feat: add cookiePath config to scope session cookies to subpath

Fixes #122.

README.md

@@ -121,6 +121,7 @@ Full reference in [docs/CONFIGURATION.md](docs/CONFIGURATION.md).
 | `enablePKCE` | `false` | PKCE on the auth code flow. |
 | `cookieDomain` | auto | Set explicitly for multi-subdomain setups (`.example.com`). |
 | `cookiePrefix` | `_oidc_raczylo_` | Unique prefix per middleware instance to isolate sessions. |
+| `cookiePath` | `/` | Restrict cookies to a path prefix. Set to the middleware's path (e.g. `/app`) to prevent the browser from sending OIDC cookies to unprotected paths, avoiding 431 "Request Header Or Cookie Too Large" errors on mixed-use domains. |
 | `sessionMaxAge` | `86400` | Session lifetime in seconds. |
 | `refreshGracePeriodSeconds` | `60` | Proactively refresh tokens this many seconds before expiry. |
 | `maxRefreshTokenAgeSeconds` | `21600` | Heuristic max stored refresh-token lifetime (6h). Past this, the plugin treats the RT as expired without contacting the IdP — returns 401 to AJAX, full re-auth on navigations. Set `0` to disable. Tune to match your IdP's RT TTL. |

main.go

@@ -335,6 +335,10 @@ func NewWithContext(ctx context.Context, config *Config, next http.Handler, name
 	// Convert sessionMaxAge from seconds to duration (0 will use default 24 hours)
 	sessionMaxAge := time.Duration(config.SessionMaxAge) * time.Second
 	t.sessionManager, _ = NewSessionManager(config.SessionEncryptionKey, config.ForceHTTPS, config.CookieDomain, config.CookiePrefix, sessionMaxAge, t.logger) // Safe to ignore: session manager creation with fallback to defaults
+	if config.CookiePath != "" {
+		t.sessionManager.cookiePath = config.CookiePath
+		t.logger.Debugf("Using configured cookie path: %s", config.CookiePath)
+	}
 	t.errorRecoveryManager = NewErrorRecoveryManager(t.logger)
 
 	// Initialize token resilience manager with default configuration

session.go

@@ -382,6 +382,7 @@ type SessionManager struct {
 	cancel         context.CancelFunc
 	cookieDomain   string
 	cookiePrefix   string
+	cookiePath     string
 	sessionMaxAge  time.Duration
 	activeSessions int64
 	poolHits       int64
@@ -851,7 +852,12 @@ func (sm *SessionManager) EnhanceSessionSecurity(options *sessions.Options, r *h
 	}
 
 	options.HttpOnly = true
-	options.Path = "/" // Ensure cookies are available on all paths for OAuth flow
+	// Use configured cookie path (default "/" for backward compatibility)
+	cookiePath := sm.cookiePath
+	if cookiePath == "" {
+		cookiePath = "/"
+	}
+	options.Path = cookiePath
 
 	if sm.cookieDomain != "" {
 		options.Domain = sm.cookieDomain

settings.go

@@ -64,23 +64,30 @@ type Config struct {
 	// IdPs do not expose RT TTL on the wire, so this is intentionally a
 	// conservative heuristic; tune to match your provider configuration.
 	// Default 21600 (6h). Set to 0 to disable the check.
-	MaxRefreshTokenAgeSeconds int    `json:"maxRefreshTokenAgeSeconds"`
-	SessionMaxAge             int    `json:"sessionMaxAge"`
-	RateLimit                 int    `json:"rateLimit"`
-	OverrideScopes            bool   `json:"overrideScopes"`
-	DisableReplayDetection    bool   `json:"disableReplayDetection,omitempty"`
-	RequireTokenIntrospection bool   `json:"requireTokenIntrospection,omitempty"`
-	AllowOpaqueTokens         bool   `json:"allowOpaqueTokens,omitempty"`
-	StrictAudienceValidation  bool   `json:"strictAudienceValidation,omitempty"`
-	EnablePKCE                bool   `json:"enablePKCE"`
-	ForceHTTPS                bool   `json:"forceHTTPS"`
-	AllowPrivateIPAddresses   bool   `json:"allowPrivateIPAddresses,omitempty"`
-	MinimalHeaders            bool   `json:"minimalHeaders,omitempty"`
-	StripAuthCookies          bool   `json:"stripAuthCookies,omitempty"`
-	EnableBackchannelLogout   bool   `json:"enableBackchannelLogout,omitempty"`
-	EnableFrontchannelLogout  bool   `json:"enableFrontchannelLogout,omitempty"`
-	BackchannelLogoutURL      string `json:"backchannelLogoutURL,omitempty"`
-	FrontchannelLogoutURL     string `json:"frontchannelLogoutURL,omitempty"`
+	MaxRefreshTokenAgeSeconds int                              `json:"maxRefreshTokenAgeSeconds"`
+	SessionMaxAge             int                              `json:"sessionMaxAge"`
+	RateLimit                 int                              `json:"rateLimit"`
+	OverrideScopes            bool                             `json:"overrideScopes"`
+	DisableReplayDetection    bool                             `json:"disableReplayDetection,omitempty"`
+	RequireTokenIntrospection bool                             `json:"requireTokenIntrospection,omitempty"`
+	AllowOpaqueTokens         bool                             `json:"allowOpaqueTokens,omitempty"`
+	StrictAudienceValidation  bool                             `json:"strictAudienceValidation,omitempty"`
+	EnablePKCE                bool                             `json:"enablePKCE"`
+	ForceHTTPS                bool                             `json:"forceHTTPS"`
+	AllowPrivateIPAddresses   bool                             `json:"allowPrivateIPAddresses,omitempty"`
+	MinimalHeaders            bool                             `json:"minimalHeaders,omitempty"`
+	StripAuthCookies          bool                             `json:"stripAuthCookies,omitempty"`
+	// CookiePath restricts session cookies to a specific path prefix instead of "/".
+	// When traefikoidc protects some but not all paths on a domain, set this to the
+	// middleware's path prefix (e.g. "/app-protegido") so the browser does not send
+	// the OIDC session cookies to unprotected paths — preventing "Request Header
+	// Or Cookie Too Large" (431) errors on those paths.
+	// Default "/" (all paths, current behaviour).
+	CookiePath                string                           `json:"cookiePath,omitempty"`
+	EnableBackchannelLogout   bool                             `json:"enableBackchannelLogout,omitempty"`
+	EnableFrontchannelLogout  bool                             `json:"enableFrontchannelLogout,omitempty"`
+	BackchannelLogoutURL      string                           `json:"backchannelLogoutURL,omitempty"`
+	FrontchannelLogoutURL     string                           `json:"frontchannelLogoutURL,omitempty"`
 	// CACertPath is an optional filesystem path to a PEM-encoded CA bundle used
 	// to verify the OIDC provider's TLS certificate. Use this when the provider
 	// is signed by an internal/private CA that is not in the system trust store.