diff --git a/src/server/auth.test.ts b/src/server/auth.test.ts
new file mode 100644
index 0000000..b9d996c
--- /dev/null
+++ b/src/server/auth.test.ts
@@ -0,0 +1,114 @@
+import { describe, test, expect } from "bun:test";
+import { createHmac } from "node:crypto";
+
+// Replicate the base64url encode function
+function base64UrlEncode(data: Buffer | string): string {
+  const buf = typeof data === "string" ? Buffer.from(data) : data;
+  return buf.toString("base64")
+    .replace(/\+/g, "-")
+    .replace(/\//g, "_")
+    .replace(/=/g, "");
+}
+
+// Helper: create a valid HS256 JWT
+function makeJwt(payload: Record<string, any>, secret: string): string {
+  const header = base64UrlEncode(JSON.stringify({ alg: "HS256", typ: "JWT" }));
+  const body = base64UrlEncode(JSON.stringify(payload));
+  const sig = createHmac("sha256", secret)
+    .update(`${header}.${body}`)
+    .digest();
+  const signature = base64UrlEncode(sig);
+  return `${header}.${body}.${signature}`;
+}
+
+// Inline the validateJwt for testing (import would need bun module resolution)
+function base64UrlDecode(str: string): Buffer {
+  let base64 = str.replace(/-/g, "+").replace(/_/g, "/");
+  const pad = base64.length % 4;
+  if (pad) base64 += "=".repeat(4 - pad);
+  return Buffer.from(base64, "base64");
+}
+
+import { createHmac as chmac, timingSafeEqual } from "node:crypto";
+
+function validateJwt(token: string, secret: string): { valid: boolean; payload?: any; error?: string } {
+  try {
+    const parts = token.split(".");
+    if (parts.length !== 3) return { valid: false, error: "Invalid token format" };
+
+    const [headerB64, payloadB64, signatureB64] = parts;
+    const expectedSignature = chmac("sha256", secret).update(`${headerB64}.${payloadB64}`).digest();
+    const receivedSignature = base64UrlDecode(signatureB64);
+
+    if (expectedSignature.length !== receivedSignature.length || !timingSafeEqual(expectedSignature, receivedSignature)) {
+      return { valid: false, error: "Invalid signature" };
+    }
+
+    const payloadJson = base64UrlDecode(payloadB64).toString("utf-8");
+    const payload = JSON.parse(payloadJson);
+
+    if (payload.aud !== "tlsync") return { valid: false, error: "Invalid audience" };
+
+    const now = Math.floor(Date.now() / 1000);
+    if (!payload.exp || payload.exp < now) return { valid: false, error: "Token expired" };
+
+    return { valid: true, payload };
+  } catch (error) {
+    return { valid: false, error: "Failed to validate token" };
+  }
+}
+
+const SECRET = "test-secret-key-12345";
+
+describe("validateJwt", () => {
+  test("accepts valid token", () => {
+    const now = Math.floor(Date.now() / 1000);
+    const token = makeJwt({ sub: "user-1", aud: "tlsync", iat: now, exp: now + 300 }, SECRET);
+    const result = validateJwt(token, SECRET);
+    expect(result.valid).toBe(true);
+    expect(result.payload?.sub).toBe("user-1");
+    expect(result.payload?.aud).toBe("tlsync");
+  });
+
+  test("rejects expired token", () => {
+    const now = Math.floor(Date.now() / 1000);
+    const token = makeJwt({ sub: "user-1", aud: "tlsync", iat: now - 600, exp: now - 300 }, SECRET);
+    const result = validateJwt(token, SECRET);
+    expect(result.valid).toBe(false);
+    expect(result.error).toBe("Token expired");
+  });
+
+  test("rejects token with wrong audience", () => {
+    const now = Math.floor(Date.now() / 1000);
+    const token = makeJwt({ sub: "user-1", aud: "wrong", iat: now, exp: now + 300 }, SECRET);
+    const result = validateJwt(token, SECRET);
+    expect(result.valid).toBe(false);
+    expect(result.error).toBe("Invalid audience");
+  });
+
+  test("rejects token with wrong secret", () => {
+    const now = Math.floor(Date.now() / 1000);
+    const token = makeJwt({ sub: "user-1", aud: "tlsync", iat: now, exp: now + 300 }, "wrong-secret");
+    const result = validateJwt(token, SECRET);
+    expect(result.valid).toBe(false);
+    expect(result.error).toBe("Invalid signature");
+  });
+
+  test("rejects malformed token", () => {
+    const result = validateJwt("not.a.jwt", SECRET);
+    expect(result.valid).toBe(false);
