diff --git a/apps/gittensory-miner-ui/README.md b/apps/gittensory-miner-ui/README.md
index 893c58624..0bd0bb4ba 100644
--- a/apps/gittensory-miner-ui/README.md
+++ b/apps/gittensory-miner-ui/README.md
@@ -15,3 +15,14 @@ Phase 6 data views (run history, portfolio cards) land in follow-up issues after
 | Env var                     | Required | Description                                                                                                                                                                                                                                            |
 | --------------------------- | -------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
 | `VITE_MINER_UI_GRAFANA_URL` | No       | If set (and non-empty), renders a footer link to your ORB/Grafana dashboard at this URL. Unset ⇒ no link. Must be `VITE_`-prefixed so Vite exposes it to the client bundle. It is a plain navigational link — no token or credential is ever appended. |
+
+## Local API authentication
+
+`/api/*` (run-state, portfolio-queue, ledgers, and any future endpoint under that prefix) requires a
+same-origin session cookie — an unauthenticated request is rejected with `401`. The dev/preview server
+(`vite-auth.ts`) generates a random token once per process and sets it as an `HttpOnly; SameSite=Strict`
+cookie on every response; a browser that has loaded this app's own page (`/`) already carries the cookie
+automatically on every subsequent same-origin `fetch()` call, so none of the client-side data fetchers need
+to know about it. A request from another local process, or from a different page/origin the user has open
+(including a DNS-rebinding attempt), has no way to obtain the cookie and is rejected. There is nothing to
+configure — this is always on for both `vite dev` and `vite preview`.
diff --git a/apps/gittensory-miner-ui/src/auth.test.ts b/apps/gittensory-miner-ui/src/auth.test.ts
new file mode 100644
index 000000000..35fdcf67a
--- /dev/null
+++ b/apps/gittensory-miner-ui/src/auth.test.ts
@@ -0,0 +1,156 @@
+import { describe, expect, it } from "vitest";
+
+import { authPlugin, handleAuthRequest, isAuthenticatedRequest } from "../vite-auth";
+
+const TOKEN = "deterministic-test-token";
+
+describe("isAuthenticatedRequest (#4858)", () => {
+  it("rejects a request with no Cookie header at all", () => {
+    expect(isAuthenticatedRequest(undefined, TOKEN)).toBe(false);
+  });
+
+  it("rejects a Cookie header that never mentions the auth cookie", () => {
+    expect(isAuthenticatedRequest("othercookie=x; another=y", TOKEN)).toBe(false);
+  });
+
+  it("skips a malformed cookie pair with no '=' separator, without throwing", () => {
+    expect(isAuthenticatedRequest(`malformed; gittensory_miner_ui_token=${TOKEN}`, TOKEN)).toBe(true);
+  });
+
+  it("skips a cookie pair with an empty name (leading '=')", () => {
+    expect(isAuthenticatedRequest(`=novalue; gittensory_miner_ui_token=${TOKEN}`, TOKEN)).toBe(true);
+  });
+
+  it("rejects the auth cookie when its value doesn't match the server's token", () => {
+    expect(isAuthenticatedRequest("gittensory_miner_ui_token=wrong-value", TOKEN)).toBe(false);
+  });
+
+  it("accepts the auth cookie when its value matches the server's token exactly", () => {
+    expect(isAuthenticatedRequest(`gittensory_miner_ui_token=${TOKEN}`, TOKEN)).toBe(true);
+  });
+});
+
+describe("handleAuthRequest (#4858)", () => {
+  it("falls through (null) for a request with no url", () => {
+    expect(handleAuthRequest(undefined, undefined, TOKEN)).toBeNull();
+  });
+
+  it("falls through (null) for a non-/api/ request regardless of cookie state", () => {
+    expect(handleAuthRequest("/", undefined, TOKEN)).toBeNull();
+    expect(handleAuthRequest("/assets/index.js", undefined, TOKEN)).toBeNull();
+  });
+
+  it("falls through (null) for an authenticated /api/* request", () => {
+    expect(handleAuthRequest("/api/portfolio-queue", `gittensory_miner_ui_token=${TOKEN}`, TOKEN)).toBeNull();
+  });
+
+  it("returns a 401 JSON body for an unauthenticated /api/* request", () => {
+    expect(handleAuthRequest("/api/portfolio-queue", undefined, TOKEN)).toEqual({
+      status: 401,
+      body: JSON.stringify({ error: "unauthenticated: missing or invalid local miner-ui session cookie" }),
+    });
+  });
+});
+
+type CapturedRequestHandler = (
+  req: { url?: string; headers: { cookie?: string } },
+  res: { statusCode: number; setHeader: (k: string, v: string) => void; end: (body: string) => void },
+  next: () => void,
+) => void;
+
+function captureMiddleware(deps = { generateToken: () => TOKEN }): CapturedRequestHandler {
+  let captured: CapturedRequestHandler | undefined;
+  const plugin = authPlugin(deps);
+  const server = { middlewares: { use: (fn: CapturedRequestHandler) => (captured = fn) } };
+  // @ts-expect-error -- the test double only implements the subset of Vite's ViteDevServer this plugin reads.
