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CVE-2026-123272mcp-go before v1.2.1 Origin Validation Error via Unvalidated Well-Known Protected Resource Metadata
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.
5.1 MEDIUMEPSS 0.19%Not in KEVNVD: Deferred
Overview
Severity
| Score | Severity | Version | Vector String |
|---|---|---|---|
| 5.1 | MEDIUM | 4.0VulnCheck (CNA) | 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 |
| 8.0 | HIGH | 3.1VulnCheck (CNA) | CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:H/A:N |
Details
- CVE ID
- CVE-2026-123272
- Status
- Deferred
- Assigner (CNA)
- VulnCheck
- Published
- Nov 5, 2026
- Last modified
- 2026-11-05 21:18 UTC
- Weaknesses
- CWE-346 · Origin Validation Error
Affected Products
As published by the CNA in the CVE record.
mark3labsmcp-go1 version range
- v0.48.0 before v1.2.1
References
- github.com https://github.com/mark3labs/mcp-go/blob/4cb0d2015f24b669726fd8e5e53fc8c6d3f57abf/client/transport/oauth.go#L799-L821
- github.com https://github.com/mark3labs/mcp-go/commit/90d247ed8d45cbb7d71d9a95e112789b78760443
- github.com https://github.com/mark3labs/mcp-go/pull/1053
- github.com https://github.com/mark3labs/mcp-go/releases/tag/v1.2.1
- github.com https://github.com/mark3labs/mcp-go/security/advisories/GHSA-xf6j-693p-wqxx
- vulncheck.com https://www.vulncheck.com/advisories/mcp-go-before-1.2.1-origin-validation-error-via-unvalidated-well-known-protected-resource-metadata