CVE-2021-39197: 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 Rai...

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.

Metrics

CVSS 3.1

8.8/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

EPSS Probability

0.61%

44.9th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-352

Affected Software

Vendor	Product	Versions
Better Errors Project	Better Errors	< 2.8.0

References

https://github.com/BetterErrors/better_errors/commit/8e8e796bfbde4aa088741823c8a3fc6df2089bb0Patch, Third Party Advisory
https://github.com/BetterErrors/better_errors/discussions/507Third Party Advisory
https://github.com/BetterErrors/better_errors/pull/474Patch, Third Party Advisory
https://github.com/BetterErrors/better_errors/security/advisories/GHSA-w3j4-76qw-wwjmThird Party Advisory
https://github.com/BetterErrors/better_errors/commit/8e8e796bfbde4aa088741823c8a3fc6df2089bb0Patch, Third Party Advisory
https://github.com/BetterErrors/better_errors/discussions/507Third Party Advisory
https://github.com/BetterErrors/better_errors/pull/474Patch, Third Party Advisory
https://github.com/BetterErrors/better_errors/security/advisories/GHSA-w3j4-76qw-wwjmThird Party Advisory

Timeline

Published: Sep 7, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-39197?

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.

How severe is CVE-2021-39197?

CVE-2021-39197 has a CVSS score of 8.8/10 (HIGH severity). The EPSS model estimates a 0.61% probability of exploitation in the next 30 days.

How do I fix CVE-2021-39197?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.