security: add maximum session limit to prevent memory exhaustion

Add configurable MaxSessions limit (default: 10000) to SessionStore.
When the limit is reached, new session creation returns ErrTooManySessions
and HTTP transport responds with 503 Service Unavailable.

This prevents attackers from exhausting server memory by creating
unlimited sessions through repeated initialize requests.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/mcp/session.go

@@ -3,6 +3,7 @@ package mcp
 import (
 	"crypto/rand"
 	"encoding/hex"
+	"fmt"
 	"sync"
 	"time"
 )
@@ -82,16 +83,32 @@ func (s *Session) Close() {
 type SessionStore struct {
 	sessions    map[string]*Session
 	ttl         time.Duration
+	maxSessions int
 	mu          sync.RWMutex
 	stopClean   chan struct{}
 	cleanDone   chan struct{}
 }
 
+// ErrTooManySessions is returned when the session limit is reached.
+var ErrTooManySessions = fmt.Errorf("too many active sessions")
+
+// DefaultMaxSessions is the default maximum number of concurrent sessions.
+const DefaultMaxSessions = 10000
+
 // NewSessionStore creates a new session store with the given TTL.
 func NewSessionStore(ttl time.Duration) *SessionStore {
+	return NewSessionStoreWithLimit(ttl, DefaultMaxSessions)
+}
+
+// NewSessionStoreWithLimit creates a new session store with TTL and max session limit.
+func NewSessionStoreWithLimit(ttl time.Duration, maxSessions int) *SessionStore {
+	if maxSessions <= 0 {
+		maxSessions = DefaultMaxSessions
+	}
 	s := &SessionStore{
 		sessions:    make(map[string]*Session),
 		ttl:         ttl,
+		maxSessions: maxSessions,
 		stopClean:   make(chan struct{}),
 		cleanDone:   make(chan struct{}),
 	}
@@ -100,14 +117,21 @@ func NewSessionStore(ttl time.Duration) *SessionStore {
 }
 
 // Create creates a new session and adds it to the store.
+// Returns ErrTooManySessions if the maximum session limit is reached.
 func (s *SessionStore) Create() (*Session, error) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	// Check session limit
+	if len(s.sessions) >= s.maxSessions {
+		return nil, ErrTooManySessions
+	}
+
 	session, err := NewSession()
 	if err != nil {
 		return nil, err
 	}
 
-	s.mu.Lock()
-	defer s.mu.Unlock()
 	s.sessions[session.ID] = session
 	return session, nil
 }

internal/mcp/session_test.go

@@ -245,6 +245,76 @@ func TestSessionStoreConcurrency(t *testing.T) {
 	wg.Wait()
 }
 
+func TestSessionStoreMaxSessions(t *testing.T) {
+	maxSessions := 5
+	store := NewSessionStoreWithLimit(30*time.Minute, maxSessions)
+	defer store.Stop()
+
+	// Create sessions up to limit
+	for i := 0; i < maxSessions; i++ {
+		_, err := store.Create()
+		if err != nil {
+			t.Fatalf("Failed to create session %d: %v", i, err)
+		}
+	}
+
+	if store.Count() != maxSessions {
+		t.Errorf("Expected %d sessions, got %d", maxSessions, store.Count())
+	}
+
+	// Try to create one more - should fail
+	_, err := store.Create()
+	if err != ErrTooManySessions {
+		t.Errorf("Expected ErrTooManySessions, got %v", err)
+	}
+
+	// Count should still be at max
+	if store.Count() != maxSessions {
+		t.Errorf("Expected %d sessions after failed create, got %d", maxSessions, store.Count())
+	}
+}
+
+func TestSessionStoreMaxSessionsWithDeletion(t *testing.T) {
+	maxSessions := 3
+	store := NewSessionStoreWithLimit(30*time.Minute, maxSessions)
+	defer store.Stop()
+
+	// Fill up the store
+	sessions := make([]*Session, maxSessions)
+	for i := 0; i < maxSessions; i++ {
+		s, err := store.Create()
+		if err != nil {
+			t.Fatalf("Failed to create session: %v", err)
+		}
+		sessions[i] = s
+	}
+
+	// Should be full
+	_, err := store.Create()
+	if err != ErrTooManySessions {
+		t.Error("Expected ErrTooManySessions when full")
+	}
+
+	// Delete one session
+	store.Delete(sessions[0].ID)
+
+	// Should be able to create again
+	_, err = store.Create()
+	if err != nil {
+		t.Errorf("Should be able to create after deletion: %v", err)
+	}
+}
+
+func TestSessionStoreDefaultMaxSessions(t *testing.T) {
+	store := NewSessionStore(30 * time.Minute)
+	defer store.Stop()
+
+	// Just verify it uses the default (don't create 10000 sessions)
+	if store.maxSessions != DefaultMaxSessions {
+		t.Errorf("Expected default max sessions %d, got %d", DefaultMaxSessions, store.maxSessions)
+	}
+}
+
 func TestGenerateSessionID(t *testing.T) {
 	ids := make(map[string]bool)
 

