- Severity: MEDIUM
- CVE: CVE-2026-126077
- Ecosystems: rubygems, rubygems
- Packages: actionpack, actionpack
### Impact
When `ActionDispatch::Cookies` reads a signed or encrypted cookie that was written under a rotated secret (`config.action_dispatch.cookies_rotations`) or in the format that a `:hybrid` cookie serializer migrates from, it writes the cookie back to the response in the current format, and that write ignores `request.session_options[:skip]`. `ActiveStorage::DisableSession` relies on that option to keep the session cookie off the public, immutable responses of the Active Storage proxy controllers, but those controllers still load the session, so a request carrying an old-format session cookie could receive the rewritten session cookie in `Set-Cookie` on a cacheable response. A shared cache that stores such a response together with its `Set-Cookie` header could then serve one user’s session to later requesters of the same URL, or serve an attacker’s session to other users.
Applications are affected when they serve Active Storage files through the proxy controllers, have a cookie rotation or a `:hybrid` serializer migration configured, and run behind a cache that stores `Set-Cookie` on public responses. This is a path around the fix for [CVE-2024-26144](https://github.com/advisories/GHSA-8h22-8cf7-hq6g).
### Details
In the fixed releases, a response that skips the session leaves an old-format signed or encrypted cookie unchanged, and the next request that does not skip the session upgrades it. An application that skips the session on every request no longer upgrades its…
Source: GitHub Security Advisories · 2026-11-12