CVE-2026-42559
DNS rebinding via unvalidated Host header in RMCP HTTP server (pre-1.4.0).
Is CVE-2026-42559 being exploited?
Not confirmed. CVE-2026-42559 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 0.21% probability of exploitation in the next 30 days.
How severe is CVE-2026-42559?
CVE-2026-42559 is rated High with a CVSS score of 8.8. 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-2026-42559?
Yes. A fix has been recorded for CVE-2026-42559. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-42559 affect?
CVE-2026-42559 affects RMCP Rust SDK (Model Context Protocol), transport/streamable_http_server, pre-1.4.0. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-42559?
Upgrade RMCP to 1.4.0+ to fix Host header validation.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
DNS rebinding enables authenticated requests to the MCP server on loopback/private network.
Patch when possible
- affected< 1.4.0
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.