+  plugin.configureServer(server);
+  if (!captured) throw new Error("authPlugin did not register a middleware");
+  return captured;
+}
+
+function fakeResponse() {
+  const headers: Record<string, string> = {};
+  let statusCode = 200;
+  let ended: string | undefined;
+  return {
+    res: {
+      get statusCode() {
+        return statusCode;
+      },
+      set statusCode(value: number) {
+        statusCode = value;
+      },
+      setHeader: (k: string, v: string) => {
+        headers[k] = v;
+      },
+      end: (body: string) => {
+        ended = body;
+      },
+    },
+    headers,
+    getEnded: () => ended,
+    getStatus: () => statusCode,
+  };
+}
+
+describe("authPlugin (#4858)", () => {
+  it("stamps the Set-Cookie header on a non-/api/ request that falls through (the initial page load)", () => {
+    const middleware = captureMiddleware();
+    const { res, headers } = fakeResponse();
+    let calledNext = false;
+    middleware({ url: "/", headers: {} }, res, () => {
+      calledNext = true;
+    });
+    expect(headers["Set-Cookie"]).toBe(`gittensory_miner_ui_token=${TOKEN}; HttpOnly; SameSite=Strict; Path=/`);
+    expect(calledNext).toBe(true);
+  });
+
+  it("rejects an unauthenticated /api/* request with 401, never calls next(), and NEVER leaks the token via Set-Cookie", () => {
+    // Regression test: an earlier version set Set-Cookie on every response BEFORE checking auth, so an
+    // unauthenticated caller could read the valid token straight off this 401's own headers and replay it --
+    // completely defeating the mechanism. The token must only ever reach a caller that is already
+    // authenticated, or a non-/api/* (page/asset) request.
+    const middleware = captureMiddleware();
+    const { res, headers, getEnded, getStatus } = fakeResponse();
+    let calledNext = false;
+    middleware({ url: "/api/portfolio-queue", headers: {} }, res, () => {
+      calledNext = true;
+    });
+    expect(getStatus()).toBe(401);
+    expect(JSON.parse(getEnded() ?? "{}")).toEqual({
+      error: "unauthenticated: missing or invalid local miner-ui session cookie",
+    });
+    expect(calledNext).toBe(false);
+    expect(headers["Set-Cookie"]).toBeUndefined();
+  });
+
+  it("lets an authenticated /api/* request fall through and re-stamps the same cookie", () => {
+    const middleware = captureMiddleware();
+    const { res, headers } = fakeResponse();
+    let calledNext = false;
+    middleware({ url: "/api/portfolio-queue", headers: { cookie: `gittensory_miner_ui_token=${TOKEN}` } }, res, () => {
+      calledNext = true;
+    });
+    expect(calledNext).toBe(true);
+    expect(headers["Set-Cookie"]).toBe(`gittensory_miner_ui_token=${TOKEN}; HttpOnly; SameSite=Strict; Path=/`);
+  });
+
+  it("uses deps.generateToken so a fixed test token is deterministic across requests", () => {
+    const middleware = captureMiddleware({ generateToken: () => "fixed-token-123" });
+    const { res } = fakeResponse();
+    let calledNext = false;
+    middleware({ url: "/api/ledgers", headers: { cookie: "gittensory_miner_ui_token=fixed-token-123" } }, res, () => {
+      calledNext = true;
+    });
+    expect(calledNext).toBe(true);
+  });
+
+  it("also attaches via configurePreviewServer for `vite preview`", () => {
+    let captured: CapturedRequestHandler | undefined;
+    const plugin = authPlugin();
+    const server = { middlewares: { use: (fn: CapturedRequestHandler) => (captured = fn) } };
+    // @ts-expect-error -- same partial test double as configureServer above.
