CVE-2021-39197: Cross-Site Request Forgery in better_errors

Severity: high (CVSS 8.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.6% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2021-09-07 · updated 2026-06-17
Weakness: CWE-352

Description

better_errors is an open source replacement for the standard Rails error page with more information rich error pages. It is also usable outside of Rails in any Rack app as Rack middleware. 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). 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". There are no known workarounds to mitigate the risk of using older releases of better_errors.

Affected products

BetterErrors better_errors: before 2.8.0 — fixed in 2.8.0
Rubygems Better Errors: before 2.8.0; 0.0.1, 0.0.2, 0.0.3, 0.0.4, 0.0.5, 0.0.6 (+28) — fixed in 2.8.0

How to fix

Upgrade to a fixed version: BetterErrors better_errors 2.8.0; Rubygems Better Errors 2.8.0. Check the vendor advisory for mitigations.

References

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