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

Quick assessment

Affected

mark3labs mcp-go

Exploitation

No confirmed in-the-wild exploitation; assess based on exposure

Recommended action

Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

Mark3labs 的 mcp-go 在 v1.2.1 之前版本中存在一个 OAuthHandler.fetchServerMetadata 函数中的验证不当漏洞。该漏洞允许恶意的 MCP 服务器通过在“well-known"元数据中声明任意资源，从而获取其他资源的访问令牌。攻击者可以将不同的 MCP 服务器声明为 RFC 8707 中定义的资源指示符（resource indicator），诱使授权服务器生成令牌，随后攻击者将这些令牌重放（replay）到目标资源上，以实现未授权访问。

CVSS 8.0 · High Updated Nov 06, 2026

Possible ATT&CK Techniques 3 AI

T1078 · Valid Accounts T1190 · Exploit Public-Facing Application T1551

Affected Version Matrix 1

Vendor	Product	Version Range	Status
mark3labs	mcp-go	`v0.48.0< v1.2.1`	affected

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Vulnerability Information

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Vulnerability Title

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

Source: CVE Program / CVE List V5

Vulnerability 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.

Source: CVE Program / CVE List V5

CVSS Information

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

Source: CVE Program / CVE List V5

Vulnerability Type

源验证错误

Source: CVE Program / CVE List V5

Affected Products

Vendor	Product	Affected Versions	CPE	Subscribe
mark3labs	mcp-go	v0.48.0 ~ v1.2.1	-

II. Public POCs for CVE-2026-123272

#	POC Description	Source Link	Shenlong Link

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No public POC found.

III. Intelligence Information for CVE-2026-123272

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Other References for CVE-2026-123272 (6)

https://github.com/mark3labs/mcp-go/blob/4cb0d2015f24b669726fd8e5e53fc8c6d3f57abf/client/transport/oauth.go#L799-L821 technical-description
https://github.com/mark3labs/mcp-go/commit/90d247ed8d45cbb7d71d9a95e112789b78760443 patch
https://github.com/mark3labs/mcp-go/pull/1053 issue-tracking patch
https://github.com/mark3labs/mcp-go/releases/tag/v1.2.1 release-notes
https://github.com/mark3labs/mcp-go/security/advisories/GHSA-xf6j-693p-wqxx vendor-advisory
https://www.vulncheck.com/advisories/mcp-go-before-1.2.1-origin-validation-error-via-unvalidated-well-known-protected-resource-metadata third-party-advisory

https://www.cve.org/CVERecord?id=CVE-2026-123272

IV. Related Vulnerabilities

Same product: mcp-go

Same vendor: mark3labs

Same weakness: CWE-346