v3: the API stops trusting what it's handed — a review of api/ turned up a server anyone could crash, uploads that ran script on the app's origin, and a server that would fetch its own network on request, and this closes them along with the smaller holes around them. A WebSocket message that wasn't an object (a bare null will do) threw inside the listener, which Deno treats as uncaught and exits on; messages are now shape-checked and the handler can't throw out. Uploaded files kept the MIME type the uploader's browser claimed, so an .html or .svg dump opened from /api/files ran with the session token in reach; only images, audio, video, PDF and plain text are served inline now, the rest download, and every file carries nosniff and a sandboxing CSP (PDFs aside, which Chrome won't render sandboxed). The image proxy passed image/svg+xml straight through the same way and now takes raster formats only. Outbound fetches for previews, the proxy and Bandcamp checked the hostname against a partial list — no 169.254.169.254, no 0.0.0.0, an IPv6 check that never matched a bracketed hostname, nothing for names that resolve inward, and redirects followed blindly; every hop now resolves and is refused if any address is private or reserved, redirects are walked by hand, the curl fallback is pinned to the checked IP, and /api/preview needs a login. PATCH /users/me accepted a new password without the current one and now takes only the description and email; a wrong password at login answered 500 because the handler caught its own 401, and now says 401. Changing or resetting a password now ends every older session through users.tokens_valid_after (migration 0015) and an iat on every token, with change-password handing back fresh tokens the modal keeps, and all token checks go through one authenticate() that also refuses deleted users. Invites and reset links are claimed atomically before the async hash rather than marked after, so one invite no longer opens several accounts, and an email address belongs to one account. File and thumbnail URLs carry a media-only token rather than the session token, so a URL in a log exposes files and not the account; older sessions fetch one on load. Private things stop leaking into broadcasts: private playlist updates go to their owner, votes and likes on someone else's private dump 404 and their counts go to the owner alone, and a private dump can't be slipped into a playlist. Smaller: selfProvider only claims this site's own host, so github.com/users/bob stops rendering our bob; the chat read marker can't be parked in the future on an empty chat; dump mutations use the resolved id so a slug works; multipart dumps get the title and comment limits; logins and reset requests are rate-limited through a shared token bucket; the password compare is constant-time; ffmpeg reads only local files in known video containers.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

api/lib/public-address.ts

@@ -0,0 +1,137 @@
+/**
+ * Guards for server-side fetches of user-supplied URLs (link previews, the
+ * image proxy, Bandcamp resolution): the server must not become a way to reach
+ * addresses only it can see — loopback, the LAN, container networks, or the
+ * cloud metadata endpoint at 169.254.169.254.
+ */
+
+function parseIPv4(s: string): number[] | null {
+  const parts = s.split(".");
+  if (parts.length !== 4) return null;
+  const bytes = parts.map((p) => /^\d{1,3}$/.test(p) ? Number(p) : NaN);
+  return bytes.every((b) => b >= 0 && b <= 255) ? bytes : null;
+}
+
+function isPrivateIPv4([a, b]: number[]): boolean {
+  return a === 0 || // "this" network
+    a === 10 ||
+    a === 127 ||
+    (a === 100 && b >= 64 && b <= 127) || // carrier-grade NAT
+    (a === 169 && b === 254) || // link-local, cloud metadata
+    (a === 172 && b >= 16 && b <= 31) ||
+    (a === 192 && b === 0) || // IETF protocol assignments (192.0.0.0/24 …)
+    (a === 192 && b === 168) ||
+    (a === 198 && (b === 18 || b === 19)) || // benchmarking
+    a >= 224; // multicast, reserved, broadcast
+}
+
+/** 16 bytes, or null when `s` isn't an IPv6 address. */
+function parseIPv6(s: string): number[] | null {
+  let addr = s.toLowerCase().replace(/%.*$/, ""); // drop a zone id
+  // An embedded dotted quad (::ffff:10.0.0.1) becomes the last two groups.
