diff --git a/.changeset/harden-remote-plugin-assets.md b/.changeset/harden-remote-plugin-assets.md
new file mode 100644
index 0000000..62797c2
--- /dev/null
+++ b/.changeset/harden-remote-plugin-assets.md
@@ -0,0 +1,5 @@
+---
+"@jmfederico/pi-web": patch
+---
+
+Harden remote machine plugin asset proxying so plugin asset URLs cannot escape the remote plugin directory.
diff --git a/src/server/app.test.ts b/src/server/app.test.ts
index 9113b5a..aebed93 100644
--- a/src/server/app.test.ts
+++ b/src/server/app.test.ts
@@ -343,6 +343,45 @@ describe("buildApp", () => {
     expect(request).toHaveBeenCalledWith("GET", "/pi-web-plugins/remote-tools/pi-web-plugin.js?v=123");
   });
 
+  it("drops unsafe remote machine plugin manifest modules", async () => {
+    const addResponse = await app.inject({ method: "POST", url: "/api/machines", payload: { name: "Remote", baseUrl: "https://remote.example.test/" } });
+    const remote = addResponse.json<{ id: string }>();
+    remoteClient = fakeRemoteClient({
+      requestJson: vi.fn(() => Promise.resolve({
+        statusCode: 200,
+        headers: { "content-type": "application/json" },
+        body: {
+          plugins: [
+            { id: "safe-tools", module: "nested/pi-web-plugin.js?v=1", source: "local", scope: "local" },
+            { id: "traversal-tools", module: "..%2F..%2Fapi%2Fconfig", source: "local", scope: "local" },
+            { id: "wrong-root", module: "/pi-web-plugins/other/pi-web-plugin.js", source: "local", scope: "local" },
+          ],
+        },
+      })),
+    });
+
+    const manifestResponse = await app.inject({ method: "GET", url: `/api/machines/${remote.id}/pi-web-plugins/manifest.json` });
+
+    expect(manifestResponse.statusCode).toBe(200);
+    expect(manifestResponse.json()).toEqual({
+      plugins: [{ id: "safe-tools", module: `/pi-web-plugins/${machineScopedPluginId(remote.id, "safe-tools")}/nested/pi-web-plugin.js?v=1`, source: "local", scope: "local" }],
+    });
+  });
+
+  it("rejects remote machine plugin asset traversal before proxying", async () => {
+    const addResponse = await app.inject({ method: "POST", url: "/api/machines", payload: { name: "Remote", baseUrl: "https://remote.example.test/" } });
+    const remote = addResponse.json<{ id: string }>();
+    const request = vi.fn(() => Promise.resolve({ statusCode: 200, headers: {}, body: Readable.from([]) }));
+    remoteClient = fakeRemoteClient({ request });
+    const scopedPluginId = machineScopedPluginId(remote.id, "remote-tools");
+
+    const response = await app.inject({ method: "GET", url: `/pi-web-plugins/${scopedPluginId}/..%2F..%2Fapi%2Fconfig` });
+
+    expect(response.statusCode).toBe(400);
+    expect(response.json()).toEqual({ error: "Invalid remote PI WEB plugin asset path" });
+    expect(request).not.toHaveBeenCalled();
+  });
+
   it("returns stable errors for invalid project requests", async () => {
     const addResponse = await app.inject({
       method: "POST",
diff --git a/src/server/machines/machinePluginProxyRoutes.ts b/src/server/machines/machinePluginProxyRoutes.ts
index 20b37ee..65856b2 100644
--- a/src/server/machines/machinePluginProxyRoutes.ts
+++ b/src/server/machines/machinePluginProxyRoutes.ts
@@ -59,8 +59,14 @@ export async function proxyMachinePluginAsset(machines: MachinePluginProxyMachin
     return true;
   }
 
