FrontMCP and mcp-from-openapi have bypass of OpenAPI external $ref SSRF fix

Exploitation

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

CVSS

8.5 · CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:L/A:N

EPSS

• CVE-2026-59973: 0.39% estimated exploitation activity in the next 30 days; percentile 33.51 (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 The published fix for GHSA-v6ph-xcq9-qxxj / CVE-2026-39885 added a direct hostname denylist for OpenAPI external $ref dereferencing, but the latest patched dependency mcp-from-openapi 2.3.0 still makes backend-origin requests to loopback when the target is reached through hostname resolution, redirects, or IPv4-mapped IPv6 syntax. FrontMCP latest release v1.2.1 and current main still call OpenAPIToolGenerator.fromURL() and OpenAPIToolGenerator.fromJSON() from mcp-from-openapi 2.3.0 when loading OpenAPI adapters. An attacker who can cause a hosted or multi-user FrontMCP deployment to load an untrusted OpenAPI spec can trigger requests from the server to localhost or private services during tool generation. This is a latest-version bypass of the previous fix. A direct http://127.0.0.1 $ref control is now denied and produces zero canary hits, while semantically equivalent loopback targets still reach the canary. Latest versions checked - frontmcp npm latest: 1.2.1 -…

Published by the source

11 Sep 2026, 22:02 UTC

Last modified by the source

11 Sep 2026, 22:15 UTC

Source last checked

26 Oct 2026, 13:02 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-65h7-9wrw-629c

2. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GHSA-65h7-9wrw-629c

3. Revision 1 · created · 23 Oct 2026

   Published from ghsa record GHSA-65h7-9wrw-629c

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# Patch Next advisory brief: CVE-2026-59973

Advisory: FrontMCP and mcp-from-openapi have bypass of OpenAPI external $ref SSRF fix
IDs: CVE-2026-59973, GHSA-65h7-9wrw-629c
Record: https://patchnext.com/advisories/cve-2026-59973 (revision 3, source checked 26 Oct 2026, 13:02 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: mcp-from-openapi 2.5.0; @frontmcp/adapters 1.5.0; frontmcp 1.5.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm mcp-from-openapi: affected >= 2.3.0, < 2.5.0; fixed in 2.5.0
- npm @frontmcp/adapters: affected >= 1.2.1, < 1.5.0; fixed in 1.5.0
- npm frontmcp: affected >= 1.2.1, < 1.5.0; fixed in 1.5.0

## Suggested inspection (read-only)
- `npm ls mcp-from-openapi --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.
- `npm ls @frontmcp/adapters --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.
- `npm ls frontmcp --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## 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-from-openapi, @frontmcp/adapters, frontmcp.
- 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://github.com/advisories/GHSA-65h7-9wrw-629c (GitHub-reviewed advisory GHSA-65h7-9wrw-629c lists affected ranges: npm mcp-from-openapi >=2.3.0 =1.2.1 =1.2.1 <1.5.0)
- Verified: https://osv.dev/vulnerability/GHSA-65h7-9wrw-629c (OSV record GHSA-65h7-9wrw-629c lists affected ranges: npm mcp-from-openapi >=2.3.0 =1.2.1 =1.2.1 <1.5.0 (fixed))

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