Unexpected Origin header ("http://localhost:3000") in POST requests inside GitHub Codespaces despite using .app.github.dev URL

[hamajyotan]

Select Topic Area

Bug

Body

I'm running a Ruby on Rails application inside GitHub Codespaces.
While testing a POST request (e.g., submitting a form), Rails rejected it with the following CSRF error:

ActionController::InvalidAuthenticityToken
HTTP Origin header (http://localhost:3000) didn't match request.base_url (https://*.app.github.dev)

🔍 Observations

• I'm accessing the app via a Codespaces-exposed domain, such as: https://solid-capybara-xxxxxxx-3000.app.github.dev
• Chrome DevTools correctly shows the Origin header as:

  Origin: https://solid-capybara-xxxxxxx-3000.app.github.dev
  

• However, within the Rails app, request.headers["Origin"] is unexpectedly:

  http://localhost:3000
  

This suggests that somewhere between the browser and the Rails app, the Origin header is being rewritten to http://localhost:3000.

❓ Is this intentional?
I understand that Codespaces may internally simulate a localhost-like environment for developer convenience. If the Origin header is intentionally rewritten, that would explain the issue.
But in web frameworks like Rails that enforce strict CSRF checks using Origin, this results in development-time errors unless config.action_controller.forgery_protection_origin_check is disabled, which is not ideal.

✅ What I'm looking for:

• Confirmation: Is this header rewriting expected behavior?
• Advice: Is there a recommended pattern for making CSRF protection work in Codespaces while preserving security?
• Optionally: Should frameworks like Rails treat Codespaces differently in their default configs?

💻 Environment:

• GitHub Codespaces
• Ruby on Rails 8.x
• Browser: Chrome (latest)
• No custom proxy or nginx in place

Thanks in advance for your time and help!

[github-actions[bot]]

💬 Your Product Feedback Has Been Submitted 🎉

Thank you for taking the time to share your insights with us! Your feedback is invaluable as we build a better GitHub experience for all our users.

Here's what you can expect moving forward ⏩

• Your input will be carefully reviewed and cataloged by members of our product teams. 
  • Due to the high volume of submissions, we may not always be able to provide individual responses.
  • Rest assured, your feedback will help chart our course for product improvements.
• Other users may engage with your post, sharing their own perspectives or experiences. 
• GitHub staff may reach out for further clarification or insight. 
  • We may 'Answer' your discussion if there is a current solution, workaround, or roadmap/changelog post related to the feedback.

Where to look to see what's shipping 👀

• Read the Changelog for real-time updates on the latest GitHub features, enhancements, and calls for feedback.
• Explore our Product Roadmap, which details upcoming major releases and initiatives.

What you can do in the meantime 💻

• Upvote and comment on other user feedback Discussions that resonate with you.
• Add more information at any point! Useful details include: use cases, relevant labels, desired outcomes, and any accompanying screenshots.

As a member of the GitHub community, your participation is essential. While we can't promise that every suggestion will be implemented, we want to emphasize that your feedback is instrumental in guiding our decisions and priorities.

Thank you once again for your contribution to making GitHub even better! We're grateful for your ongoing support and collaboration in shaping the future of our platform. ⭐

[k4ml]

Bumped into this as well with django (we just migrated from gitpod). Looks like this is a won't fix issue from codespaces dev. The only way is to add localhost to the trusted origins.

https://github.com/orgs/community/discussions/147513