Summary

A nil pointer dereference vulnerability in the GitHub MCP Server causes it to crash when receiving a malformed completion/complete request with missing or empty parameters. This allows any unauthenticated client to cause a complete denial of service.

Details

The CompletionsHandler function in pkg/github/server.go:198 accesses params.Ref without checking if it's nil first. When a client sends a completion/complete request with a missing ref field, the handler dereferences nil and the Go runtime panics.

The crash occurs before any authentication or token validation, so even requests with fake tokens can trigger it.

PoC

After completing the MCP initialization handshake, send either:

Empty params:

{"jsonrpc":"2.0","id":2,"method":"completion/complete","params":{}}

Missing ref field:

{"jsonrpc":"2.0","id":2,"method":"completion/complete","params":{"argument":{"name":"x","value":"y"}}}

Result:

panic: runtime error: invalid memory address or nil pointer dereference
goroutine 42 [running]:
github.com/github/github-mcp-server/pkg/github.NewMCPServer.CompletionsHandler.func1(...)
    pkg/github/server.go:198 +0x24

Impact

Any unauthenticated client that can send JSON-RPC messages to the server can crash it immediately. This is a complete denial of service - the panic is unrecoverable and kills the process.

Automated fuzzing with mcpsec found 108 crashes out of 925 test cases (11.7% crash rate).

Timeline

• Feb 21, 2026 - Initial report sent to opensource-security@github.com
• Mar 03, 2026 - Follow-up email sent, no response
• Mar 21, 2026 - Re-verified on v0.33.0, sent detailed report with PoC, no response
• Apr 06, 2026 - GHSA filed after 44 days without acknowledgment

Suggested Fix

func (s *Server) CompletionsHandler(ctx context.Context, params *mcp.CompleteParams) (*mcp.CompleteResult, error) {
    if params == nil || params.Ref == nil {
        return nil, fmt.Errorf("invalid request: missing ref parameter")
    }
    // ... rest of handler
}