mitigations.md

Mitigation Strategies Analysis for bigskysoftware/htmx

Mitigation Strategy: Strategic Use of hx-swap

1. Strategic Use of hx-swap

• Description:

  1. Analyze HTML injection points: For each htmx interaction, carefully consider where the received HTML fragment will be inserted into the DOM.
  2. Choose the safest hx-swap value: Select the hx-swap value that provides the necessary functionality with the least risk of script execution.
     • Prefer: innerHTML, outerHTML, beforebegin, afterbegin, beforeend, afterend, delete.
     • Use with extreme caution: morph (if you don't fully control the content), none (if you're not carefully managing the response).
  3. Avoid unnecessary script execution: If you don't need to execute scripts within the swapped content, choose a hx-swap value that prevents it.
  4. Combine with server-side sanitization: Even with careful hx-swap selection, always sanitize and encode server-side output.

• Threats Mitigated:

  • Cross-Site Scripting (XSS): (Severity: Critical) - By limiting the ways HTML can be injected, you reduce the attack surface for XSS. Certain hx-swap values are inherently safer than others.
  • HTML Injection: (Severity: High) - Similar to XSS, controlling the injection method limits the potential for disruptive HTML manipulation.

• Impact:

  • XSS: Risk reduced from Critical to Medium (must be combined with server-side encoding for Low risk).
  • HTML Injection: Risk reduced from High to Medium.

• Currently Implemented:

  • hx-swap="innerHTML" is used in most places.

• Missing Implementation:

  • hx-swap="outerHTML" is used in one instance (/product/update) where innerHTML would be sufficient and safer. This should be reviewed.
  • No consistent review process for hx-swap choices.

Mitigation Strategy: htmx-Specific CSRF Protection with hx-headers

2. htmx-Specific CSRF Protection with hx-headers

• Description:

  1. Generate CSRF tokens (server-side): Ensure your server-side framework generates unique, unpredictable CSRF tokens for each user session.
  2. Include token in hx-headers: For every htmx request that modifies data (POST, PUT, DELETE, PATCH), use the hx-headers attribute to include the CSRF token.
     • Example: <button hx-post="/delete-item" hx-headers='{"X-CSRF-Token": "{{ csrf_token }}"}'>Delete</button>
     • JavaScript alternative: If you're using JavaScript to trigger htmx requests, you can add the headers dynamically: htmx.ajax('POST', '/delete-item', {headers: {'X-CSRF-Token': getCsrfToken()}});
  3. Server-side validation: Your server must validate the CSRF token on every corresponding request.

• Threats Mitigated:

  • Cross-Site Request Forgery (CSRF): (Severity: High) - Prevents attackers from forging requests that appear to come from legitimate users.

• Impact:

  • CSRF: Risk reduced from High to Low (when combined with server-side validation).

• Currently Implemented:

  • No hx-headers usage for CSRF tokens.

• Missing Implementation:

  • This is a critical missing implementation across all htmx requests that modify data.

Mitigation Strategy: Safe Redirects with hx-redirect (and Server-Side Validation)

3. Safe Redirects with hx-redirect (and Server-Side Validation)

• Description:

  1. Avoid user input directly in hx-redirect: The ideal scenario is to avoid using any user-supplied data to determine the redirect URL. Use server-side logic.
  2. If user input is unavoidable:
     • Whitelist: Maintain a server-side whitelist of allowed redirect URLs or URL patterns.
     • Strict validation: Before setting the hx-redirect header on the server, rigorously validate the target URL against the whitelist and/or a strict, predefined pattern.
     • No open redirects: Never allow a completely arbitrary URL provided by the user.
  3. Server-side control: The hx-redirect attribute itself is just an instruction; the server sets the actual Location header. The security responsibility lies entirely on the server.

• Threats Mitigated:

  • Open Redirects: (Severity: Medium) - Prevents attackers from using your application to redirect users to malicious websites.

• Impact:

  • Open Redirects: Risk reduced from Medium to Low (with proper server-side validation).

• Currently Implemented:

  • hx-redirect is used, but without any server-side validation of the redirect target.

• Missing Implementation:

  • The crucial server-side validation and whitelisting are completely missing.

Mitigation Strategy: Error Handling with hx-swap-oob

4. Error Handling with hx-swap-oob

• Description:

  1. Dedicated error element: Create a designated HTML element (e.g., a <div>) in your layout specifically for displaying error messages. Give it a unique ID.
  2. hx-swap-oob="true": On this error element, add the attribute hx-swap-oob="true".
  3. Server-side error handling: When an htmx request fails on the server:
     • Catch the error: Use appropriate error handling mechanisms (try-except blocks).
     • Log details (server-side): Log the full error details for debugging.
     • Return a generic error message: In the htmx response, return an HTML fragment containing a generic error message. Do not include any sensitive information.
     • Target the error element: Ensure the returned HTML fragment has the same ID as your dedicated error element. htmx will automatically swap it into place because of hx-swap-oob="true".
  4. Example:

     <div id="error-message" hx-swap-oob="true" style="display: none;"></div>
     
     <!-- ... other content ... -->
     
     <!-- Server-side (Python/Flask example): -->
     @app.route('/my-htmx-endpoint', methods=['POST'])
     def my_htmx_endpoint():
         try:
             # ... process the request ...
         except Exception as e:
             logging.exception("Error in /my-htmx-endpoint")  # Log details
             return '<div id="error-message">An unexpected error occurred.</div>', 500

• Threats Mitigated:

  • Information Disclosure: (Severity: Medium) - Prevents sensitive error details from being displayed to the user.
  • Improved User Experience: (Severity: Low) - Provides a cleaner way to handle errors without disrupting the main UI flow.

• Impact:

  • Information Disclosure: Risk reduced from Medium to Low.
  • User Experience: Improved.

• Currently Implemented:

  • Not implemented. Errors are often displayed inline, potentially revealing sensitive information.

• Missing Implementation:

  • Complete implementation is missing.

Mitigation Strategy: Controlling Request Behavior with hx-request

5. Controlling Request Behavior with hx-request

• Description:

  1. Review hx-request usage: Examine all instances where hx-request is used.
  2. include option: Use the include option to specify exactly which elements' values should be included in the request. This prevents unintended data from being sent.
  3. withCredentials option: Carefully consider the withCredentials option. If set to true, cookies and other credentials will be sent with the request. Ensure this is necessary and desired. If not, set it to false (the default).
  4. Server-Side Validation is Key: Remember that hx-request only controls what is sent; the server must still validate all received data.

• Threats Mitigated:

  • Data Leakage: (Severity: Low to Medium) - By controlling which data is included in requests, you reduce the risk of accidentally sending sensitive information.
  • CSRF (Indirectly): (Severity: High) - Proper use of withCredentials, combined with other CSRF protections, helps prevent CSRF attacks.

• Impact:

  • Data Leakage: Risk reduced.
  • CSRF: Contributes to overall CSRF mitigation.

• Currently Implemented:

  • hx-request is not used extensively.

• Missing Implementation:

  • A review of existing htmx requests to determine if hx-request could be used to improve security is needed.