Impact

better_errors prior to 2.8.0 did not implement CSRF protection for its internal requests. It also did not enforce the correct "Content-Type" header for these requests, which allowed a cross-origin "simple request" to be made without CORS protection. These together left an application with better_errors enabled open to cross-origin attacks.

As a developer tool, better_errors documentation strongly recommends addition only to the development bundle group, so this vulnerability should only affect development environments. Please ensure that your project limits better_errors to the development group (or the non-Rails equivalent).

Patches

Starting with release 2.8.x, CSRF protection is enforced. It is recommended that you upgrade to the latest release, or minimally to "~> 2.8.3".

Workarounds

There are no known workarounds to mitigate the risk of using older releases of better_errors.

References

• Chris Moberly provided an example attack that uses a now-patched vulnerability of webpack-dev-server in conjunction with Better Errors

For more information

If you have any questions or comments about this advisory, please

• Add to the discussion in better_errors
• Open an issue in better_errors

References

• GHSA-w3j4-76qw-wwjm
• BetterErrors/better_errors#474
• BetterErrors/better_errors@8e8e796
• https://nvd.nist.gov/vuln/detail/CVE-2021-39197
• BetterErrors/better_errors#507
• https://github.com/rubysec/ruby-advisory-db/blob/master/gems/better_errors/CVE-2021-39197.yml