diff --git a/shared/google-oauth.ts b/shared/google-oauth.ts
index b76bcf06..ce9a9c72 100644
--- a/shared/google-oauth.ts
+++ b/shared/google-oauth.ts
@@ -14,6 +14,8 @@
  * ```
  */
 
+import { assertAllowedRedirectUri } from "./redirect-allowlist.ts";
+
 export interface GoogleOAuthConfig {
   /**
    * Google OAuth scopes required by this MCP
@@ -61,6 +63,8 @@ export function createGoogleOAuth(config: GoogleOAuthConfig) {
      * Handles Google's requirement for clean redirect_uri (without state param)
      */
     authorizationUrl: (callbackUrl: string, env?: any) => {
+      assertAllowedRedirectUri(callbackUrl);
+
       const callbackUrlObj = new URL(callbackUrl);
       const state = callbackUrlObj.searchParams.get("state");
 
@@ -104,6 +108,7 @@ export function createGoogleOAuth(config: GoogleOAuthConfig) {
           "redirect_uri is required for Google OAuth token exchange",
         );
       }
+      assertAllowedRedirectUri(redirect_uri);
 
       const params = new URLSearchParams({
         code,
diff --git a/shared/redirect-allowlist.test.ts b/shared/redirect-allowlist.test.ts
new file mode 100644
index 00000000..81e95789
--- /dev/null
+++ b/shared/redirect-allowlist.test.ts
@@ -0,0 +1,88 @@
+import { describe, expect, test } from "bun:test";
+import {
+  assertAllowedRedirectUri,
+  isAllowedRedirectUri,
+} from "./redirect-allowlist.ts";
+
+describe("isAllowedRedirectUri", () => {
+  test("accepts decocms.com apex and arbitrary subdomains over https", () => {
+    expect(isAllowedRedirectUri("https://decocms.com/cb")).toBe(true);
+    expect(
+      isAllowedRedirectUri(
+        "https://google-gmail-mcp.decocms.com/oauth/callback",
+      ),
+    ).toBe(true);
+    expect(isAllowedRedirectUri("https://a.b.decocms.com/cb")).toBe(true);
+  });
+
+  test("accepts deco.site apex and arbitrary subdomains over https", () => {
+    expect(isAllowedRedirectUri("https://deco.site/cb")).toBe(true);
+    expect(
+      isAllowedRedirectUri(
+        "https://sites-youtube-channel-admin.deco.site/oauth/callback",
+      ),
+    ).toBe(true);
+    expect(isAllowedRedirectUri("https://a.b.deco.site/cb")).toBe(true);
+  });
+
+  test("allows loopback over http for local dev", () => {
+    expect(isAllowedRedirectUri("http://localhost:8787/oauth/callback")).toBe(
+      true,
+    );
+    expect(isAllowedRedirectUri("http://127.0.0.1:8787/cb")).toBe(true);
+  });
+
+  test("accepts deco.host apex and arbitrary subdomains over https", () => {
+    expect(isAllowedRedirectUri("https://deco.host/cb")).toBe(true);
+    expect(
+      isAllowedRedirectUri("https://localhost-c056dce8.deco.host/cb"),
+    ).toBe(true);
+  });
+
+  test("rejects non-loopback http", () => {
+    expect(isAllowedRedirectUri("http://sites-google-drive.deco.site/cb")).toBe(
+      false,
+    );
+    expect(isAllowedRedirectUri("http://tunnel.deco.host/cb")).toBe(false);
+  });
+
+  test("rejects look-alike and suffix-confusion hosts", () => {
+    expect(isAllowedRedirectUri("https://evildecocms.com/cb")).toBe(false);
+    expect(isAllowedRedirectUri("https://decocms.com.attacker.io/cb")).toBe(
+      false,
+    );
+    expect(isAllowedRedirectUri("https://notdeco.site/cb")).toBe(false);
+    expect(isAllowedRedirectUri("https://deco.site.attacker.io/cb")).toBe(
+      false,
+    );
+    expect(isAllowedRedirectUri("https://notdeco.host/cb")).toBe(false);
+    expect(isAllowedRedirectUri("https://deco.host.attacker.io/cb")).toBe(
+      false,
+    );
