Vulnerability in Mark3labs Mcp-Go
CVE-2026-123272
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.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 8.0 (High). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:H/A:N.
EPSS exploit prediction
No EPSS score from FIRST.org yet. New CVEs usually get one within a few days; no score means no signal yet, not low risk. How EPSS works.
Affected products
Weakness classification (CWE)
References
- github.com/mark3labs/mcp-go/blob/4cb0d2015f24b669726fd8e5e53fc8c6d3f57abf/clien… (technical-description)
- github.com/mark3labs/mcp-go/commit/90d247ed8d45cbb7d71d9a95e112789b78760443 (patch)
- github.com/mark3labs/mcp-go/pull/1053 (issue-tracking, patch)
- mcp-go v1.2.1 Release Notes (release-notes)
- GitHub Security Advisory (GHSA-xf6j-693p-wqxx) (vendor-advisory)
- VulnCheck Advisory: mcp-go before v1.2.1 Origin Validation Error via Unvalidated Well-Known Protected Resource Metadata (third-party-advisory)
Frequently asked questions
- What is CVE-2026-123272?
- CVE-2026-123272 is a high-severity vulnerability in Mark3labs Mcp-Go, classified under Origin Validation Error. CVSS score: 8.0/10. Published 2026-11-05.
- How severe is CVE-2026-123272?
- High severity. CVSS v3 base score is 8.0 out of 10.