CVE-2026-123272
mcp-go before v1.2.1 Origin Validation Error via Unvalidated Well-Known Protected Resource Metadata
CVSS Score
8.0
EPSS Score
0.0%
EPSS Percentile
0th
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.
| CWE | CWE-346 |
| Vendor | mark3labs |
| Product | mcp-go |
| Published | Nov 5, 2026 |
| Last Updated | Nov 5, 2026 |
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CVSS v3 Breakdown
CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:H/A:N Attack Vector
Network
Attack Complexity
High
Privileges Required
None
User Interaction
Required
Scope
Changed
Confidentiality
High
Integrity
High
Availability
None
Affected Versions
mark3labs / mcp-go
v0.48.0 < v1.2.1
References
github.com: https://github.com/mark3labs/mcp-go/security/advisories/GHSA-xf6j-693p-wqxx github.com: https://github.com/mark3labs/mcp-go/pull/1053 github.com: https://github.com/mark3labs/mcp-go/commit/90d247ed8d45cbb7d71d9a95e112789b78760443 github.com: https://github.com/mark3labs/mcp-go/releases/tag/v1.2.1 github.com: https://github.com/mark3labs/mcp-go/blob/4cb0d2015f24b669726fd8e5e53fc8c6d3f57abf/client/transport/oauth.go#L799-L821 vulncheck.com: https://www.vulncheck.com/advisories/mcp-go-before-1.2.1-origin-validation-error-via-unvalidated-well-known-protected-resource-metadata
Credits
Theodoros Lampropoulos Ed Zynda (ezynda3)