Vulnerability · Published Nov 5, 2026 · Assigned by VulnCheck

CVE-2026-123272mcp-go before v1.2.1 Origin Validation Error via Unvalidated Well-Known Protected Resource Metadata

High 8.0

In plain English

A high-severity security flaw in mcp-go from mark3labs. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 9% of all known vulnerabilities).

Severity (CVSS 3.1)8.0High

Exploit odds (30 days)0.2%top 90.6% of all CVEs

Exploited in the wildNo reportsCISA SSVC: none

Attack toolingNoneno public tooling known

Official description

Mark3labs mcp-go before v1.2.1 contains an improper validation vulnerability in OAuthHandler.fetchServerMetadata that allows malicious MCP servers to obtain tokens for other resources by declaring arbitrary resources in well-known metadata. Attackers can declare a different MCP server as the RFC 8707 resource indicator, causing the authorization server to mint tokens the attacker replays against that resource.

Affected products (as reported)

Vendor	Product	Versions
mark3labs	mcp-go	v0.48.0 – before v1.2.1

Weakness: CWE-346 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:H/A:N

References

GitHub Security Advisory (GHSA-xf6j-693p-wqxx) vendor-advisory
https://github.com/mark3labs/mcp-go/pull/1053 issue-tracking, patch
https://github.com/mark3labs/mcp-go/commit/90d247ed8d45cbb7d71d9a95e112789b78760443 patch
mcp-go v1.2.1 Release Notes release-notes
https://github.com/mark3labs/mcp-go/blob/4cb0d2015f24b669726fd8e5e53fc8c6d3f57abf/client/transport/oauth.go#L799-L821 technical-description
VulnCheck Advisory: mcp-go before v1.2.1 Origin Validation Error via Unvalidated Well-Known Protected Resource Metadata third-party-advisory