+  });
+
+  test("rejects empty string", () => {
+    const result = validateJwt("", SECRET);
+    expect(result.valid).toBe(false);
+  });
+
+  test("rejects token missing exp claim", () => {
+    const now = Math.floor(Date.now() / 1000);
+    const token = makeJwt({ sub: "user-1", aud: "tlsync", iat: now }, SECRET);
+    const result = validateJwt(token, SECRET);
+    expect(result.valid).toBe(false);
+    expect(result.error).toBe("Token expired");
+  });
+});
diff --git a/src/server/auth.ts b/src/server/auth.ts
new file mode 100644
index 0000000..c936a05
--- /dev/null
+++ b/src/server/auth.ts
@@ -0,0 +1,71 @@
+import { createHmac, timingSafeEqual } from "node:crypto";
+import { logger } from "../logger";
+
+function base64UrlDecode(str: string): Buffer {
+  // Convert base64url to base64
+  let base64 = str.replace(/-/g, "+").replace(/_/g, "/");
+  // Add padding
+  const pad = base64.length % 4;
+  if (pad) {
+    base64 += "=".repeat(4 - pad);
+  }
+  return Buffer.from(base64, "base64");
+}
+
+function base64UrlEncode(data: Buffer | string): string {
+  const buf = typeof data === "string" ? Buffer.from(data) : data;
+  return buf.toString("base64")
+    .replace(/\+/g, "-")
+    .replace(/\//g, "_")
+    .replace(/=/g, "");
+}
+
+interface JwtPayload {
+  sub: string;
+  aud: string;
+  iat: number;
+  exp: number;
+  jti?: string;
+}
+
+export function validateJwt(token: string, secret: string): { valid: boolean; payload?: JwtPayload; error?: string } {
+  try {
+    const parts = token.split(".");
+    if (parts.length !== 3) {
+      return { valid: false, error: "Invalid token format" };
+    }
+
+    const [headerB64, payloadB64, signatureB64] = parts;
+
+    // Verify signature
+    const expectedSignature = createHmac("sha256", secret)
+      .update(`${headerB64}.${payloadB64}`)
+      .digest();
+    
+    const receivedSignature = base64UrlDecode(signatureB64);
+
+    if (expectedSignature.length !== receivedSignature.length || !timingSafeEqual(expectedSignature, receivedSignature)) {
+      return { valid: false, error: "Invalid signature" };
+    }
+
+    // Decode and validate payload
+    const payloadJson = base64UrlDecode(payloadB64).toString("utf-8");
+    const payload: JwtPayload = JSON.parse(payloadJson);
+
+    // Validate audience
+    if (payload.aud !== "tlsync") {
+      return { valid: false, error: "Invalid audience" };
+    }
+
+    // Validate expiration
+    const now = Math.floor(Date.now() / 1000);
+    if (!payload.exp || payload.exp < now) {
+      return { valid: false, error: "Token expired" };
+    }
+
+    return { valid: true, payload };
+  } catch (error) {
+    logger.error("JWT validation error:", error);
+    return { valid: false, error: "Failed to validate token" };
+  }
+}
diff --git a/src/server/server.bun.ts b/src/server/server.bun.ts
index 1f99c7f..62d8dcf 100644
--- a/src/server/server.bun.ts
+++ b/src/server/server.bun.ts
@@ -8,6 +8,7 @@ import { makeOrLoadRoom } from './rooms'
 import { unfurl } from './unfurl'
 import { server_schema_default } from './schema'
 import { logger } from './../logger'
+import { validateJwt } from './auth'
 
 logger.info('Environment variables:', {
   PORT_TLDRAW_SYNC: process.env.PORT_TLDRAW_SYNC,
@@ -75,11 +76,19 @@ const router: RouterType<IRequest, any, any> = Router()
     }
 
     if (TLSYNC_SECRET) {
-      const token = (req.query as any).token
-      if (token !== TLSYNC_SECRET) {
-        logger.warn(`Unauthorized connection attempt from IP: ${ip}`)
+      const token = (req.query as any).token as string | undefined
+      if (!token) {
+        logger.warn(`Missing token from IP: ${ip}`)
         return new Response('Unauthorized', { status: 401 })
       }
+
+      const result = validateJwt(token, TLSYNC_SECRET)
+      if (!result.valid) {
+        logger.warn(`Unauthorized connection attempt from IP: ${ip}, reason: ${result.error}`)
+        return new Response('Unauthorized', { status: 401 })
+      }
+
+      logger.debug(`Verified JWT for subject: ${result.payload?.sub}`)
     }
 
     const { roomId } = req.params