fix: malformed stream path escape returns routing, not a 500

CHANGELOG.md

@@ -39,6 +39,10 @@ refused in every mode, including `allow_all`, and the browser's WebRTC traffic n
 filter instead of around it. A computer run without an API server can set
 `EGRESS_POLICY_REQUIRED=0` to keep the old behaviour.
 
+### A malformed `%` in a stream URL no longer returns a 500
+
+A request to `/api/computers/<id>/stream` whose id held a broken percent-escape, such as `%zz`, made the server throw and answer 500. It is now treated as not matching the stream route and goes through normal routing. Valid ids behave as before.
+
 ### `start.sh` names the port to change on macOS
 
 When the API server's or the app's port was held by another process, `start.sh` was meant to say

server/src/computer/stream-path.ts

@@ -0,0 +1,18 @@
+/**
+ * Which Bot's screen. The Bot is named in the path and its computer is located the same way every
+ * other call locates it, so the live stream cannot point at a different Bot's browser.
+ *
+ * Its own module so it can be tested without starting a server: `index.ts` calls `serve()` at
+ * module scope, so importing it to reach one pure function binds a port.
+ */
+export function streamPathBotId(pathname: string): string | null {
+  const match = pathname.match(/^\/api\/computers\/([^/]+)\/stream$/);
+  if (!match?.[1]) return null;
+  try {
+    return decodeURIComponent(match[1]);
+  } catch {
+    // A malformed escape is not a Bot id. Decoding it used to throw URIError out of the fetch
+    // handler, turning one bad URL into a 500; let normal routing answer it instead.
+    return null;
+  }
+}

server/src/index.ts

@@ -73,6 +73,7 @@ import {
   parseApprovalContinuation,
   parseApprovalResult,
 } from "./approvals/types";
+import { streamPathBotId } from "./computer/stream-path";
 import {
   type AuditInitiator,
   createAuditReader,
@@ -3057,15 +3058,6 @@ const toStreamUrl = (baseUrl: string, botId: string) =>
   // be reachable without it.
   `${baseUrl.replace(/^http/, "ws").replace(/\/$/, "")}/stream?bot=${encodeURIComponent(botId)}&token=${encodeURIComponent(config.computer?.token ?? "")}`;
 
-/**
- * Which Bot's screen. The Bot is named in the path and its computer is located the same way every
- * other call locates it, so the live stream cannot point at a different Bot's browser.
- */
-const streamPathBotId = (pathname: string): string | null => {
-  const match = pathname.match(/^\/api\/computers\/([^/]+)\/stream$/);
-  return match?.[1] ? decodeURIComponent(match[1]) : null;
-};
-
 /** What each proxied socket carries: where to connect inward, and the socket once opened. */
 type StreamData = {
   upstream: string;

server/tests/computer-stream-path.test.ts

@@ -0,0 +1,23 @@
+import { describe, expect, test } from "bun:test";
+import { streamPathBotId } from "../src/computer/stream-path";
+
+describe("which Bot a stream path names", () => {
+  test("a plain id reads as itself", () => {
+    expect(streamPathBotId("/api/computers/my-bot/stream")).toBe("my-bot");
+  });
+
+  test("a percent-encoded id decodes", () => {
+    expect(streamPathBotId("/api/computers/my%20bot/stream")).toBe("my bot");
+  });
+
+  test("a malformed escape is not a Bot id, not a 500", () => {
+    expect(streamPathBotId("/api/computers/%zz/stream")).toBeNull();
+    expect(streamPathBotId("/api/computers/%E2%28/stream")).toBeNull();
+  });
+
+  test("other paths name no Bot", () => {
+    expect(streamPathBotId("/api/computers/my-bot/files")).toBeNull();
+    expect(streamPathBotId("/api/computers/stream")).toBeNull();
+    expect(streamPathBotId("/")).toBeNull();
+  });
+});