chore: bring back x-auth-checks with a length limit (#19928)

coderd/coderd.go

@@ -489,16 +489,8 @@ func New(options *Options) *API {
 	r := chi.NewRouter()
 	// We add this middleware early, to make sure that authorization checks made
 	// by other middleware get recorded.
-	//nolint:revive,staticcheck // This block will be re-enabled, not going to remove it
 	if buildinfo.IsDev() {
-		// TODO: Find another solution to opt into these checks.
-		//   If the header grows too large, it breaks `fetch()` requests.
-		//   Temporarily disabling this until we can find a better solution.
-		//	 One idea is to include checking the request for `X-Authz-Record=true`
-		//   header. To opt in on a per-request basis.
-		//   Some authz calls (like filtering lists) might be able to be
-		//   summarized better to condense the header payload.
-		// r.Use(httpmw.RecordAuthzChecks)
+		r.Use(httpmw.RecordAuthzChecks)
 	}
 
 	ctx, cancel := context.WithCancel(context.Background())

coderd/httpapi/authz.go

@@ -9,6 +9,14 @@ import (
 	"github.com/coder/coder/v2/coderd/rbac"
 )
 
+// The x-authz-checks header can end up being >10KB in size on certain queries.
+// Many HTTP clients will fail if a header or the response head as a whole is
+// too long to prevent malicious responses from consuming all of the client's
+// memory. I've seen reports that browsers have this limit as low as 4KB for the
+// entire response head, so we limit this header to a little less than 2KB,
+// ensuring there's still plenty of room for the usual smaller headers.
+const maxHeaderLength = 2000
+
 // This is defined separately in slim builds to avoid importing the rbac
 // package, which is a large dependency.
 func SetAuthzCheckRecorderHeader(ctx context.Context, rw http.ResponseWriter) {
@@ -23,6 +31,11 @@ func SetAuthzCheckRecorderHeader(ctx context.Context, rw http.ResponseWriter) {
 		//   configured on server startup for development and testing builds.
 		// - If this header is missing from a response, make sure the response is
 		//   being written by calling `httpapi.Write`!
-		rw.Header().Set("x-authz-checks", rec.String())
+		checks := rec.String()
+		if len(checks) > maxHeaderLength {
+			checks = checks[:maxHeaderLength]
+			checks += "<truncated>"
+		}
+		rw.Header().Set("x-authz-checks", checks)
 	}
 }