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CVE-2026-123272

mcp-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.

CVSS 4.0 · 5.1 Medium EPSS < 1% KEV · Not listed
Vendors
Mark3labs
Products
Mcp-go
CVE-2026-108859

mcp-go through 1.2.1 Denial of Service via Unbounded POST Body Buffering

mcp-go through 1.2.1 contains a denial of service vulnerability in StreamableHTTPServer.ServeHTTP that allows remote unauthenticated attackers to exhaust memory by sending oversized POST bodies. Attackers can send arbitrarily large or many concurrent POST requests, read fully via io.ReadAll before validation, to degrade or OOM-kill the server process.

CVSS 4.0 · 8.7 High EPSS < 1% KEV · Not listed
Vendors
Mark3labs
Products
Mcp-go
CVE-2026-81092

mcp-go before 0.56.0 Missing Host Header Validation Enables DNS Rebinding

mcp-go accepted requests on its HTTP transports without checking the Host header. StreamableHTTPServer.ServeHTTP in server/streamable_http.go and SSEServer.ServeHTTP in server/sse.go served any request arriving over a loopback connection regardless of the host it named, and the SSE transport's cross-origin default allowed any origin. A page in a browser could therefore point a name it controlled at the loopback address and reach a server listening there, invoking tools and reading resources that the server exposed on the assumption that only local software could connect. No release before 0.56.0 validated the header on either transport; 0.56.0 adds server/http_localhost.go, which rejects a loopback-bound request carrying a host that is not a loopback name, and wires it into both transports.

CVSS 4.0 · 7.6 High EPSS < 1% KEV · Not listed
Vendors
Mark3labs
Products
Mcp-go