CVE-2026-67431
Session fixation in MCP Ruby SDK allows attackers to hijack sessions and execute requests.
Is CVE-2026-67431 being exploited?
Not confirmed. CVE-2026-67431 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 0.29% probability of exploitation in the next 30 days.
How severe is CVE-2026-67431?
CVE-2026-67431 is rated High. 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-2026-67431?
Yes. A fix has been recorded for CVE-2026-67431. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-67431 affect?
CVE-2026-67431 affects mcp gem < 0.23.0, MCP Ruby SDK, MCP::Server::Transports::StreamableHTTPTransport. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-67431?
Upgrade mcp gem to >=0.23.0 and rotate/revoke affected session IDs.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Attacker with a stolen session ID can send tools/call requests that execute in the victim's session, enabling unauthorized actions or code execution.
Immediate action required
- affected< 0.23.0
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.