#712: read the invite link from the URL fragment (#join=), never sent to the web host; ignore overlong links

apps/figcraft-web/src/main.rs

@@ -13,10 +13,12 @@
 //!   (browsers block `ws://` from secure pages); serve it over `http://` on a trusted network to
 //!   join from other machines.
 //!
-//! URL query: `?webgl` forces the WebGL2 backend instead of WebGPU; `?join=<invite link or join
-//! code>` joins that live session once the page has loaded, so a bookmark opens a shared file.
-//! `join` takes the rest of the query: put it last. The link carries the session's secret, so the
-//! URL (and the browser history) is as private as the link itself.
+//! URL query flag: `?webgl` forces the WebGL2 backend instead of WebGPU.
+//!
+//! URL fragment: `#join=<invite link or join code>` joins that live session once the page has
+//! loaded, so a bookmark opens a shared file. `join` takes the rest of the fragment: put it last.
+//! The link carries the session's secret; a fragment is never sent to the web host, but the address
+//! bar and the browser history keep it, so the URL is as private as the link itself.
 
 #[cfg(any(target_arch = "wasm32", test))]
 mod query;

apps/figcraft-web/src/query.rs

@@ -1,10 +1,15 @@
-//! The page's URL query: which flags it sets and the invite link it names.
+//! The page's URL: the invite link its fragment names.
 
-/// The invite link in `?join=<link>` (`search` is `location.search`, with or without its `?`), still
-/// escaped. `join` takes the rest of the query, so a pasted `figcraft://join?host=…&file=…&token=…`
-/// keeps its own `&`s; put it last.
-pub fn join_link(search: &str) -> Option<&str> {
-    let query = search.strip_prefix('?').unwrap_or(search);
-    let start = if query.starts_with("join=") { 0 } else { query.find("&join=")? + 1 };
-    query.get(start + "join=".len()..).filter(|link| !link.is_empty())
+/// The invite link in `#join=<link>` (`fragment` is `location.hash`, with or without its `#`), still
+/// escaped. The fragment, unlike the query, never leaves the browser, so the link's secret stays out
+/// of the web host's logs and `Referer` headers. `join` takes the rest of the fragment, so a pasted
+/// `figcraft://join?host=…&file=…&token=…` keeps its own `&`s. Anything longer than an escaped invite
+/// link can be is ignored; what is left is checked by `collab.join` like a link the desktop app is
+/// opened with.
+pub fn join_link(fragment: &str) -> Option<&str> {
+    // An invite link is at most 2 KiB; every byte of it escaped is three.
+    const MAX: usize = 3 * 2048;
+    let fragment = fragment.strip_prefix('#').unwrap_or(fragment);
+    let start = if fragment.starts_with("join=") { 0 } else { fragment.find("&join=")? + 1 };
+    fragment.get(start + "join=".len()..).filter(|link| !link.is_empty() && link.len() <= MAX)
 }

apps/figcraft-web/src/query_tests.rs

@@ -1,21 +1,29 @@
-//! `?join=` in the page's URL.
+//! `#join=` in the page's URL.
 
 use crate::query::join_link;
 
 #[test]
-fn join_takes_the_rest_of_the_query() {
+fn join_takes_the_rest_of_the_fragment() {
     let link = "figcraft://join?host=192.168.1.20:47900&file=abc&token=AbCdEfGhIjKlMnOpQrStUv";
-    assert_eq!(join_link(&format!("?join={link}")), Some(link), "a pasted link keeps its own &s");
-    assert_eq!(join_link(&format!("?webgl&join={link}")), Some(link), "after other flags");
-    assert_eq!(join_link("?join=figcraft%3A%2F%2Fjoin%3Fhost%3Dh"), Some("figcraft%3A%2F%2Fjoin%3Fhost%3Dh"), "escaped, decoded later");
-    assert_eq!(join_link("join=K7M-2QX-9TD"), Some("K7M-2QX-9TD"), "a join code, no leading ?");
+    assert_eq!(join_link(&format!("#join={link}")), Some(link), "a pasted link keeps its own &s");
+    assert_eq!(join_link(&format!("#x=1&join={link}")), Some(link), "after other entries");
+    assert_eq!(join_link("#join=figcraft%3A%2F%2Fjoin%3Fhost%3Dh"), Some("figcraft%3A%2F%2Fjoin%3Fhost%3Dh"), "escaped, decoded later");
+    assert_eq!(join_link("join=K7M-2QX-9TD"), Some("K7M-2QX-9TD"), "a join code, no leading #");
 }
 
 #[test]
-fn no_join_without_the_parameter() {
+fn no_join_without_the_entry() {
     assert_eq!(join_link(""), None);
-    assert_eq!(join_link("?webgl"), None);
-    assert_eq!(join_link("?join="), None);
-    assert_eq!(join_link("?rejoin=x"), None, "only the whole parameter name");
-    assert_eq!(join_link("?xjoin=x&a=1"), None);
+    assert_eq!(join_link("#"), None);
+    assert_eq!(join_link("#join="), None);
+    assert_eq!(join_link("#rejoin=x"), None, "only the whole entry name");
+    assert_eq!(join_link("#xjoin=x&a=1"), None);
+    assert_eq!(join_link("?join=x"), None, "the query isn't read: it goes to the web host");
+}
+
+#[test]
+fn an_overlong_link_is_ignored() {
+    let long = format!("#join=figcraft://join?host=h&file=f&token={}", "a".repeat(7000));
+    assert_eq!(join_link(&long), None);
+    assert_eq!(join_link(&format!("#join={}", "é".repeat(4000))), None, "length counts bytes");
 }

apps/figcraft-web/src/web.rs

@@ -41,9 +41,9 @@ pub fn start() {
                     let ctx = cc.egui_ctx.clone();
                     session.live.install(Box::new(figcraft_collab_net::NetLink::new(std::sync::Arc::new(move || ctx.request_repaint()))));
                     let mut fig = FigApp::new(session, Box::new(WebServices { reduce_motion, ctx: cc.egui_ctx.clone() }));
-                    // `?join=<link>`: join that session, as opening an invite link does on the desktop.
-                    let search = query();
-                    if let Some(link) = crate::query::join_link(&search) {
+                    // `#join=<link>`: join that session, as opening an invite link does on the desktop.
+                    let hash = web_sys::window().and_then(|w| w.location().hash().ok()).unwrap_or_default();
+                    if let Some(link) = crate::query::join_link(&hash) {
                         let link = js_sys::decode_uri_component(link).ok().and_then(|s| s.as_string()).unwrap_or_else(|| link.to_string());
                         figcraft_ui_egui::live::open_invite(&mut fig, &link);
                     }