attack-surface.md

Attack Surface Analysis for carrierwaveuploader/carrierwave

Attack Surface: Inadequate File Type Validation

Description: Carrierwave, by design, delegates file type validation to the developer. If developers fail to implement robust server-side validation, attackers can bypass intended file type restrictions.

Carrierwave Contribution: Carrierwave's flexibility in validation means that weak or missing validation logic directly stems from how Carrierwave is implemented in the application.

Example: A Carrierwave uploader is configured to allow "images," but only checks the file extension on the client-side. An attacker uploads a malicious PHP script renamed to image.jpg. Without server-side content type verification, this script could be processed by the server if placed in a web-accessible directory.

Impact: Remote Code Execution (RCE), Cross-Site Scripting (XSS), data corruption, system compromise.

Risk Severity: Critical

Mitigation Strategies:

• Implement Server-Side Content Type Validation: Use libraries like MIME::Types in Ruby to verify the actual content type of uploaded files based on their magic numbers, not just file extensions.
• Whitelist Allowed MIME Types: Define a strict whitelist of allowed MIME types in your Carrierwave uploader and reject any file that doesn't match.
• Avoid Client-Side Validation as Primary Security: Client-side validation is easily bypassed and should only be used for user experience, not security.

Attack Surface: Insecure Direct Object Reference (IDOR) to Uploaded Files (due to predictable naming)

Description: Carrierwave's default or poorly configured filename generation can lead to predictable file URLs. This allows attackers to guess and access files they are not authorized to view.

Carrierwave Contribution: If developers rely on default filename generation or implement predictable naming schemes within Carrierwave uploaders, they directly create this vulnerability.

Example: Carrierwave is configured to store files with sequential IDs. User profile pictures are accessible at URLs like /uploads/user/avatar/1.jpg, /uploads/user/avatar/2.jpg, etc. An attacker can easily iterate through these URLs to access other users' private profile pictures.

Impact: Information Disclosure, unauthorized access to sensitive user data.

Risk Severity: High

Mitigation Strategies:

• Use UUID or Random Filenames: Configure Carrierwave to generate unique and unpredictable filenames using UUIDs or random string generators. Carrierwave's :uuid storage option is a good starting point.
• Implement Access Control: Ensure proper authentication and authorization checks are in place to control access to uploaded files at the application level, even if filenames are unpredictable.
• Consider Private Storage: Utilize private cloud storage buckets or restrict direct web access to the upload directory, serving files through application logic with access control.

Attack Surface: Path Traversal Vulnerabilities in Storage Paths (due to misconfiguration)

Description: Improper handling of file paths within Carrierwave configuration or application code can allow attackers to manipulate paths and potentially access or overwrite files outside the intended upload directory.

Carrierwave Contribution: While Carrierwave itself doesn't inherently cause path traversal, misconfigurations in how developers define storage paths, process filenames, or handle user-provided input related to file paths within Carrierwave uploaders can introduce this vulnerability.

Example: A Carrierwave uploader incorrectly concatenates user-provided input with the base upload path without proper sanitization. An attacker crafts a filename like ../../../etc/passwd which, when processed by the vulnerable code, could lead to attempts to write or read system files.

Impact: Unauthorized file access, file overwriting, potential system compromise.

Risk Severity: Critical

Mitigation Strategies:

• Sanitize User Input: Thoroughly sanitize and validate any user-provided input that influences file paths or filenames used in Carrierwave.
• Use Absolute Paths: Define storage paths using absolute paths and avoid relative path manipulations within Carrierwave configuration and application code.
• Restrict File Operations: Limit file system operations performed by the application to the intended upload directory and prevent access to parent directories.

Attack Surface: Vulnerabilities in Image Processing Libraries (via Carrierwave integration)

Description: Carrierwave often integrates with image processing libraries like ImageMagick or MiniMagick. Exploitable vulnerabilities in these libraries can be triggered by uploading specially crafted image files through Carrierwave.

Carrierwave Contribution: Carrierwave's direct integration with these libraries means that vulnerabilities within them become part of Carrierwave's attack surface in applications that utilize image processing features.

Example: An application uses Carrierwave and MiniMagick for image resizing. An attacker uploads a specially crafted image file designed to exploit a known vulnerability in MiniMagick (e.g., ImageTragick), potentially leading to Remote Code Execution on the server.

Impact: Remote Code Execution (RCE), Denial of Service (DoS), system compromise.

Risk Severity: Critical

Mitigation Strategies:

• Keep Image Processing Libraries Updated: Regularly update ImageMagick, MiniMagick, and any other image processing libraries used by Carrierwave to the latest versions to patch known vulnerabilities.
• Restrict Image Processing Functionality: Disable or restrict potentially dangerous features of image processing libraries, such as delegate policies in ImageMagick, if not strictly necessary.
• Input Sanitization for Image Processing: Sanitize and validate image files before passing them to image processing libraries to prevent exploitation of vulnerabilities through malicious file formats.

Attack Surface: Cross-Site Scripting (XSS) through Uploaded Files (due to improper serving)

Description: If Carrierwave is used to serve user-uploaded files directly without proper Content-Type headers and sanitization, attackers can upload malicious files (e.g., HTML, SVG, JavaScript) that execute scripts in other users' browsers when accessed.

Carrierwave Contribution: Carrierwave's role in managing file storage and retrieval means that if developers don't configure proper serving mechanisms and rely on direct access to uploaded files, Carrierwave becomes a pathway for delivering malicious content.

Example: An attacker uploads a malicious SVG file containing embedded JavaScript. If the application serves this SVG file with an incorrect or missing Content-Type header, browsers might render it inline, executing the embedded JavaScript and potentially leading to XSS attacks against users who view the file.

Impact: Cross-Site Scripting (XSS), account compromise, data theft, defacement.

Risk Severity: High

Mitigation Strategies:

• Set Correct Content-Type Headers: Ensure that your application and web server are configured to serve uploaded files with accurate Content-Type headers based on the detected file type.
• Force Download for Potentially Executable Files: For file types that could contain executable code (e.g., HTML, SVG, JavaScript), serve them with Content-Type: application/octet-stream and Content-Disposition: attachment headers to force browsers to download them instead of rendering them inline.
• Content Security Policy (CSP): Implement a strong Content Security Policy to further mitigate the impact of XSS attacks, even if malicious files are served.
• Separate Domain for User Content: Serve user-uploaded content from a separate domain or subdomain to isolate it from the main application domain and limit the potential damage from XSS.