CVE-2021-41263: Secure/signed cookies share secrets between sites in rails_multisite

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: 2021-11-15 · updated 2026-06-17
Weakness: CWE-200

Description

rails_multisite provides multi-db support for Rails applications. In affected versions this vulnerability impacts any Rails applications using `rails_multisite` alongside Rails' signed/encrypted cookies. Depending on how the application makes use of these cookies, it may be possible for an attacker to re-use cookies on different 'sites' within a multi-site Rails application. The issue has been patched in v4 of the `rails_multisite` gem. Note that this upgrade will invalidate all previous signed/encrypted cookies. The impact of this invalidation will vary based on the application architecture.

Affected products

discourse rails_multisite: before 4.0.0 — fixed in 4.0.0

How to fix

Upgrade to a fixed version: discourse rails_multisite 4.0.0. Check the vendor advisory for mitigations.

References

Get alerted when CVE-2021-41263 or the next vulnerability affects your stack