CVE-2026-123272

    Dashboard / Vulnerabilities / CVE-2026-123272

    CVE-2026-123272

    Published: 5 Nov 2026Last Modified: 7 Nov 2026

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

    Details: 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://github.com/CVEProject/cvelistV5/tree/main/cves/2026/123xxx/CVE-2026-123272.json, https://github.com/mark3labs/mcp-go/releases/tag/v1.2.1, https://github.com/mark3labs/mcp-go/security/advisories/GHSA-xf6j-693p-wqxx, https://nvd.nist.gov/vuln/detail/CVE-2026-123272, 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/commit/90d247ed8d45cbb7d71d9a95e112789b78760443, https://github.com/mark3labs/mcp-go/pull/1053, https://github.com/mark3labs/mcp-go, https://github.com/mark3labs/mcp-go/blob/4cb0d2015f24b669726fd8e5e53fc8c6d3f57abf/client/transport/oauth.go#L799-L821

    Affected packages

    Package

    Name:

    Purl:

    Affected ranges

    Type: GIT

    Events:

    Introduced- c99de0d8995745b82ef4851419f91cbb48897d12
    Fixed -90d247ed8d45cbb7d71d9a95e112789b78760443

    Affected versions

    v1.2.0

    Common Vulnerability Scoring System

    Attack Vector
    Network
    Adjacent
    Local
    Physical
    Privileges Required
    None
    Low
    High
    User Interaction
    None
    Required
    Scope
    Unchanged
    Changed
    Confidentiality
    None
    Low
    High
    Integrity
    None
    Low
    High
    Availability
    None
    Low
    High
    CVE-2026-123272 | CVE-DB