fix: read latest CSRF token from cookie

[asadali145]

Issue Links

https://github.com/mitodl/hq/issues/10704

Description

Fixes intermittent 403 CSRF errors encountered by learners when submitting problem answers in the Learning MFE.

The Learning MFE renders course units inside an LMS-served iframe. When a learner submits a problem, the page sends POST requests to the LMS (e.g., /event and /courses/{course_key}/xblock/{usage_key}/handler/xmodule_handler/problem_check). These requests require a valid CSRF token in the X-CSRFToken header.

Root Cause

The CSRF token was being cached at page load:

• The LMS initialized jQuery AJAX with a static headers block
• $.cookie('csrftoken') was read once when the iframe loaded
• If the user re-authenticated in another tab (or CSRF rotated), the cookie updated but the header did not
• Subsequent requests sent a stale CSRF token, resulting in HTTP 403 errors

Server logs confirmed:

Forbidden (CSRF token from the 'X-Csrftoken' HTTP header incorrect.)

Fix

Update AJAX setup to read the CSRF cookie immediately before each unsafe same-origin request (via beforeSend) instead of caching it at load time.

This ensures:

• The X-CSRFToken header always matches the latest csrftoken cookie
• Requests remain valid even after re-authentication or CSRF rotation

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Impact

• Learners: Fixes intermittent failures when submitting problems in Learning MFE
• Developers / Operators: No action required

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Testing Instructions

Reproduction Steps

1. Start LMS with this branch (ensure updated static assets are served)
2. Log in and open a course unit with a problem (Learning MFE)
3. Keep this tab open
4. In another tab:
   • Log out
   • Log back in (CSRF token rotates)
5. Return to the original tab (without refreshing)
6. Submit an answer

Expected Behavior

Before fix:

• Submission may fail with HTTP 403
• Logs show CSRF token mismatch

After fix:

• Submission succeeds
• Request uses the latest CSRF token

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Additional Verification

• Open DevTools → Network tab
• Inspect problem_check request
• Confirm:
  • X-CSRFToken header matches current csrftoken cookie

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Regression Checks

• Normal problem submission works without re-login flow

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[pdpinch]

Are there tests for this? Is that even possible?

[asadali145]

Are there tests for this? Is that even possible?

@pdpinch There are no existing tests, that's why I skipped that part.

[pdpinch]

Is this going to add overhead to every ajax request?