Malicious Actors Exploit CVE-2023-27350 in PaperCut MF and NG – CISA

SUMMARY

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.

Download the PDF version of this report (653kb):

For a downloadable copy of IOCs (55kb), see:

TECHNICAL DETAILS

Vulnerability Overview

CVE-2023-27350 allows a remote actor to bypass authentication and conduct remote code execution on the following affected installations of PaperCut:[1]

• Version 8.0.0 to 19.2.7
• Version 20.0.0 to 20.1.6
• Version 21.0.0 to 21.2.10
• Version 22.0.0 to 22.0.8

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:

• Using the print scripting interface to execute shell commands.
• Using the User/Group Sync interface to execute a living-off-the-land-style attack.

FBI and CISA note that actors may develop other methods for RCE.

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.

This CVE was added to CISA’s Known Exploited Vulnerabilities (KEV) Catalog on April 21, 2023.

Threat Actor Activity

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).

According to FBI information, legitimate remote management and maintenance (RMM) software was downloaded and executed on victim systems via commands issued through PaperCut’s print scripting interface. External network communications through Tor and/or other proxies from inside victim networks helped Bl00dy Gang ransomware actors mask their malicious network traffic. The FBI also identified information relating to the download and execution of command and control (C2) malware such as DiceLoader, TrueBot, and Cobalt Strike Beacons, although it is unclear at which stage in the attack these tools were executed.

DETECTION METHODS

Network defenders should focus detection efforts on three key areas:

• Network traffic signatures – Look for network traffic attempting to access the SetupCompleted page of an exposed and vulnerable PaperCut server.
• System monitoring – Look for child processes spawned from a PaperCut server’s pc-app.exe process.
• Server settings and log files – Look for evidence of malicious activity in PaperCut server settings and log files.

Network Traffic Signatures

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. 

alert http any any -> $HOME_NET any (\
  msg:"ET EXPLOIT PaperCut MF/NG SetupCompleted Authentication Bypass (CVE-2023-27350)"; \
  flow:established,to_server; \
  http.method; content:"GET"; \
  http.uri; content:"/app?service=page/SetupCompleted"; bsize:32; fast_pattern; \
  reference:cve,2023-27350; \
  classtype:attempted-admin; \

alert http any any -> $HOME_NET any (msg:"ET EXPLOIT PaperCut MF/NG SetupCompleted Authentication Bypass (CVE-2023-27350)"; flow:established,to_server; http.method; content:"GET"; http.uri; content:"page/SetupCompleted"; fast_pattern; reference:url,www.huntress.com/blog/critical-vulnerabilities-in-papercut-print-management-software; reference:cve,2023-27350; classtype:attempted-admin; metadata:attack_target Server, cve CVE_2023_27350, deployment Perimeter, deployment Internal, deployment SSLDecrypt, former_category EXPLOIT, performance_impact Low, confidence High, signature_severity Major, updated_at 2023_05_05;)

Note that these signatures and other rule-based detections, including YARA rules, may fail to detect more advanced iterations of CVE-2023-27350 exploits. Actors are known to adapt exploits to circumvent rule-based detections formulated for the original iterations of exploits observed in the wild. For example, the first rule above detected some of the first known exploits of CVE-2023-27350, but a slight modification of the exploit’s GET request can evade that rule. The second rule was designed to detect a broader range of activity than the first rule.

The following additional Emerging Threat Suricata signatures are designed to detect Domain Name System (DNS) lookups of known malicious domains associated with recent PaperCut exploitation:

alert dns $HOME_NET any -> any any (msg:"ET TROJAN Possible PaperCut MF/NG Post Exploitation Domain in DNS Lookup (windowcsupdates .com)"; dns_query; content:"windowcsupdates.com"; nocase; isdataat:!1,relative; pcre:"/(?:^|\.)windowcsupdates\.com$/"; reference:url,www.huntress.com/blog/critical-vulnerabilities-in-papercut-print-management-software; classtype:trojan-activity; metadata:affected_product Windows_XP_Vista_7_8_10_Server_32_64_Bit, attack_target Client_Endpoint, deployment Perimeter, former_category MALWARE, performance_impact Low, signature_severity Major, updated_at 2023_04_21;)