+    plugin.configurePreviewServer(server);
+    expect(captured).toBeTypeOf("function");
+  });
+});
diff --git a/apps/gittensory-miner-ui/vite-auth.ts b/apps/gittensory-miner-ui/vite-auth.ts
new file mode 100644
index 000000000..b885d4548
--- /dev/null
+++ b/apps/gittensory-miner-ui/vite-auth.ts
@@ -0,0 +1,114 @@
+import { randomBytes } from "node:crypto";
+import type { Plugin } from "vite";
+
+// Local miner-ui API auth (#4858): the miner-ui's /api/* endpoints previously relied on undocumented, implicit
+// loopback-only trust -- and even that was inconsistent (vite-run-state-api.ts's own isLoopbackAddress gated
+// only ONE of the three API plugins; vite-portfolio-queue-api.ts and vite-ledgers-api.ts had no gate at all).
+// Neither is real authentication: loopback-IP checks don't stop another local process, or a malicious page the
+// user has open in the SAME browser, from hitting the loopback API.
+//
+// This adds a minimal-but-real mechanism instead of touching each API file individually: a random token
+// generated ONCE per dev-server process, delivered to the browser as a same-origin HttpOnly SameSite=Strict
+// cookie on every response the caller is authorized to receive (so the very first page load already carries
+// it going forward -- NOT on an unauthenticated /api/* request's own 401, which must never leak the token it
+// is rejecting the caller for lacking), and required on every /api/* request thereafter. HttpOnly keeps it
+// unreachable from any XSS in the SPA itself; SameSite=Strict
+// means a cross-origin page -- including one from a DNS-rebinding attack, which resolves an ATTACKER-CONTROLLED
+// hostname to 127.0.0.1 rather than reusing this dev server's own origin -- never has it attached automatically
+// by the browser. Because it rides on the browser's own cookie jar, no client-side fetch call needs to change:
+// the browser attaches it automatically to every same-origin request, including the existing fetchPortfolioQueue/
+// fetchLedgers/fetchRunState calls.
+//
+// Registered as the FIRST plugin in vite.config.ts so its middleware runs before runStateApiPlugin/
+// portfolioQueueApiPlugin/ledgersApiPlugin's own middlewares in the Connect chain: an unauthenticated /api/*
+// request never reaches any of them. This also means any FUTURE /api/* endpoint (e.g. a write action) is
+// covered automatically, with no per-endpoint auth wiring required.