+  });
+
+  test("rejects other schemes and malformed input", () => {
+    expect(isAllowedRedirectUri("javascript:alert(1)")).toBe(false);
+    expect(isAllowedRedirectUri("ftp://decocms.com/cb")).toBe(false);
+    expect(isAllowedRedirectUri("not a url")).toBe(false);
+    expect(isAllowedRedirectUri("")).toBe(false);
+  });
+
+  test("is case-insensitive on the host", () => {
+    expect(isAllowedRedirectUri("https://Sites-Gmail.Deco.Site/cb")).toBe(true);
+  });
+});
+
+describe("assertAllowedRedirectUri", () => {
+  test("passes for an allowed uri", () => {
+    expect(() =>
+      assertAllowedRedirectUri("https://sites-google-docs.deco.site/cb"),
+    ).not.toThrow();
+  });
+
+  test("throws for a disallowed uri", () => {
+    expect(() => assertAllowedRedirectUri("https://attacker.io/cb")).toThrow(
+      /Refusing OAuth redirect_uri/,
+    );
+  });
+});
diff --git a/shared/redirect-allowlist.ts b/shared/redirect-allowlist.ts
new file mode 100644
index 00000000..90315423
--- /dev/null
+++ b/shared/redirect-allowlist.ts
@@ -0,0 +1,66 @@
+/**
+ * OAuth `redirect_uri` allowlist, shared by every OAuth-based MCP.
+ *
+ * The runtime hands `authorizationUrl()` a callback URL that we forward to
+ * the identity provider (e.g. Google) as the `redirect_uri`. The provider
+ * delivers the authorization `code` to whatever host that URL points at, so
+ * if the origin is ever attacker-influenced (spoofed Host header, an
+ * injected query/redirect param, a misconfigured route) the code — and the
+ * access token minted from it — leaks to that host.
+ *
+ * We close this by refusing any `redirect_uri` whose host isn't one of our
+ * own domains (or a subdomain of one). MCPs are hosted under either
+ * decocms.com or deco.site depending on how they were deployed; deco.host is
+ * additionally allowed since it's the local-dev tunnel domain (cloudflared-
+ * style), needed to test OAuth flows against providers that reject
+ * http://localhost redirect URIs. Loopback hosts are allowed over http for
+ * local dev (RFC 8252 §7.3); everything else must be https.
+ */
+
+export const ALLOWED_REDIRECT_HOST_SUFFIXES = [
+  "decocms.com",
+  "deco.site",
+  "deco.host",
+] as const;
+
+const LOOPBACK_HOSTS = new Set(["localhost", "127.0.0.1", "[::1]", "::1"]);
+
+/** Returns true if `redirectUri` is a well-formed URL pointing at an allowed host. */
+export function isAllowedRedirectUri(redirectUri: string): boolean {
+  let url: URL;
+  try {
+    url = new URL(redirectUri);
+  } catch {
+    return false;
+  }
+
+  const host = url.hostname.toLowerCase();
+  const isLoopback = LOOPBACK_HOSTS.has(host);
+
+  // https everywhere; http only for loopback dev hosts.
+  if (url.protocol === "https:") {
+    // ok
+  } else if (url.protocol === "http:" && isLoopback) {
+    return true;
+  } else {
+    return false;
+  }
+
+  if (isLoopback) return true;
+
+  // `endsWith(".decocms.com")` enforces a label boundary, so "evildecocms.com"
+  // and "decocms.com.attacker.io" are both rejected.
+  return ALLOWED_REDIRECT_HOST_SUFFIXES.some(
+    (suffix) => host === suffix || host.endsWith(`.${suffix}`),
+  );
+}
+
+/** Throws if `redirectUri` is not on the allowlist. */
+export function assertAllowedRedirectUri(redirectUri: string): void {
+  if (!isAllowedRedirectUri(redirectUri)) {
+    throw new Error(
+      `Refusing OAuth redirect_uri outside the allowed domains ` +
+        `(${ALLOWED_REDIRECT_HOST_SUFFIXES.join(", ")}): ${redirectUri}`,
+    );
+  }
+}