Dissecting CVE-20 Exploitation Techniques

Exploiting CVE-2023-27350: PaperCut NG/MF Authentication Bypass

Exploiting CVE-2023-27350 involves an authentication bypass vulnerability within PaperCut NG/MF, which can lead to remote code execution when chained with other vulnerabilities. This critical flaw allows unauthenticated attackers to gain administrative privileges on affected systems. The Common Vulnerability Scoring System (CVSS) v3.1 rates this vulnerability with a score of 9.8, categorizing it as Critical.

What the Vulnerability Is and Its Impact

CVE-2023-27350 is an authentication bypass in PaperCut NG and PaperCut MF. It affects versions 8.0 to 19.2.7, 20.0 to 20.1.7, 21.0 to 21.2.10, and 22.0 to 22.0.7. This vulnerability provides unauthenticated access to sensitive setup functions within the PaperCut application. Such access enables an attacker to manipulate internal configurations, effectively elevating their privileges to that of an administrator. The direct impact is full control over the PaperCut application, including user management, print job control, and system settings.

When combined with CVE-2023-27351, a path traversal vulnerability, the authentication bypass becomes a precursor to remote code execution (RCE). An attacker first bypasses authentication using CVE-2023-27350, then exploits CVE-2023-27351 to upload arbitrary files and execute code on the underlying server. This chain of vulnerabilities presents a severe risk, allowing complete system compromise.

Technical Root Cause Analysis

The core of CVE-2023-27350 lies in improper access control within specific servlets of the PaperCut application. The `/app?service=page/SetupCompleted` endpoint is particularly vulnerable. This endpoint is designed for the initial setup process of PaperCut, which typically requires no authentication. However, the application fails to restrict access to this endpoint even after the setup process concludes.

The `SetupCompleted` class, and other related classes, contain methods that the application exposes through reflection. During normal operation, these methods are called internally or by authenticated users. The vulnerability arises because the unauthenticated `/app?service=page/SetupCompleted` endpoint can be coerced into calling arbitrary methods within the `Config` class and other critical components.

An attacker can leverage this reflection capability by passing specific parameters in the URL query string. These parameters directly map to method calls and arguments within the application's internal structure. The absence of an authentication check at this crucial stage allows an attacker to invoke sensitive methods that should only be accessible to legitimate administrators.

Specifically, methods related to user source configuration are exposed. The `storeUserSourceConfig` method, for example, allows an administrator to define how PaperCut integrates with external user directories like Active Directory or LDAP. An unauthenticated attacker can call this method to modify or create new user source configurations, effectively changing the authentication mechanism or adding malicious user sources.

Exploitation Mechanics

Exploiting CVE-2023-27350 begins with an unauthenticated HTTP GET request to the vulnerable endpoint. The attacker targets `/app?service=page/SetupCompleted`. This initial request bypasses any standard authentication mechanisms. The server processes this request without verifying the client's identity.

The attacker then crafts subsequent requests to interact with the application's internal configuration. A common method involves manipulating the `config.properties` file or other configuration data. The vulnerability allows an attacker to invoke methods that set or modify system properties. For instance, an attacker can use reflection to call methods that manage user authentication sources.

Consider a scenario where an attacker wants to gain administrative access. They can send a GET request containing parameters that instruct the application to add a new administrator account or modify an existing one. An example parameter might resemble `&config.property.key=admin.password&config.property.value=newpassword`. While this is a simplified example, the underlying mechanism involves calling methods like `setConfigValue` or `storeUserSourceConfig` with attacker-controlled data.

The authentication bypass (CVE-2023-27350) is often the first step in a chain. Once administrative control is gained, the path traversal vulnerability (CVE-2023-27351) becomes exploitable. An attacker, now acting as an administrator, can upload a malicious file. This file could be a Java Server Page (JSP) web shell or another executable payload. The path traversal allows placing this file in a web-accessible directory.

For example, an attacker could upload a JSP file to a directory like `/webapps/app/WEB-INF/web.xml/../../../custom/reports/`. This allows direct execution of the uploaded file by navigating to its URL. This two-stage attack demonstrates how the initial authentication bypass provides the necessary privileges to facilitate arbitrary code execution on the server.