+
+const COOKIE_NAME = "gittensory_miner_ui_token";
+
+export type AuthDeps = {
+  /** Injectable so tests get a deterministic token instead of a real random one. */
+  generateToken: () => string;
+};
+
+const defaultDeps: AuthDeps = {
+  generateToken: () => randomBytes(24).toString("hex"),
+};
+
+function parseCookieHeader(cookieHeader: string | undefined): Record<string, string> {
+  if (!cookieHeader) return {};
+  const cookies: Record<string, string> = {};
+  for (const pair of cookieHeader.split(";")) {
+    const separatorIndex = pair.indexOf("=");
+    if (separatorIndex === -1) continue;
+    const name = pair.slice(0, separatorIndex).trim();
+    const value = pair.slice(separatorIndex + 1).trim();
+    if (name) cookies[name] = value;
+  }
+  return cookies;
+}
+
+/** True when the incoming request carries the server's own auth cookie. Exported so the plugin's request
+ *  handling can be exercised directly in tests without a real HTTP server. */
+export function isAuthenticatedRequest(cookieHeader: string | undefined, token: string): boolean {
+  return parseCookieHeader(cookieHeader)[COOKIE_NAME] === token;
+}
+
+/** The request handler, factored out of the Vite plugin shape so tests drive it directly (mirrors the sibling
+ *  API files' handleXRequest pattern). Returns the 401 body when an /api/* request lacks a valid cookie, or
+ *  null when the request should fall through to the next middleware (either it's authenticated, or it isn't
+ *  an /api/* request at all and only needs the Set-Cookie header, applied by the caller). */
+export function handleAuthRequest(
+  url: string | undefined,
+  cookieHeader: string | undefined,
+  token: string,
+): { status: number; body: string } | null {
+  if (!url?.startsWith("/api/")) return null;
+  if (isAuthenticatedRequest(cookieHeader, token)) return null;
+  return {
+    status: 401,
+    body: JSON.stringify({ error: "unauthenticated: missing or invalid local miner-ui session cookie" }),
+  };
+}
+
+/** Vite dev/preview middleware: generates one token per process and (a) rejects any /api/* request that
+ *  doesn't present it, before (b) stamping the cookie onto a response that's allowed to proceed. The ORDER
+ *  matters: handleAuthRequest returns non-null (a rejection) ONLY for an unauthenticated /api/* request, so
+ *  the Set-Cookie header must never be set on that response -- otherwise the token itself would ride along
+ *  with the very 401 that was supposed to deny the caller, letting anyone who can reach the loopback port
+ *  read the token off a single unauthenticated request and replay it, defeating the whole mechanism. Every
+ *  request that reaches the setHeader call below is either already authenticated or isn't an /api/* request
+ *  at all (the initial page/asset load a real browser session is meant to obtain the cookie from). */
+export function authPlugin(deps: AuthDeps = defaultDeps): Plugin {
+  const token = deps.generateToken();
+  const attach = (middlewares: {
+    use: (
+      fn: (
+        req: { url?: string; headers: { cookie?: string } },
+        res: { statusCode: number; setHeader: (k: string, v: string) => void; end: (body: string) => void },
+        next: () => void,
+      ) => void,
+    ) => void;
+  }) => {
+    middlewares.use((req, res, next) => {
+      const rejection = handleAuthRequest(req.url, req.headers.cookie, token);
+      if (rejection) {
+        res.statusCode = rejection.status;
+        res.setHeader("Content-Type", "application/json");
+        res.end(rejection.body);
+        return;
+      }
+      res.setHeader("Set-Cookie", `${COOKIE_NAME}=${token}; HttpOnly; SameSite=Strict; Path=/`);
+      next();
+    });
+  };
+  return {
+    name: "gittensory-miner-ui:auth",
+    configureServer(server) {
+      attach(server.middlewares);
+    },
+    configurePreviewServer(server) {
+      attach(server.middlewares);
+    },
+  };
+}
diff --git a/apps/gittensory-miner-ui/vite.config.ts b/apps/gittensory-miner-ui/vite.config.ts
index e4c188f9c..097dd093c 100644
--- a/apps/gittensory-miner-ui/vite.config.ts
+++ b/apps/gittensory-miner-ui/vite.config.ts
@@ -4,6 +4,7 @@ import react from "@vitejs/plugin-react";
 import { defineConfig } from "vite";
 import tsconfigPaths from "vite-tsconfig-paths";
 
+import { authPlugin } from "./vite-auth";
 import { ledgersApiPlugin } from "./vite-ledgers-api";
 import { portfolioQueueApiPlugin } from "./vite-portfolio-queue-api";
 import { runStateApiPlugin } from "./vite-run-state-api";
@@ -14,6 +15,9 @@ export default defineConfig({
     react(),
     tailwindcss(),
     tsconfigPaths(),
+    // Must run before the three API plugins below: it rejects any unauthenticated /api/* request before
+    // their own middlewares are reached (#4858).
+    authPlugin(),
     runStateApiPlugin(),
     portfolioQueueApiPlugin(),
     ledgersApiPlugin(),