diff --git a/src/web/middleware/csrf.test.ts b/src/web/middleware/csrf.test.ts
index d056ebd..24da36f 100644
--- a/src/web/middleware/csrf.test.ts
+++ b/src/web/middleware/csrf.test.ts
@@ -55,4 +55,19 @@ describe("csrfOriginCheck middleware", () => {
       .set("Referer", "https://evil.com/some/path");
     expect(res.status).toBe(403);
   });
+
+  // Documents the server side of the QR-login outage: a `no-referrer` document
+  // policy makes the browser send the literal `Origin: null` on same-origin
+  // POSTs, which this guard cannot parse a host from and therefore rejects.
+  // The fix lives in the frontend (referrer policy -> same-origin); this test
+  // pins the gate behavior so the interaction stays understood. See
+  // src/web/referrer-policy.test.ts.
+  it('rejects POST with the literal Origin: "null" (no-referrer downgrade)', async () => {
+    const res = await request(app)
+      .post("/")
+      .set("Host", "example.com")
+      .set("Origin", "null");
+    expect(res.status).toBe(403);
+    expect(res.body).toEqual({ error: "bad origin" });
+  });
 });
diff --git a/src/web/referrer-policy.test.ts b/src/web/referrer-policy.test.ts
new file mode 100644
index 0000000..02ca30f
--- /dev/null
+++ b/src/web/referrer-policy.test.ts
@@ -0,0 +1,46 @@
+import { describe, it, expect } from "vitest";
+import fs from "node:fs";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+
+/**
+ * Regression guard for the QR-login / cookie-save outage (and in fact every
+ * mutating WebUI action). On 2026-05-27 the WebUI-auth feature added the
+ * same-origin CSRF gate `app.use("/api", csrfOriginCheck)` in server.ts, and
+ * the same day a `<meta name="referrer" content="no-referrer">` was added to
+ * web/index.html so cross-origin CDN cover thumbnails would load.
+ *
+ * Those two changes conflict: per the WHATWG Fetch "Append a request Origin
+ * header" algorithm, the `no-referrer` policy sets the Origin header to the
+ * literal string "null" on same-origin non-GET requests. csrfOriginCheck then
+ * fails to parse a host (`new URL("null")` throws) and returns 403 "bad
+ * origin", so POST /api/auth/qrcode (and every other POST/PUT/DELETE under
+ * /api/* except /api/session/*) never reaches its handler.
+ *
+ * `same-origin` is the correct policy: it keeps the real Origin on same-origin
+ * requests (CSRF passes) while still sending no Referer cross-origin (CDN
+ * thumbnails keep loading). Never switch this back to `no-referrer`.
+ */
+describe("frontend referrer policy (CSRF / Origin-header regression)", () => {
+  const indexHtmlPath = path.resolve(
+    path.dirname(fileURLToPath(import.meta.url)),
+    "../../web/index.html"
+  );
+  const html = fs.readFileSync(indexHtmlPath, "utf-8");
+
+  const referrerMeta = html.match(
+    /<meta\s+name=["']referrer["']\s+content=["']([^"']+)["']\s*\/?>/i
+  );
+
+  it("declares a referrer policy meta tag", () => {
+    expect(referrerMeta).not.toBeNull();
+  });
+
+  it("uses same-origin (NOT no-referrer, which sends Origin: null and 403s every POST)", () => {
+    expect(referrerMeta?.[1]).toBe("same-origin");
+  });
+
+  it("does not contain no-referrer anywhere in the document head", () => {
+    expect(html).not.toMatch(/content=["']no-referrer["']/i);
+  });
+});
diff --git a/web/index.html b/web/index.html
index bf7535c..5c0042c 100644
--- a/web/index.html
+++ b/web/index.html
@@ -3,10 +3,18 @@
 <head>
   <meta charset="UTF-8">
   <meta name="viewport" content="width=device-width, initial-scale=1.0">
-  <!-- Bilibili / NetEase / QQ image CDNs reject requests whose Referer is not on their whitelist.
-       Setting no-referrer at the document level covers <img> tags AND CSS background-image fetches.
-       Our own /api/* CSRF check uses Origin (not Referer), so this doesn't break auth. -->
-  <meta name="referrer" content="no-referrer">
+  <!-- Bilibili / NetEase / QQ image CDNs reject requests whose Referer is not on
+       their whitelist, so we must not leak a Referer cross-origin. "same-origin"
+       does exactly that: full Referer for our own requests, none for cross-origin
+       ones — so cover thumbnails (<img> AND CSS background-image) still load.
+       Do NOT switch this back to "no-referrer": per the WHATWG Fetch spec
+       ("Append a request Origin header") no-referrer downgrades the Origin header
+       to the literal string "null" on same-origin non-GET requests. The /api/*
+       CSRF guard (src/web/middleware/csrf.ts) then can't parse a host from it and
+       responds 403 "bad origin", silently breaking EVERY POST/PUT/DELETE — QR
+       login, cookie save, playback controls, bot management, user admin, etc.
+       "same-origin" keeps the real Origin on same-origin requests, so CSRF passes. -->
+  <meta name="referrer" content="same-origin">
   <title>TSMusicBot</title>
   <link rel="preconnect" href="https://fonts.googleapis.com">
   <link href="https://fonts.googleapis.com/css2?family=Barlow:wght@400;500;600;700;800&display=swap" rel="stylesheet">