GHSA-w4q6-qw23-4rg7

### 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 }