internal/mcp/transport_http.go

@@ -17,6 +17,7 @@ type HTTPConfig struct {
 	Endpoint          string        // MCP endpoint path (e.g., "/mcp")
 	AllowedOrigins    []string      // Allowed Origin headers for CORS (empty = localhost only)
 	SessionTTL        time.Duration // Session TTL (default: 30 minutes)
+	MaxSessions       int           // Maximum concurrent sessions (default: 10000)
 	TLSCertFile       string        // TLS certificate file (optional)
 	TLSKeyFile        string        // TLS key file (optional)
 	MaxRequestSize    int64         // Maximum request body size in bytes (default: 1MB)
@@ -55,6 +56,9 @@ func NewHTTPTransport(server *Server, config HTTPConfig) *HTTPTransport {
 	if config.SessionTTL == 0 {
 		config.SessionTTL = 30 * time.Minute
 	}
+	if config.MaxSessions == 0 {
+		config.MaxSessions = DefaultMaxSessions
+	}
 	if config.MaxRequestSize == 0 {
 		config.MaxRequestSize = DefaultMaxRequestSize
 	}
@@ -74,7 +78,7 @@ func NewHTTPTransport(server *Server, config HTTPConfig) *HTTPTransport {
 	return &HTTPTransport{
 		server:   server,
 		config:   config,
-		sessions: NewSessionStore(config.SessionTTL),
+		sessions: NewSessionStoreWithLimit(config.SessionTTL, config.MaxSessions),
 	}
 }
 
@@ -231,6 +235,11 @@ func (t *HTTPTransport) handleInitialize(w http.ResponseWriter, r *http.Request,
 	// Create a new session
 	session, err := t.sessions.Create()
 	if err != nil {
+		if err == ErrTooManySessions {
+			t.server.logger.Printf("Session limit reached")
+			http.Error(w, "Service unavailable: too many active sessions", http.StatusServiceUnavailable)
+			return
+		}
 		t.server.logger.Printf("Failed to create session: %v", err)
 		http.Error(w, "Failed to create session", http.StatusInternalServerError)
 		return

internal/mcp/transport_http_test.go

@@ -582,6 +582,50 @@ func TestHTTPTransportRequestBodyTooLarge(t *testing.T) {
 	}
 }
 
+func TestHTTPTransportSessionLimitReached(t *testing.T) {
+	_, ts := testHTTPTransport(t, HTTPConfig{
+		MaxSessions: 2, // Very low limit for testing
+	})
+
+	initReq := Request{
+		JSONRPC: "2.0",
+		ID:      1,
+		Method:  MethodInitialize,
+		Params:  json.RawMessage(`{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"test","version":"1.0"}}`),
+	}
+	body, _ := json.Marshal(initReq)
+
+	// Create sessions up to the limit
+	for i := 0; i < 2; i++ {
+		req, _ := http.NewRequest("POST", ts.URL+"/mcp", bytes.NewReader(body))
+		req.Header.Set("Content-Type", "application/json")
+
+		resp, err := http.DefaultClient.Do(req)
+		if err != nil {
+			t.Fatalf("Request %d failed: %v", i, err)
+		}
+		resp.Body.Close()
+
+		if resp.StatusCode != http.StatusOK {
+			t.Errorf("Request %d: expected 200, got %d", i, resp.StatusCode)
+		}
+	}
+
+	// Third request should fail with 503
+	req, _ := http.NewRequest("POST", ts.URL+"/mcp", bytes.NewReader(body))
+	req.Header.Set("Content-Type", "application/json")
+
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatalf("Request failed: %v", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusServiceUnavailable {
+		t.Errorf("Expected 503 when session limit reached, got %d", resp.StatusCode)
+	}
+}
+
 func TestHTTPTransportRequestBodyWithinLimit(t *testing.T) {
 	_, ts := testHTTPTransport(t, HTTPConfig{
 		MaxRequestSize: 10000, // Reasonable limit