fix(mesh): validate redirect_uri in oauth-proxy to prevent OAuth hijacking

apps/mesh/src/api/app.ts

@@ -668,6 +668,36 @@ export async function createApp(options: CreateAppOptions = {}) {
     // 2. Cookies are set on the correct domain
     // 3. The user can interact with the consent screen
     if (endpoint === "authorize") {
+      // Validate redirect_uri to prevent OAuth hijacking — only allow our own origin.
+      // Use .get() to grab the first value, then .set() to canonicalize to exactly
+      // one redirect_uri param, preventing parser-differential bypasses via duplicates.
+      const redirectUri = targetUrl.searchParams.get("redirect_uri");
+      if (redirectUri) {
+        const allowedOrigin = getSettings().baseUrl ?? reqUrl.origin;
+        try {
+          const redirectOrigin = new URL(redirectUri).origin;
+          if (redirectOrigin !== new URL(allowedOrigin).origin) {
+            return c.json(
+              {
+                error: "invalid_request",
+                error_description: "redirect_uri is not allowed",
+              },
+              400,
+            );
+          }
+        } catch {
+          return c.json(
+            {
+              error: "invalid_request",
+              error_description: "redirect_uri is malformed",
+            },
+            400,
+          );
+        }
+        // Collapse any duplicate redirect_uri params to the single validated value
+        targetUrl.searchParams.set("redirect_uri", redirectUri);
+      }
+
       // IMPORTANT: Rewrite the 'resource' parameter to point to the origin MCP endpoint
       // Some auth servers (like Supabase) validate that the resource is their actual endpoint,
       // not our proxy. We keep the proxy URL for redirect_uri since that's where we handle the callback.