CVE-2026-123272

medium

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.

References

https://www.vulncheck.com/advisories/mcp-go-before-1.2.1-origin-validation-error-via-unvalidated-well-known-protected-resource-metadata

https://github.com/mark3labs/mcp-go/security/advisories/GHSA-xf6j-693p-wqxx

https://github.com/mark3labs/mcp-go/releases/tag/v1.2.1

https://github.com/mark3labs/mcp-go/pull/1053

https://github.com/mark3labs/mcp-go/commit/90d247ed8d45cbb7d71d9a95e112789b78760443

https://github.com/mark3labs/mcp-go/blob/4cb0d2015f24b669726fd8e5e53fc8c6d3f57abf/client/transport/oauth.go#L799-L821

https://euvd.enisa.europa.eu/vulnerability/EUVD-2026-117725

Details

Source: Mitre, NVD

Published: 2026-11-05

Updated: 2026-11-05

Risk Information

CVSS v2

Base Score: 7.1

Vector: CVSS2#AV:N/AC:H/Au:N/C:C/I:C/A:N

Severity: High

CVSS v3

Base Score: 8

Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:H/A:N

Severity: High

CVSS v4

Base Score: 5.1

Vector: 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

Severity: Medium

EPSS

EPSS: 0.00196