CVE-2026-123272: mark3labs mcp-go

mcp-go before v1.2.1 Origin Validation Error via Unvalidated Well-Known Protected Resource Metadata

CVSS 5.1 Medium

Published by the CVE Program (CNA: VulnCheck) at 5 Nov 2026, 18:45 UTC. agentprime has not analysed this yet, so whether it affects any app is not known. This CVE names a vendor and product rather than a package, so the lockfile check doesn’t match it.

Affected products, as the CNA wrote them
Vendor	Product	Versions
mark3labs	mcp-go pkg:golang/github.com/mark3labs/mcp-go	affected v0.48.0 to before v1.2.1 semver Other versions: unaffected

Identifiers

CVE-2026-123272

CNA

VulnCheck

Weakness (CWE)

CWE-346

Not analysed yet

We haven’t published our own analysis of this CVE. If a person reviews it, this page will say what’s affected and what has to be true for it to matter.

Severity and exploitation

Score	Rating	CVSS	Vector	Source
5.1	Medium	4.0	CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:A/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N	CNA (VulnCheck)Used for this record
8.0	High	3.1	CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:H/A:N	CNA (VulnCheck)

CISA KEV Not listed.
EPSS No score.

Timeline

5 Nov 2026, 18:45 UTC Confirmed by the CVE List
5 Nov 2026, 18:54 UTC First seen by agentprime
5 Nov 2026, 19:17 UTC Confirmed by NVD

Sources

References from the CNA

github.com/mark3labs/mcp-go/security/advisories/GHSA-xf6j-693p-wqxx vendor-advisory
github.com/mark3labs/mcp-go/pull/1053 issue-trackingpatch
github.com/mark3labs/mcp-go/commit/90d247ed8d45cbb7d71d9a95e112789b78760443 patch
github.com/mark3labs/mcp-go/releases/tag/v1.2.1 release-notes
github.com/mark3labs/mcp-go/blob/4cb0d2015f24b669726fd8e5e53fc8c6d3f57abf/client/transport/oauth.go#L799-L821 technical-description
www.vulncheck.com/advisories/mcp-go-before-1.2.1-origin-validation-error-via-unvalidated-well-known-protected-resource-metadata third-party-advisory

Facts on this page come from the CVE List as the CNA published them, with CVSS from the sources named above, CISA KEV and FIRST EPSS. Vendors, products and versions are shown as the CNA wrote them. We don’t reproduce the CVE’s description.