MCP Ruby SDK: Streamable HTTP transport lacks DNS-rebinding (Host/Origin) protection

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

6.9 · CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:A/VC:H/VI:N/VA:N/SC:H/SI:N/SA:N · score source

EPSS

• CVE-2026-63118: 0.24% estimated exploitation activity in the next 30 days; percentile 15.01 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary MCP::Server::Transports::StreamableHTTPTransport (the Rack-mountable Streamable HTTP transport in the mcp gem) processes every incoming JSON-RPC request without ever inspecting the HTTP Host or Origin request headers. There is no AllowedHosts/AllowedOrigins allowlist and no DNS-rebinding guard anywhere in the transport. A local MCP server that binds a loopback or LAN HTTP port is therefore reachable by any web origin a victim's browser visits, via a DNS-rebinding attack: a malicious page rebinds its own hostname to 127.0.0.1, then drives the local MCP server cross-origin to enumerate and invoke its tools and exfiltrate their output. This is the standard browser-driven local-service attack that the MCP Streamable HTTP guidance exists to prevent. Impact - An attacker who can get a victim to open a web page can reach any MCP server the victim runs locally over the Streamable HTTP transport (e.g. a developer-tools or filesystem MCP server on localhost). - Because the transport…

Published by the source

30 Jul 2026, 14:41 UTC

Last modified by the source

30 Jul 2026, 14:45 UTC

Source last checked

26 Oct 2026, 15:52 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-rjr6-rcgv-9m7m

2. Revision 2 · amended · 25 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-rjr6-rcgv-9m7m

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# Patch Next advisory brief: CVE-2026-63118

Advisory: MCP Ruby SDK: Streamable HTTP transport lacks DNS-rebinding (Host/Origin) protection
IDs: CVE-2026-63118, GHSA-rjr6-rcgv-9m7m
Record: https://patchnext.com/advisories/cve-2026-63118 (revision 3, source checked 26 Oct 2026, 15:52 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: mcp 0.23.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- RubyGems mcp: affected <= 0.22.0; fixed in 0.23.0

## Suggested inspection (read-only)
- RubyGems mcp: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on mcp.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-rjr6-rcgv-9m7m (OSV record GHSA-rjr6-rcgv-9m7m lists affected ranges: RubyGems mcp >=0 <0.23.0 (fixed))
- Verified: https://github.com/advisories/GHSA-rjr6-rcgv-9m7m (GitHub-reviewed advisory GHSA-rjr6-rcgv-9m7m lists affected ranges: RubyGems mcp >=0 <=0.22.0)

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