diff --git a/server/types/config.ts b/server/types/config.ts
index acebb9c..f369ccb 100644
--- a/server/types/config.ts
+++ b/server/types/config.ts
@@ -1,7 +1,5 @@
 import {ArrayField, Field, RawJsonField} from "sparkson"
 
-export type Version = string;
-
 // TODO: Replace string types by more specific types like URL, Password, etc.
 
 export class LoggingConfig {
diff --git a/server/types/index.ts b/server/types/index.ts
index 8b27f9d..bc0e85e 100644
--- a/server/types/index.ts
+++ b/server/types/index.ts
@@ -1,5 +1,6 @@
 export * from "./config";
 export * from "./logger";
+export * from "./shared";
 
 // TODO: Token type.
 export type Token = string;
@@ -48,16 +49,6 @@ export type NodeSecrets = {
     monitoringToken?: MonitoringToken,
 };
 
-export type NodeStatistics = {
-    registered: number,
-    withVPN: number,
-    withCoords: number,
-    monitoring: {
-        active: number,
-        pending: number
-    }
-};
-
 export type MailId = string;
 export type MailData = any;
 export type MailType = string;
diff --git a/server/types/shared.ts b/server/types/shared.ts
new file mode 100644
index 0000000..4d143bd
--- /dev/null
+++ b/server/types/shared.ts
@@ -0,0 +1,45 @@
+// Types shared with the client.
+
+export function isObject(arg: unknown): arg is object {
+    return arg !== null && typeof arg === "object";
+}
+
+export type Version = string;
+
+export function isVersion(arg: unknown): arg is Version {
+    // Should be good enough for now.
+    return typeof arg === "string";
+}
+
+export type NodeStatistics = {
+    registered: number;
+    withVPN: number;
+    withCoords: number;
+    monitoring: {
+        active: number;
+        pending: number;
+    };
+};
+
+export function isNodeStatistics(arg: unknown): arg is NodeStatistics {
+    if (!isObject(arg)) {
+        return false;
+    }
+    const stats = arg as NodeStatistics;
+    return (
+        typeof stats.registered === "number" &&
+        typeof stats.withVPN === "number" &&
+        typeof stats.withCoords === "number" &&
+        typeof stats.monitoring === "object" &&
+        typeof stats.monitoring.active === "number" &&
+        typeof stats.monitoring.pending === "number"
+    );
+}
+
+export interface Statistics {
+    nodes: NodeStatistics;
+}
+
+export function isStatistics(arg: unknown): arg is Statistics {
+    return isObject(arg) && isNodeStatistics((arg as Statistics).nodes);
+}
diff --git a/shell.nix b/shell.nix
index 6934ce6..96db906 100644
--- a/shell.nix
+++ b/shell.nix
@@ -11,6 +11,7 @@ stdenv.mkDerivation rec {
     nasm
     nodejs-16_x
     rsync
+    sass
     sqlite
     yarn
     zlib