CVE-2021-39197
HIGHOlder releases of better_errors open to Cross-Site Request Forgery attack
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Blast Radius
better_errorsReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects RubyGems packages — download data is not available via public APIs for these ecosystems.
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 Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 💎RubyGems | better_errors | all versions | 2.8.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for better_errors. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update better_errors to 2.8.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-39197 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2021-39197 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2021-39197. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2021-39197 in your dependencies?
O3 detects CVE-2021-39197 across RubyGems dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.