alert dns $HOME_NET any -> any any (msg:"ET ATTACK_RESPONSE Possible PaperCut MF/NG Post Exploitation Domain in DNS Lookup (anydeskupdate .com)"; dns_query; content:"anydeskupdate.com"; nocase; isdataat:!1,relative; pcre:"/(?:^|\.)anydeskupdate\.com$/"; reference:url,www.huntress.com/blog/critical-vulnerabilities-in-papercut-print-management-software; classtype:trojan-activity; metadata:affected_product Windows_XP_Vista_7_8_10_Server_32_64_Bit, attack_target Client_Endpoint, deployment Perimeter, former_category MALWARE, performance_impact Low, signature_severity Major, updated_at 2023_04_21;)

alert dns $HOME_NET any -> any any (msg:"ET TROJAN Possible PaperCut MF/NG Post Exploitation Domain in DNS Lookup (anydeskupdates .com)"; dns_query; content:"anydeskupdates.com"; nocase; isdataat:!1,relative; pcre:"/(?:^|\.)anydeskupdates\.com$/"; reference:url,www.huntress.com/blog/critical-vulnerabilities-in-papercut-print-management-software; classtype:trojan-activity; metadata:affected_product Windows_XP_Vista_7_8_10_Server_32_64_Bit, attack_target Client_Endpoint, deployment Perimeter, former_category MALWARE, performance_impact Low, signature_severity Major, updated_at 2023_04_21;)

alert dns $HOME_NET any -> any any (msg:"ET TROJAN Possible PaperCut MF/NG Post Exploitation Domain in DNS Lookup (windowservicecemter .com)"; dns_query; content:"windowservicecemter.com"; nocase; isdataat:!1,relative; pcre:"/(?:^|\.)windowservicecemter\.com$/"; reference:url,www.huntress.com/blog/critical-vulnerabilities-in-papercut-print-management-software; classtype:trojan-activity; metadata:affected_product Windows_XP_Vista_7_8_10_Server_32_64_Bit, attack_target Client_Endpoint, deployment Perimeter, former_category MALWARE, performance_impact Low, signature_severity Major, updated_at 2023_04_21;)

alert dns $HOME_NET any -> any any (msg:"ET ATTACK_RESPONSE Possible PaperCut MF/NG Post Exploitation Domain in DNS Lookup (winserverupdates .com)"; dns_query; content:"winserverupdates.com"; nocase; isdataat:!1,relative; pcre:"/(?:^|\.)winserverupdates\.com$/"; reference:url,www.huntress.com/blog/critical-vulnerabilities-in-papercut-print-management-software; classtype:trojan-activity; metadata:affected_product Windows_XP_Vista_7_8_10_Server_32_64_Bit, attack_target Client_Endpoint, deployment Perimeter, former_category MALWARE, performance_impact Low, signature_severity Major, updated_at 2023_04_21;)

alert dns $HOME_NET any -> any any (msg:"ET TROJAN Possible PaperCut MF/NG Post Exploitation Domain in DNS Lookup (netviewremote .com)"; dns_query; content:"netviewremote.com"; nocase; isdataat:!1,relative; pcre:"/(?:^|\.)netviewremote\.com$/"; reference:url,www.huntress.com/blog/critical-vulnerabilities-in-papercut-print-management-software; classtype:trojan-activity; metadata:affected_product Windows_XP_Vista_7_8_10_Server_32_64_Bit, attack_target Client_Endpoint, deployment Perimeter, former_category MALWARE, performance_impact Low, signature_severity Major, updated_at 2023_04_21;)

alert dns $HOME_NET any -> any any (msg:"ET TROJAN Possible PaperCut MF/NG Post Exploitation Domain in DNS Lookup (updateservicecenter .com)"; dns_query; content:"updateservicecenter.com"; nocase; isdataat:!1,relative; pcre:"/(?:^|\.)updateservicecenter\.com$/"; reference:url,www.huntress.com/blog/critical-vulnerabilities-in-papercut-print-management-software; classtype:trojan-activity; metadata:affected_product Windows_XP_Vista_7_8_10_Server_32_64_Bit, attack_target Client_Endpoint, deployment Perimeter, former_category MALWARE, performance_impact Low, signature_severity Major, updated_at 2023_04_21;)

