CVE-2026-126077 is a medium-severity (CVSS 6.8) Information Exposure vulnerability in actionpack. A fix is available for actionpack — see the affected versions and patch details below.
Action Pack: Possible sensitive session information leak in Active Storage proxy mode
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
actionpack💎actionpackReal-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
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.
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 signed or encrypted cookies. Cookies that the application sets explicitly are still written. See the Action Pack CHANGELOG.
Workarounds
Configure caching proxies not to store responses that carry a Set-Cookie header, or complete the cookie rotation or serializer migration and remove it from the configuration.
Releases
The fixed releases are available at the normal locations.
Patches for those who cannot upgrade at once:
- 8.1: https://github.com/rails/rails/commit/849a6c30eacbb6ca4722132f43c4333a93188556.patch
- 8.0: https://github.com/rails/rails/commit/0e6cf8faa89caf4c64311d35ca2bc88c1458d4e3.patch
Credit
This issue was responsibly reported by HackerOne researcher flashmyname.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 💎RubyGems | actionpack | ≥ 8.1.0.beta1&&< 8.1.4.1 | 8.1.4.1bundle update actionpack --conservative |
| 💎RubyGems | actionpack | all versions | 8.0.5.2bundle update actionpack --conservative |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for actionpack, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update actionpack to 8.1.4.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-126077 is resolved across your whole dependency graph.
Workarounds
Assume what was exposed is already known: rotate any credential, token or key that the affected component could return, restrict the endpoint to callers that genuinely need it, and strip sensitive fields from responses and error output at the boundary rather than relying on the client not to read them.
Frequently Asked Questions
Is CVE-2026-126077 in your dependencies?
Find it across RubyGems, including transitive dependencies.