+  const requestPath = remotePluginAssetRequestPath(remotePlugin, assetPath, requestUrl);
+  if (requestPath === undefined) {
+    await reply.code(400).send({ error: "Invalid remote PI WEB plugin asset path" });
+    return true;
+  }
+
   try {
-    const upstream = await client.request("GET", remotePluginAssetRequestPath(remotePlugin, assetPath, requestUrl));
+    const upstream = await client.request("GET", requestPath);
     reply.code(upstream.statusCode);
     applySafeHeaders(reply, upstream.headers);
     if (upstream.body === undefined) await reply.send();
@@ -87,29 +93,57 @@ function rewriteRemotePluginManifest(machineId: string, manifest: RemotePluginMa
 
 function remotePluginModulePath(pluginId: string, module: string): { path: string; query: string } | undefined {
   if (!isPiWebPluginId(pluginId)) return undefined;
+  const prefix = `/pi-web-plugins/${encodeURIComponent(pluginId)}/`;
+  const base = new URL(prefix, "http://pi-web.local");
   try {
-    const url = new URL(module, "http://pi-web.local");
-    const prefix = `/pi-web-plugins/${encodeURIComponent(pluginId)}/`;
-    if (url.pathname.startsWith(prefix)) {
-      return { path: url.pathname.slice(prefix.length), query: url.search };
-    }
-    if (!module.startsWith("/") && !/^https?:\/\//iu.test(module)) {
-      const [path, query = ""] = module.split("?", 2);
-      if (path !== undefined && path !== "") return { path, query: query === "" ? "" : `?${query}` };
-    }
+    const url = new URL(module, base);
+    if (url.origin !== base.origin || !url.pathname.startsWith(prefix)) return undefined;
+    const path = safeRemotePluginAssetPath(url.pathname.slice(prefix.length));
+    return path === undefined ? undefined : { path, query: url.search };
   } catch {
     return undefined;
   }
-  return undefined;
 }
 
-function remotePluginAssetRequestPath(remotePlugin: MachineScopedPluginIdParts, assetPath: string, requestUrl: string): string {
+function remotePluginAssetRequestPath(remotePlugin: MachineScopedPluginIdParts, assetPath: string, requestUrl: string): string | undefined {
+  const path = safeRemotePluginAssetPath(assetPath);
+  if (path === undefined) return undefined;
   const query = requestUrl.includes("?") ? requestUrl.slice(requestUrl.indexOf("?")) : "";
-  return `/pi-web-plugins/${encodeURIComponent(remotePlugin.pluginId)}/${encodePathSegments(assetPath)}${query}`;
+  return `/pi-web-plugins/${encodeURIComponent(remotePlugin.pluginId)}/${path}${query}`;
 }
 
-function encodePathSegments(path: string): string {
-  return path.split("/").map((segment) => encodeURIComponent(segment)).join("/");
+function safeRemotePluginAssetPath(path: string): string | undefined {
+  const segments: string[] = [];
+  for (const rawSegment of path.split("/")) {
+    const segment = safeRemotePluginAssetPathSegment(rawSegment);
+    if (segment === undefined) return undefined;
+    if (segment === "") continue;
+    segments.push(segment);
+  }
+  if (segments.length === 0) return undefined;
+  return segments.map((segment) => encodeURIComponent(segment)).join("/");
+}
+
+function safeRemotePluginAssetPathSegment(rawSegment: string): string | undefined {
+  if (rawSegment === "" || rawSegment === ".") return "";
+  if (/%(?:2f|5c)/iu.test(rawSegment)) return undefined;
+  let segment: string;
+  try {
+    segment = decodeURIComponent(rawSegment);
+  } catch {
+    return undefined;
+  }
+  if (segment === "" || segment === ".") return "";
+  if (segment === ".." || segment.includes("/") || segment.includes("\\") || hasControlCharacter(segment)) return undefined;
+  return segment;
+}
+
+function hasControlCharacter(value: string): boolean {
+  for (let index = 0; index < value.length; index += 1) {
+    const code = value.charCodeAt(index);
+    if (code <= 0x1f || code === 0x7f) return true;
+  }
+  return false;
 }
 
 function parseRemoteManifest(value: unknown): RemotePluginManifest {