CVE-2021-39197

Older releases of better_errors open to Cross-Site Request Forgery attack

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

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).

A victim's authenticated browser session is used to submit forged requests to an application that cannot distinguish them from legitimate ones. Typical impact: state-changing actions performed as the victim without their consent.

CVE-2021-39197 has a CVSS score of 6.3 (Medium). The vector is network-reachable, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.8.0); upgrading removes the vulnerable code path.

Kodem intelligence

Already deployed Kodem?

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".

1. What is CVE-2021-39197? CVE-2021-39197 is a medium-severity cross-site request forgery (CSRF) vulnerability in better_errors (rubygems), affecting versions < 2.8.0. It is fixed in 2.8.0. A victim's authenticated browser session is used to submit forged requests to an application that cannot distinguish them from legitimate ones.
2. How severe is CVE-2021-39197? CVE-2021-39197 has a CVSS score of 6.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of better_errors are affected by CVE-2021-39197? better_errors (rubygems) versions < 2.8.0 is affected.
4. Is there a fix for CVE-2021-39197? Yes. CVE-2021-39197 is fixed in 2.8.0. Upgrade to this version or later.
5. Is CVE-2021-39197 exploitable, and should I be worried? Whether CVE-2021-39197 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2021-39197 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2021-39197? Upgrade better_errors to 2.8.0 or later.