fix(fleet): bound the token an enrolling server can make us write

src/fleet/enrol-client.ts

@@ -13,6 +13,17 @@ import { resolveMachineId, type MachineIdentityDeps } from "./machine-identity.j
 
 export const ENROL_PATH = "/api/agent/enrol";
 const DEFAULT_TIMEOUT_MS = 30_000;
+
+/**
+ * The longest token this will accept and write to disk.
+ *
+ * A real one is `cgm_` plus 32 base64url bytes — under fifty characters. The
+ * bound exists because the value comes from whatever server the operator
+ * pointed at: without it, a hostile or broken one can return a gigabyte of
+ * string and this writes all of it into the user's home directory. Generous
+ * enough that a longer token format would still fit.
+ */
+const MAX_TOKEN_LENGTH = 1024;
 /** The token is a fleet credential; nobody else on the machine needs it. */
 const FILE_MODE = 0o600;
 const DIR_MODE = 0o700;
@@ -90,6 +101,13 @@ export async function enrolMachine(
     return { ok: false, reason: `${url} did not return a token` };
   }
 
+  if (body.token.length > MAX_TOKEN_LENGTH) {
+    return {
+      ok: false,
+      reason: `${url} returned a token of ${body.token.length} characters, which is not a token this server should be issuing`,
+    };
+  }
+
   const configPath = fleetConfigPath(deps);
   const contents = `${JSON.stringify({ server, token: body.token }, null, 2)}\n`;
 

tests/fleet/enrol-client.test.ts

@@ -1,4 +1,4 @@
-import { mkdtempSync, readFileSync, statSync } from "node:fs";
+import { existsSync, mkdtempSync, readFileSync, statSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
 import { describe, expect, it } from "vitest";
@@ -201,6 +201,31 @@ describe("enrolMachine", () => {
     expect(result.reason).toContain("did not return a token");
   });
 
+  // The token is whatever the server sends, and this writes it into the
+  // user's home directory. A hostile or broken server returning an enormous
+  // string should be refused rather than persisted.
+  it("refuses a token far larger than any real one, instead of writing it", async () => {
+    const home = tempHome();
+    const huge = `cgm_${"A".repeat(200_000)}`;
+
+    const result = await enrolMachine(
+      { server: "https://g.example", code: "C" },
+      {
+        homeDir: () => home,
+        fetch: (async () =>
+          new Response(JSON.stringify({ token: huge }), {
+            status: 200,
+            headers: { "content-type": "application/json" },
+          })) as unknown as typeof fetch,
+      },
+    );
+
+    expect(result.ok).toBe(false);
+    if (result.ok) return;
+    expect(result.reason).toContain("not a token this server should be issuing");
+    expect(existsSync(fleetConfigPath({ homeDir: () => home }))).toBe(false);
+  });
+
   it("says so when enrolment worked but the config could not be written", async () => {
     const result = await enrolMachine(
       { server: "https://g.example", code: "C" },