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attack-surface.md

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Attack Surface Analysis for nextcloud/server

Attack Surface: Cross-Site Scripting (XSS)

  • Description: Injection of malicious scripts into web pages viewed by other users.
  • Server Contribution: Nextcloud server renders user-generated content and application interfaces, potentially without proper sanitization, allowing malicious scripts to be embedded. Vulnerabilities in core Nextcloud code or apps running on the server can introduce XSS.
  • Example: A user uploads a file with a filename containing a malicious JavaScript payload. When another user views the file list served by the Nextcloud server, the script executes in their browser, potentially stealing session cookies or redirecting them to a phishing site.
  • Impact: Account compromise, data theft, defacement, malware distribution.
  • Risk Severity: High
  • Mitigation Strategies:
    • Developers: Implement robust input sanitization and output encoding in all Nextcloud core and app code running on the server. Use Content Security Policy (CSP) headers configured by the server to restrict script execution sources. Regularly update Nextcloud server and apps to patch known XSS vulnerabilities. Conduct thorough server-side security testing, including XSS vulnerability scanning.

Attack Surface: Authentication and Authorization Bypass

  • Description: Circumventing security mechanisms on the server to gain unauthorized access to resources or functionalities.
  • Server Contribution: Vulnerabilities in Nextcloud server's authentication or authorization logic, or server misconfigurations, can allow attackers to bypass login procedures or access resources they should not be permitted to see or modify on the server.
  • Example: A bug in Nextcloud server's user authentication code allows an attacker to log in as another user without knowing their password. Or, a misconfiguration in file sharing permissions on the server allows public access to private files stored on the server.
  • Impact: Data breach, unauthorized access to sensitive information stored on the server, account takeover, system compromise.
  • Risk Severity: Critical
  • Mitigation Strategies:
    • Developers: Implement strong and secure authentication and authorization mechanisms within the Nextcloud server codebase. Follow secure coding practices to prevent authentication bypass vulnerabilities in server-side code. Regularly review and audit authentication and authorization code on the server. Enforce principle of least privilege in access control configurations on the server. Implement robust password policies and 2FA on the server.
    • Users (Server Administrators): Use strong and unique passwords for server administrator accounts. Enable two-factor authentication (2FA) for server administrator accounts. Regularly review server access logs for suspicious logins. Properly configure file sharing permissions on the server.

Attack Surface: Server-Side Request Forgery (SSRF)

  • Description: An attacker exploits the Nextcloud server to make requests to unintended locations, often internal resources.
  • Server Contribution: Nextcloud server features that fetch external resources (e.g., file previews, external storage) can be vulnerable to SSRF if input processed by the server is not properly validated, allowing attackers to control the destination of server-side requests originating from the Nextcloud server.
  • Example: An attacker crafts a malicious URL that, when processed by Nextcloud server for a file preview, forces the server to make a request to an internal network resource, potentially exposing internal services or data to the attacker via the Nextcloud server.
  • Impact: Access to internal network resources via the Nextcloud server, information disclosure from internal systems through the server, potential remote code execution in vulnerable internal services accessible by the Nextcloud server.
  • Risk Severity: High
  • Mitigation Strategies:
    • Developers: Implement strict input validation and sanitization for URLs and hostnames used in server-side requests originating from the Nextcloud server. Use allowlists of permitted domains or protocols for external requests made by the server. Disable or restrict server features that are prone to SSRF if not essential. Implement network segmentation to limit the impact of SSRF originating from the Nextcloud server.
    • Users (Server Administrators): Restrict Nextcloud server's network access as much as possible using firewalls and network configurations. Monitor server logs for unusual outbound network connections originating from the Nextcloud server.

Attack Surface: File Handling and Storage Vulnerabilities

  • Description: Exploiting weaknesses in how the Nextcloud server handles, processes, and stores files.
  • Server Contribution: Nextcloud server manages file uploads, downloads, previews, and storage. Vulnerabilities in these server-side functionalities can lead to path traversal on the server, arbitrary file upload to the server, denial of service on the server, or information disclosure from the server's file system.
  • Example: A path traversal vulnerability in the file download functionality of the Nextcloud server allows an attacker to download files outside of the intended Nextcloud data directory on the server. Or, a vulnerability in image processing on the server allows uploading a malicious image that triggers a buffer overflow and potentially remote code execution on the server.
  • Impact: Data breach from the server's file system, data manipulation on the server, denial of service on the server, potentially remote code execution on the server.
  • Risk Severity: High to Critical (depending on the specific vulnerability)
  • Mitigation Strategies:
    • Developers: Implement secure file handling practices in the Nextcloud server codebase, including input validation for filenames and file paths processed by the server. Sanitize file content and metadata processed by the server where necessary. Use secure file storage permissions on the server's file system. Regularly update file processing libraries used by the server to patch known vulnerabilities. Implement file type validation and size limits on the server.
    • Users (Server Administrators): Regularly update Nextcloud server and apps. Implement proper file system permissions on the server. Monitor server storage usage and file access logs for suspicious activity. Consider using server-side antivirus scanning for uploaded files.