Detection: How to Check If You're Affected

Organizations must identify if their PaperCut NG/MF installations are vulnerable. The primary check involves verifying the installed version number. Any PaperCut NG/MF instance running versions 8.0 to 19.2.7, 20.0 to 20.1.7, 21.0 to 21.2.10, or 22.0 to 22.0.7 is vulnerable to CVE-2023-27350.

Administrators should review their PaperCut installation logs for suspicious activity. Look for unauthenticated access attempts to the `/app?service=page/SetupCompleted` endpoint from external or unusual IP addresses. Examine server access logs for any POST or GET requests to this specific path that are not associated with legitimate setup procedures.

Monitor configuration files for unauthorized modifications. Key files to inspect include `server.properties`, `user-source.properties`, and other files within the PaperCut installation directory. Unexpected changes to administrator accounts, user source configurations, or database connection strings indicate potential compromise.

Automated tools can assist in detection. Secably offers a free website vulnerability scanner that can help identify outdated software versions and exposed services. Running a scan against your PaperCut instance's public IP or hostname can reveal if the application is running a vulnerable version. The free port scanner from Secably can also confirm if PaperCut services are exposed to the internet, increasing the attack surface. Additionally, tools like Zondex can perform internet-wide scans to identify exposed PaperCut instances, providing an external view of your attack surface.

For deeper analysis, check for the presence of unexpected files in PaperCut's web directories, particularly in `[PaperCut MF install directory]/server/custom/`. The presence of JSP files or other executable scripts not part of the standard installation could indicate successful exploitation and a deployed web shell.

Remediation Steps

Immediate remediation for CVE-2023-27350 is to update PaperCut NG/MF to a patched version. PaperCut released patches for all affected branches. Update to versions 22.0.8, 21.2.11, 20.1.8, 19.2.8, or any newer release. These updates address the improper access control vulnerability, preventing unauthenticated access to the setup endpoint.

Restrict network access to your PaperCut instance. If possible, place the PaperCut server behind a firewall and limit access to trusted internal networks only. PaperCut services should not be directly exposed to the internet unless absolutely necessary. If external access is required, implement a Virtual Private Network (VPN) solution, such as VPNWG, to ensure only authenticated and authorized users can reach the application. This reduces the attack surface significantly.

After patching, conduct a thorough security audit of your PaperCut environment. Review all administrator accounts for any unauthorized additions or changes. Verify that all user source configurations are legitimate and haven't been tampered with. Rotate any compromised credentials.

Implement robust logging and monitoring. Ensure that your security information and event management (SIEM) system ingests logs from your PaperCut server and any web application firewalls (WAFs) protecting it. Configure alerts for suspicious activity, such as repeated access attempts to the `/app?service=page/SetupCompleted` endpoint or unexpected file uploads to the PaperCut directories. Regularly review these logs for anomalies.

As a proactive measure, consider implementing a Web Application Firewall (WAF) in front of your PaperCut instance. A WAF can help detect and block malicious requests targeting known vulnerabilities, including attempts at exploiting CVE-2023-27350, even before patches are applied. Configure WAF rules to specifically block access to the `/app?service=page/SetupCompleted` endpoint from untrusted sources.

Timeline of Disclosure

The timeline surrounding CVE-2023-27350 highlights the rapid response required for critical vulnerabilities. Initial discovery of the authentication bypass, and its potential for remote code execution, occurred in April 2023. Third-party security researchers identified the flaws and responsibly disclosed them to PaperCut.

PaperCut acknowledged the vulnerabilities promptly. They worked to develop and test patches for all affected versions of PaperCut NG and PaperCut MF. The vendor released security updates on April 26, 2023, addressing both CVE-2023-27350 and CVE-2023-27351.

Public disclosure followed shortly after the patch release. PaperCut published a comprehensive security advisory on April 27, 2023. This advisory detailed the vulnerabilities, their impact, and provided clear remediation instructions for customers. The rapid disclosure after patch availability aimed to minimize the window of opportunity for attackers.

Despite the quick response, active exploitation of CVE-2023-27350 and the chained RCE (CVE-2023-27351) was observed in the wild shortly after public disclosure. This emphasizes the urgency for organizations to apply patches immediately upon release.