Widely used Rack web framework component; exploitation could disrupt access control across many apps.
CVE-2026-39324
Unauthenticated crafted cookie accepted via fallback, enabling session manipulation.
Is CVE-2026-39324 being exploited?
Not confirmed. CVE-2026-39324 does not appear in CISA's Known Exploited Vulnerabilities catalog, which records only exploitation that has been observed and reported publicly. That is evidence of absence of a report, not evidence the flaw is unattacked. EPSS currently estimates a 0.27% probability of exploitation in the next 30 days.
How severe is CVE-2026-39324?
CVE-2026-39324 is rated Critical with a CVSS score of 9.3. Severity describes how bad exploitation would be, not how likely it is: pair it with exploitation evidence before deciding what to patch first.
Is there a patch for CVE-2026-39324?
Yes. A fix has been recorded for CVE-2026-39324. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-39324 affect?
CVE-2026-39324 affects Rack::Session::Cookie <= 2.1.1, Rack Ruby framework. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-39324?
Upgrade Rack to 2.1.2+ immediately
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Forge valid session data without secret; attackers can hijack sessions and access restricted resources.
Immediate action required
- affected>= 2.0.0 and < 2.1.2
As published in the CVE Program record. A version outside these ranges is not a statement that it is unaffected — vendors sometimes understate a range, and distribution-backported builds carry upstream numbers that do not reflect what was patched into them.