configuration files for running the Monero miner.
Figure 5 Zip package retrieved by miner setup script.
Non-Sucking Service Manager, renamed to dsm.exe is used to create a service for the miner binary.
Figure 6 Creation of the dom_miner service using NSSM (dsm.exe).
There are several references to “mimu” within the setup file. For example the script prepends the string “mimu.” to the hostname and configures the miner pool to update.mimu-me[.]cyou:80. Interesting this domain resolves to 106.251.252.226, an IP previously seen in association with exploitation of Atlassian Confluence Vulnerability CVE-2022-26134 to deploy coin miners.
Figure 7 Password generation function. "Mimu" is prepended to the system's hostname.Figure 8 Dynamic updates to the miner's configuration file.
“Mimu” is likely a reference to the Mimu/Mimo Miner botnet. The wallet id mentioned above and overall deployment behavior aligns with community reporting from previous critical vulnerabilities in Log4J.
Other public mentions of this recent campaign suggest the operators behind this botnet attempted to widely exploit CVE-2023–27350 around mid-to-late April 2023.
What did we do?
eSentire Threat Response Unit (TRU) initiated threat hunts for unusual activity under the pc-app.exe process and developed new detections and investigation runbooks for eSentire MDR for Endpoint and MDR for Network.
We added known malicious IP addresses to the eSentire Global Deny List.
We also released a security advisory addressing the risk of exploitation for our customers and provided guidance to mitigate the threat.
What can you learn from this TRU positive?
CVE-2023–27350 exploitation followed a similar pattern seen with prior RCE vulnerabilities where threat actors aimed to capitalize on patching gaps and recruit vulnerable servers into cryptocurrency mining botnets or deploy backdoors for future access.
Initial exploitation is typically opportunistic, and backdoors aren’t immediately activated for intrusion actions by hands-on-key-board actors. Defenders do have an opportunity to contain intrusions granted they have the visibility and capability to respond to exploitation events in a timely manner.
Exploitation can lead to more serious threats outside of cryptocurrency miners.
Microsoft reported in late April that Clop ransomware affiliate Lace Tempest exploited PaperCut servers as early as April 13.
Exploitation artifacts followed a similar pattern to the case above (pc-app.exe spawning PowerShell with an encoded command line) but used PowerShell to execute a Truebot DLL alongside remote access software Atera and eventually Cobalt Strike.
Recommendations from our Threat Response Unit (TRU) Team:
Public and private educational institutions that rely on PaperCut may be targeted for exploitation if their application is vulnerable. We highly recommend immediately checking to see if their application has been compromised. In addition, eSentire TRU also recommends:
Consider deploying endpoint security technologies (e.g., endpoint detection and response) to externally facing servers. These tools can aid in identification and response to exploitation events at your perimeter and increase resilience to zero-day exploits.
After performing a business impact review, apply the relevant security patches provided by PaperCut.
Restrict access to the PaperCut web interface and consider limiting access to local network or VPN-joined hosts to reduce exposure.
Adopt a comprehensive Vulnerability Management program that enables continuous awareness of the threat landscape, proactive vulnerability scanning to understand which systems are inadvertently exposed, and disciplined patch management.
eSentire’s Threat Response Unit (TRU) is a world-class team of threat researchers who develop new detections enriched by original threat intelligence and leverage new machine learning models that correlate multi-signal data and automate rapid response to advanced threats.
If you are not currently engaged with an MDR provider, eSentire MDR can help you reclaim the advantage and put your business ahead of disruption.
Learn what it means to have an elite team of Threat Hunters and Researchers that works for you. Connect with an eSentire Security Specialist.
eSentire Threat Response Unit (TRU)
Our industry-renowned Threat Response Unit (TRU) is an elite team of threat hunters and researchers, that supports our 24/7 Security Operations Centers (SOCs), builds detection models across our Atlas XDR Cloud Platform, and works as an extension of your security team to continuously improve our Managed Detection and Response service. TRU has been recognized for its threat hunting, original research and content development capabilities. TRU is strategically organized into cross-functional groups to protect you against advanced and emerging threats, allowing your organization to gain leading threat intelligence and incredible cybersecurity acumen.
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