CVE-2025-27409 is a high-severity path traversal vulnerability (CWE-22) affecting versions of the open-source note-taking application Joplin prior to 3.3.3. Joplin Server serves static files, including plugin assets located under the paths starting with 'css/pluginAssets' or 'js/pluginAssets'. The vulnerability arises from improper validation in the 'findLocalFile' function within the default route handler. This function calls 'localFileFromUrl' to resolve special plugin asset paths. If 'localFileFromUrl' returns a path, it is used directly without further checks for path traversal sequences such as '../'. Consequently, an attacker can craft a specially crafted URL to access files outside the intended directories, potentially reading arbitrary files on the server's filesystem. The vulnerability does not require authentication or user interaction and can be exploited remotely over the network. The CVSS 3.1 base score is 7.5 (high), reflecting the ease of exploitation (network vector, low attack complexity), no privileges required, and the impact on confidentiality (high), while integrity and availability remain unaffected. No known exploits are reported in the wild as of the publication date (April 30, 2025). The issue was patched in Joplin version 3.3.3 by adding proper path traversal checks to restrict file access to intended directories only.

For European organizations using Joplin Server versions prior to 3.3.3, this vulnerability poses a significant risk to confidentiality. Attackers can remotely read arbitrary files on the server, potentially exposing sensitive information such as configuration files, credentials, or user data stored on the server. This can lead to further compromise if sensitive secrets are disclosed. Since Joplin is often used for personal and organizational note-taking and task management, exposure of notes or internal documents could result in data leaks, intellectual property theft, or privacy violations. The integrity and availability of the system are not directly impacted by this vulnerability. However, the breach of confidentiality alone can have severe regulatory and reputational consequences, especially under GDPR requirements in Europe. Organizations relying on Joplin Server for collaboration or data storage should consider this vulnerability critical to address promptly to prevent unauthorized data disclosure.

1. Upgrade all Joplin Server instances to version 3.3.3 or later immediately to apply the official patch that properly restricts file path resolution and prevents path traversal. 2. If immediate upgrade is not feasible, implement web server or reverse proxy rules to restrict access to the vulnerable 'css/pluginAssets' and 'js/pluginAssets' paths, blocking suspicious path traversal patterns (e.g., '..' sequences) in URLs. 3. Conduct a thorough audit of server file permissions to ensure the Joplin Server process has minimal read access, limiting exposure if exploitation occurs. 4. Monitor server logs for unusual requests targeting pluginAssets paths or containing path traversal patterns to detect potential exploitation attempts. 5. Educate administrators to avoid exposing Joplin Server instances publicly without proper access controls or network segmentation. 6. Consider deploying Web Application Firewalls (WAFs) with rules to detect and block path traversal attempts targeting Joplin Server endpoints. 7. Review and rotate any sensitive credentials or secrets that may have been stored on affected servers as a precautionary measure.