alert dns $HOME_NET any -> any any (msg:"ET TROJAN Possible PaperCut MF/NG Post Exploitation Domain in DNS Lookup (windowservicecenter .com)"; dns_query; content:"windowservicecenter.com"; nocase; isdataat:!1,relative; pcre:"/(?:^|\.)windowservicecenter\.com$/"; reference:url,www.huntress.com/blog/critical-vulnerabilities-in-papercut-print-management-software; classtype:trojan-activity; metadata:affected_product Windows_XP_Vista_7_8_10_Server_32_64_Bit, attack_target Client_Endpoint, deployment Perimeter, former_category MALWARE, performance_impact Low, signature_severity Major, updated_at 2023_04_21;)

alert dns $HOME_NET any -> any any (msg:"ET TROJAN Possible PaperCut MF/NG Post Exploitation Domain in DNS Lookup (windowservicecentar .com)"; dns_query; content:"windowservicecentar.com"; nocase; isdataat:!1,relative; pcre:"/(?:^|\.)windowservicecentar\.com$/"; reference:url,www.huntress.com/blog/critical-vulnerabilities-in-papercut-print-management-software; classtype:trojan-activity; metadata:affected_product Windows_XP_Vista_7_8_10_Server_32_64_Bit, attack_target Client_Endpoint, deployment Perimeter, former_category ATTACK_RESPONSE, performance_impact Low, signature_severity Major, updated_at 2023_04_21;)

Note that these signatures may also not work if the actor modified activity to evade detection by known rules.

System Monitoring

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].

title: PaperCut MF/NG Vulnerability 
authors: Huntress DE&TH Team
description: Detects suspicious code execution from vulnerable PaperCut versions MF and NG 
logsource:
  category: process_creation 
  product: windows 
detection: 
  selection: 
    ParentImage|endswith: “\\pc-app.exe” 
    Image|endswith:  
      - “\\cmd.exe” 
      - “\\powershell.exe” 
  condition: selection 
level: high 
falsepositives:     
  - Expected admin activity

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.

Server Settings and Log Files

Network defenders may be able to identify suspicious activity by reviewing the PaperCut server options to identify unfamiliar print scripts or User/Group Sync settings.

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:

• User "admin" updated the config key “print.script.sandboxed”
• User "admin" updated the config key “device.script.sandboxed”
• Admin user "admin" modified the print script on printer
• User/Group Sync settings changed by "admin"

Indicators of Compromise

See Table 1 through Table 6 for IOCs obtained from FBI investigations and open-source information as of early May 2023.

INCIDENT RESPONSE

If compromise is suspected or detected, organizations should:

1. Create a backup of the current PaperCut server(s).
2. Wipe the PaperCut Application Server and/or Site Server and rebuild it.
3. Restore the database from a “safe” backup point. Using a backup dated prior to April 2023 would be prudent, given that exploitation in-the-wild exploitation began around early April.
4. Execute additional security response procedures and carry out best practices around potential compromise.
5. Report the compromise to CISA via CISA’s 24/7 Operations Center (report@cisa.gov or 888-282-0870). The FBI encourages recipients of this document to report information concerning suspicious or criminal activity to their local FBI field office or IC3.gov. Regarding specific information that appears in this communication, the context and individual indicators, particularly those of a non-deterministic or ephemeral nature (such as filenames or IP addresses), may not be indicative of a compromise. Indicators should always be evaluated in light of an organization’s complete information security situation. 

MITIGATIONS

FBI and CISA recommend organizations:

• Upgrade PaperCut to the latest version.
• If unable to immediately patch, ensure vulnerable PaperCut servers are not accessible over the internet and implement one of the following network controls:
  • Option 1: External controls: Block all inbound traffic from external IP addresses to the web management portal (port 9191 and 9192 by default).
  • Option 2: Internal and external controls: Block all traffic inbound to the web management portal. Note: The server cannot be managed remotely after this step.
• Follow best cybersecurity practices in your production and enterprise environments, including mandating phishing-resistant multifactor authentication (MFA) for all staff and for all services. For additional best practices, see CISA’s Cross-Sector Cybersecurity Performance Goals (CPGs). The CPGs, developed by CISA and the National Institute of Standards and Technology (NIST), are a prioritized subset of IT and OT security practices that can meaningfully reduce the likelihood and impact of known cyber risks and common TTPs. Because the CPGs are a subset of best practices, CISA and FBI also recommend all organizations implement a comprehensive information security program based on a recognized framework, such as the NIST Cybersecurity Framework (CSF).

ACKNOWLEDGMENTS

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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