diff --git a/docs/collaboration.md b/docs/collaboration.md
index ca254aba7..dc8cf60b5 100644
--- a/docs/collaboration.md
+++ b/docs/collaboration.md
@@ -257,7 +257,13 @@ ephemeral and never written to a project file.
 
 > **Operator note:** `POST /sessions` validates the request `Origin` (or
 > `Referer`) against `ALLOWED_ORIGINS` via `isAllowedOrigin` (defaults to the
-> app's own domains plus `localhost` for development) as browser-origin filtering
+> hosted origins (`geolibre.app`, `web.geolibre.app`, its legacy
+> `viewer.geolibre.app` alias, `studio.geolibre.app`, and
+> `collab.geolibre.app`), single-label HTTPS deployment hosts under
+> `*.geolibre-preview.pages.dev`, loopback hosts (`localhost` and
+> `127.0.0.1`), and `tauri://localhost`). Nested or custom-port preview hosts
+> and look-alike domains are rejected; the shared `opengeos.org` GitHub Pages
+> preview origin is deliberately not trusted) as browser-origin filtering
 > and defense-in-depth (not authentication or a general server-side access gate)
 > and enforces a per-IP `checkRateLimit` (10 requests / 60 s). `Access-Control-Allow-Origin: *` is
 > still sent on responses so non-browser clients (e.g. Tauri) are not blocked by
diff --git a/workers/collab-node/src/server.ts b/workers/collab-node/src/server.ts
index 2eef822d4..9a19b52e1 100644
--- a/workers/collab-node/src/server.ts
+++ b/workers/collab-node/src/server.ts
@@ -65,6 +65,9 @@ function isAllowedOrigin(
         .filter(Boolean)
     : [
         "https://geolibre.app",
+        "https://web.geolibre.app",
+        "https://viewer.geolibre.app",
+        "https://studio.geolibre.app",
         "https://collab.geolibre.app",
         "http://localhost",
         "http://127.0.0.1",
@@ -74,7 +77,19 @@ function isAllowedOrigin(
   try {
     const originUrl = new URL(originHeader);
     const host = originUrl.hostname;
-    if (host === "localhost" || host === "127.0.0.1" || host.endsWith(".localhost")) return true;
+    if (!envAllowed) {
+      if (host === "localhost" || host === "127.0.0.1" || host.endsWith(".localhost")) return true;
+      const previewSuffix = ".geolibre-preview.pages.dev";
+      const previewLabel = host.endsWith(previewSuffix) ? host.slice(0, -previewSuffix.length) : "";
+      if (
+        originUrl.protocol === "https:" &&
+        !originUrl.port &&
+        previewLabel &&
+        !previewLabel.includes(".")
+      ) {
+        return true;
+      }
+    }
     return allowedList.some((allowed) => {
       if (allowed === "*") return true;
       try {
diff --git a/workers/collab-node/test/relay.test.ts b/workers/collab-node/test/relay.test.ts
index 50e14b1c9..621405fdc 100644
--- a/workers/collab-node/test/relay.test.ts
+++ b/workers/collab-node/test/relay.test.ts
@@ -101,6 +101,72 @@ describe("Node collaboration relay", () => {
     await assert.rejects(connect(http, "NOTFOUND"), /Unexpected server response: 404/);
   });
 
+  it("allows the hosted GeoLibre web origins to create sessions", async () => {
+    const { http } = await start();
+
+    for (const origin of [
+      "https://geolibre.app",
+      "https://web.geolibre.app",
+      "https://viewer.geolibre.app",
+      "https://studio.geolibre.app",
+      "https://50e58010.geolibre-preview.pages.dev",
+    ]) {
+      const response = await fetch(`${http}/sessions`, {
+        method: "POST",
+        headers: { origin },
+      });
+      assert.equal(response.status, 200, `${origin} should be allowed`);
+    }
+
+    const rejected = await fetch(`${http}/sessions`, {
+      method: "POST",
+      headers: { origin: "https://web.geolibre.app.example.com" },
+    });
+    assert.equal(rejected.status, 403);
+
+    const rejectedPreviewLookalike = await fetch(`${http}/sessions`, {
+      method: "POST",
+      headers: { origin: "https://preview.geolibre-preview.pages.dev.example.com" },
+    });
+    assert.equal(rejectedPreviewLookalike.status, 403);
+
+    for (const origin of [
+      "https://opengeos.org",
+      "https://a.b.geolibre-preview.pages.dev",
+      "https://preview.geolibre-preview.pages.dev:8443",
+    ]) {
+      const response = await fetch(`${http}/sessions`, {
+        method: "POST",
+        headers: { origin },
+      });
+      assert.equal(response.status, 403, `${origin} should be rejected`);
+    }
+  });
+
+  it("makes a configured origin allowlist authoritative", async () => {
+    const { http } = await start();
+    const previous = process.env.ALLOWED_ORIGINS;
+    process.env.ALLOWED_ORIGINS = "https://allowed.example";
+    try {
+      const allowed = await fetch(`${http}/sessions`, {
+        method: "POST",
+        headers: { origin: "https://allowed.example" },
+      });
+      assert.equal(allowed.status, 200);
+
+      for (const origin of ["http://localhost:5173", "https://pr-1.geolibre-preview.pages.dev"]) {
+        const response = await fetch(`${http}/sessions`, {
+          method: "POST",
+          headers: { origin },
+        });
+        assert.equal(response.status, 403, `${origin} should require explicit configuration`);
+      }
+    } finally {
+      if (previous === undefined) delete process.env.ALLOWED_ORIGINS;
+      else process.env.ALLOWED_ORIGINS = previous;
+    }
+  });
+
   it("rejects an oversized session-create body by declared length and by count", async () => {
     const { http } = await start();
 
diff --git a/workers/collab/src/index.ts b/workers/collab/src/index.ts
index e34f9e415..ec55b23c9 100644
--- a/workers/collab/src/index.ts
+++ b/workers/collab/src/index.ts
@@ -51,6 +51,9 @@ function isAllowedOrigin(originHeader: string | null, envAllowed?: string): bool
         .filter(Boolean)
     : [
         "https://geolibre.app",
+        "https://web.geolibre.app",
+        "https://viewer.geolibre.app",
+        "https://studio.geolibre.app",
         "https://collab.geolibre.app",
         "http://localhost",
         "http://127.0.0.1",
@@ -60,7 +63,19 @@ function isAllowedOrigin(originHeader: string | null, envAllowed?: string): bool
   try {
     const originUrl = new URL(originHeader);
     const host = originUrl.hostname;
-    if (host === "localhost" || host === "127.0.0.1" || host.endsWith(".localhost")) return true;
+    if (!envAllowed) {
+      if (host === "localhost" || host === "127.0.0.1" || host.endsWith(".localhost")) return true;
+      const previewSuffix = ".geolibre-preview.pages.dev";
+      const previewLabel = host.endsWith(previewSuffix) ? host.slice(0, -previewSuffix.length) : "";
+      if (
+        originUrl.protocol === "https:" &&
+        !originUrl.port &&
+        previewLabel &&
+        !previewLabel.includes(".")
+      ) {
+        return true;
+      }
+    }
     return allowedList.some((allowed) => {
       if (allowed === "*") return true;
       try {