CVE-2021-39197
Cross-Site Request Forgery in better_errors
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.
Attack profile
How easy it is to exploit
- Attack vector
- Network
- Complexity
- High
- Privileges needed
- Low
- User interaction
- None
What an attacker can affect
- Confidentiality
- High
- Integrity
- None
- Availability
- None
Exploit likelihood
What each source reports
CVE List Score used
Cross-Site Request Forgery in better_errors
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.
- Record
CVE-2021-39197- CVSS 3.1
- 6.3 Medium
CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:N/A:N - Weakness
- CWE-352
- Updated
- 2026-11-02
EUVDEUVD-2021-2122
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.
- Record
EUVD-2021-2122- CVSS 3.1
- 6.3 Medium
CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:N/A:N - Updated
- 2026-11-03
GitHub AdvisoriesGHSA-w3j4-76qw-wwjm
Older releases of better_errors open to Cross-Site Request Forgery attack
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
- Record
GHSA-w3j4-76qw-wwjm- CVSS 3.1
- 6.3 Medium
CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:N/A:N - Weakness
- CWE-352
- Updated
- 2026-11-02
NVD
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.
- Record
CVE-2021-39197- CVSS 3.1
- 6.3 Medium
CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:N/A:N - CVSS 3.1
- 8.8 High
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H - CVSS 2.0
- 6.8 Medium
AV:N/AC:M/Au:N/C:P/I:P/A:P - Weakness
- CWE-352
- Updated
- 2026-11-02
OSV
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.
- Record
CVE-2021-39197- CVSS 3.1
- 8.8 High
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H - Updated
- 2026-11-02
OSVGHSA-w3j4-76qw-wwjm
Older releases of better_errors open to Cross-Site Request Forgery attack
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
- Record
GHSA-w3j4-76qw-wwjm- CVSS 3.1
- 6.3 Medium
CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:N/A:N - Weakness
- CWE-352
- Updated
- 2026-11-02
Affected products
Affected packages
| Package | Affected | Fixed |
|---|---|---|
better_errorsRubyGems | before 2.8.0 | 2.8.0 |
better_errorsRubyGems | before 2.8.0, plus listed versions 0.0.10.0.20.0.30.0.40.0.5 Show 29 moreShow less0.0.60.0.70.0.80.1.00.1.10.2.00.3.00.3.20.5.00.6.00.7.00.7.20.8.00.9.01.0.01.0.0.rc11.0.11.1.02.0.02.1.02.1.12.2.02.3.02.4.02.5.02.5.12.6.02.7.02.7.1 | 2.8.0 |
History
| Recorded | Changed | Sources |
|---|---|---|
| aliases | EUVD | |
| first indexed | CVE List, GitHub Advisories, NVD, OSV |
Dates are when Every CVE recorded each change. Upstream dates may differ.
In short
CVE-2021-39197 is a medium vulnerability (CVSS 6.3) in Better Errors Project Errors, published September 7, 2021. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 0.64%, the 51st percentile. Fixed in better_errors 2.8.0.
References
github.com/BetterErrors/better_errors/security/advisories/GHSA-w3j4-76qw-wwjm
nvd.nist.gov/vuln/detail/CVE-2021-39197