Papercut Mf Exploit

[Sueann]

We have received two vulnerability reports from a 3rd party cyber security company (Trend Micro), for high/critical severity security issues in PaperCut MF/NG. We have evidence to suggest that unpatched servers are being exploited in the wild.

We have confirmed that under certain circumstances this allows for an unauthenticated attacker to get Remote Code Execution (RCE) on a PaperCut Application Server. This could be done remotely and without the need to log in.

PaperCut has conducted analysis on all customer reports, and the earliest signature of suspicious activity on a customer server potentially linked to this vulnerability is 14th April 01:29 AEST / 13th April 15:29 UTC

Yes, the New PaperCut RCE exploit created that bypasses existing detections article is referring to exploiting the same vulnerability, in a way that the activity is not easily detected in the Sysmon or PaperCut MF application log. The method of exploiting PaperCut MF mentioned in the article is mitigated in versions 20.1.7, 21.2.11, and 22.0.9 and later.

We currently recommend looking for the following Indicators of Compromise (IOCs) to determine if it is likely that the vulnerability has been used to install malware on the system. Depending on your systems, logging and endpoint protection software you may be able to detect the following.

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The Federal Bureau of Investigation (FBI) and Cybersecurity and Infrastructure Security Agency (CISA) are releasing this joint Cybersecurity Advisory (CSA) in response to the active exploitation of CVE-2023-27350. This vulnerability occurs in certain versions of PaperCut NG and PaperCut MF and enables an unauthenticated actor to execute malicious code remotely without credentials. PaperCut released a patch in March 2023.

According to FBI observed information, malicious actors exploited CVE-2023-27350 beginning in mid-April 2023 and continuing through the present. In early May 2023, also according to FBI information, a group self-identifying as the Bl00dy Ransomware Gang attempted to exploit vulnerable PaperCut servers against the Education Facilities Subsector.

This joint advisory provides detection methods for exploitation of CVE-2023-27350 as well and indicators of compromise (IOCs) associated with Bl00dy Ransomware Gang activity. FBI and CISA strongly encourage users and administrators to immediately apply patches, and workarounds if unable to patch. FBI and CISA especially encourage organizations who did not patch immediately to assume compromise and hunt for malicious activity using the detection signatures in this CSA. If potential compromise is detected, organizations should apply the incident response recommendations included in this CSA.

PaperCut servers vulnerable to CVE-2023-27350 implement improper access controls in the SetupCompleted Java class, allowing malicious actors to bypass user authentication and access the server as an administrator. After accessing the server, actors can leverage existing PaperCut software features for remote code execution (RCE). There are currently two publicly known proofs of concept for achieving RCE in vulnerable PaperCut software:

The PaperCut server process pc-app.exe runs with SYSTEM- or root-level privileges. When the software is exploited to execute other processes such as cmd.exe or powershell.exe, these child processes are created with the same privileges. Commands supplied with the execution of these processes will also run with the same privileges. As a result, a wide range of post-exploitation activity is possible following initial access and compromise.

Education Facilities Subsector entities maintained approximately 68% of exposed, but not necessarily vulnerable, U.S.-based PaperCut servers. In early May 2023, according to FBI information, the Bl00dy Ransomware Gang gained access to victim networks across the Education Facilities Subsector where PaperCut servers vulnerable to CVE-2023-27350 were exposed to the internet. Ultimately, some of these operations led to data exfiltration and encryption of victim systems. The Bl00dy Ransomware Gang left ransom notes on victim systems demanding payment in exchange for decryption of encrypted files (see Figure 1).

To exploit CVE-2023-27350, a malicious actor must first visit the SetupCompleted page of the intended target, which will provide the adversary with authentication to the targeted PaperCut server. Deploy the following Emerging Threat Suricata signatures to detect when GET requests are sent to the SetupCompleted page. (Be careful of improperly formatted double-quotation marks if copying and pasting signatures from this advisory.)

Note that some of the techniques identified in this section can affect the availability or stability of a system. Defenders should follow organizational policies and incident response best practices to minimize the risk to operations while threat hunting.

A child process is spawned under pc-app.exe when the vulnerable PaperCut software is used to execute another process, which is the PaperCut server process. Malicious activity against PaperCut servers in mid-April used the RCE to supply commands to a cmd.exe or powershell.exe child process, which were then used to conduct further network exploitation. The following YARA rule may detect malicious activity[2].

More advanced versions of the exploit can drop a backdoor executable, use living-off-the-land binaries, or attempt to evade the above YARA rule by spawning an additional child process in-between pc-app.exe and a command-line interpreter.

If the PaperCut Application Server logs have debug mode enabled, lines containing SetupCompleted at a time not correlating with the server installation or upgrade may be indicative of a compromise. Server logs can be found in [app-path]/server/logs/*.* where server.log is normally the most recent log file.
Any of the following server log entries may be indicative of a compromise:

The Multi-State Information Sharing and Analysis Center (MS-ISAC) contributed to this advisory.
REFERENCES
[1] PaperCut: URGENT PaperCut MF/NG vulnerability bulletin (March 2023)
[2] Huntress: Critical Vulnerabilities in PaperCut Print Management Software

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