+  const lastColon = addr.lastIndexOf(":");
+  if (lastColon === -1) return null;
+  const tail = addr.slice(lastColon + 1);
+  if (tail.includes(".")) {
+    const v4 = parseIPv4(tail);
+    if (!v4) return null;
+    addr = `${addr.slice(0, lastColon + 1)}${
+      ((v4[0] << 8) | v4[1]).toString(16)
+    }:${((v4[2] << 8) | v4[3]).toString(16)}`;
+  }
+  const halves = addr.split("::");
+  if (halves.length > 2) return null;
+  const head = halves[0] ? halves[0].split(":") : [];
+  const rest = halves.length === 2 && halves[1] ? halves[1].split(":") : [];
+  const fill = 8 - head.length - rest.length;
+  if (halves.length === 1 ? fill !== 0 : fill < 1) return null;
+  const groups = [...head, ...Array(halves.length === 2 ? fill : 0).fill("0"), ...rest];
+  if (!groups.every((g) => /^[0-9a-f]{1,4}$/.test(g))) return null;
+  return groups.flatMap((g) => {
+    const n = parseInt(g, 16);
+    return [n >> 8, n & 0xff];
+  });
+}
+
+function isPrivateIPv6(b: number[]): boolean {
+  const zeroPrefix = (n: number) => b.slice(0, n).every((x) => x === 0);
+  // IPv4 carried inside IPv6 is judged as the IPv4 address it carries.
+  if (zeroPrefix(10) && b[10] === 0xff && b[11] === 0xff) { // ::ffff:a.b.c.d
+    return isPrivateIPv4(b.slice(12));
+  }
+  if (zeroPrefix(12)) { // ::, ::1 and the deprecated ::a.b.c.d
+    return true;
+  }
+  if ( // 64:ff9b::/96 NAT64
+    b[0] === 0 && b[1] === 0x64 && b[2] === 0xff && b[3] === 0x9b &&
+    b.slice(4, 12).every((x) => x === 0)
+  ) return isPrivateIPv4(b.slice(12));
+  if (b[0] === 0x20 && b[1] === 0x02) { // 2002::/16 6to4
+    return isPrivateIPv4(b.slice(2, 6));
+  }
+  return (b[0] & 0xfe) === 0xfc || // fc00::/7 unique local
+    (b[0] === 0xfe && (b[1] & 0xc0) === 0x80) || // fe80::/10 link-local
+    (b[0] === 0xfe && (b[1] & 0xc0) === 0xc0) || // fec0::/10 site-local
+    b[0] === 0xff; // multicast
+}
+
+/** True for an IP literal outside the public internet. Non-IPs are false. */
+export function isPrivateAddress(ip: string): boolean {
+  const v4 = parseIPv4(ip);
+  if (v4) return isPrivateIPv4(v4);
+  const v6 = parseIPv6(ip);
+  return v6 ? isPrivateIPv6(v6) : false;
+}
+
+function isIpLiteral(host: string): boolean {
+  return parseIPv4(host) !== null || parseIPv6(host) !== null;
+}
+
+/** `URL.hostname` keeps the brackets around an IPv6 literal. */
+function bareHostname(url: URL): string {
+  return url.hostname.replace(/^\[(.*)\]$/, "$1");
+}
+
+/**
+ * The synchronous part of the check: a hostname that is a private IP literal,
+ * or a name that is local by definition. Names still need resolving (see
+ * resolvePublicAddress) — this only rejects what is obvious without DNS.
+ */
+export function isPrivateHostname(url: URL): boolean {
+  const host = bareHostname(url).toLowerCase();
+  if (host === "localhost" || host.endsWith(".localhost")) return true;
+  return isPrivateAddress(host);
+}
+
+/**
+ * Resolves `url`'s host and returns one of its addresses, or throws if the
+ * host is private or any address it resolves to is (a name with both a public
+ * and a private record is refused outright, since the fetch may pick either).
+ *
+ * The fetch itself resolves again, so a DNS server answering differently the
+ * second time (rebinding) isn't fully excluded; the curl fallback pins the
+ * address this returns.
+ */
+export async function resolvePublicAddress(url: URL): Promise<string> {
+  if (isPrivateHostname(url)) {
+    throw new Error(`Refusing to fetch private address ${url.hostname}`);
+  }
+  const host = bareHostname(url);
+  if (isIpLiteral(host)) return host;
+
+  const results = await Promise.allSettled([
+    Deno.resolveDns(host, "A"),
+    Deno.resolveDns(host, "AAAA"),
+  ]);
+  const addresses = results.flatMap((r) =>
+    r.status === "fulfilled" ? r.value : []
+  );
+  if (addresses.length === 0) {
+    throw new Error(`Could not resolve ${host}`);
+  }
+  const blocked = addresses.find(isPrivateAddress);
+  if (blocked) {
+    throw new Error(`Refusing to fetch ${host}: resolves to ${blocked}`);
+  }
+  return addresses[0];
+}