diff --git a/lib/request-security.test.mjs b/lib/request-security.test.mjs
index 9a4e8195f..d265814a3 100644
--- a/lib/request-security.test.mjs
+++ b/lib/request-security.test.mjs
@@ -190,3 +190,77 @@ test("recognizes JSON request content types", async () => {
     headers: { "content-type": "text/plain" },
   })), false);
 });
+
+test("allows a proxy that reports the scheme out-of-band and rewrites Origin", async () => {
+  const { isApiRequestAllowed } = await loadSubject();
+  // Azure Dev Tunnels normalizes Host and Origin onto the backend authority
+  // but keeps the external scheme in x-forwarded-proto.
+  const request = new Request("https://localhost:30141/api/test", {
+    method: "POST",
+    headers: {
+      host: "localhost:30141",
+      origin: "http://localhost:30141",
+      "x-forwarded-proto": "https",
+      "sec-fetch-site": "same-origin",
+    },
+  });
+  assert.equal(isApiRequestAllowed(request), true);
+});
+
+test("still rejects a foreign origin when a proxy is in front", async () => {
+  const { isApiRequestAllowed } = await loadSubject();
+  const foreignHost = new Request("https://localhost:30141/api/test", {
+    method: "POST",
+    headers: {
+      host: "localhost:30141",
+      origin: "http://attacker.example",
+      "x-forwarded-proto": "https",
+      "sec-fetch-site": "same-site",
+    },
+  });
+  const foreignPort = new Request("https://localhost:30141/api/test", {
+    method: "POST",
+    headers: {
+      host: "localhost:30141",
+      origin: "http://localhost:30142",
+      "x-forwarded-proto": "https",
+      "sec-fetch-site": "same-site",
+    },
+  });
+  const alternateLoopback = new Request("https://localhost:30141/api/test", {
+    method: "POST",
+    headers: {
+      host: "localhost:30141",
+      origin: "http://127.0.0.1:30141",
+      "x-forwarded-proto": "https",
+      "sec-fetch-site": "same-site",
+    },
+  });
+  const opaque = new Request("https://localhost:30141/api/test", {
+    method: "POST",
+    headers: {
+      host: "localhost:30141",
+      origin: "null",
+      "x-forwarded-proto": "https",
+      "sec-fetch-site": "same-site",
+    },
+  });
+
+  assert.equal(isApiRequestAllowed(foreignHost), false);
+  assert.equal(isApiRequestAllowed(foreignPort), false);
+  assert.equal(isApiRequestAllowed(alternateLoopback), false);
+  assert.equal(isApiRequestAllowed(opaque), false);
+});
+
+test("does not relax the scheme without a proxy in front", async () => {
+  const { isApiRequestAllowed } = await loadSubject();
+  const request = new Request("https://localhost:30141/api/test", {
+    method: "POST",
+    headers: {
+      host: "localhost:30141",
+      origin: "http://localhost:30141",
+      "sec-fetch-site": "same-origin",
+    },
+  });
+  assert.equal(isApiRequestAllowed(request), false);
+});
diff --git a/lib/request-security.ts b/lib/request-security.ts
index 5d6d35da9..90bd3aebb 100644
--- a/lib/request-security.ts
+++ b/lib/request-security.ts
@@ -20,6 +20,20 @@ function hostnameFromAuthority(value: string): string | null {
   }
 }
 
+function normalizeAuthority(value: string): string | null {
+  if (!value || /[\s/@\\]/.test(value)) return null;
+  try {
+    const parsed = new URL(`http://${value}`);
+    if (parsed.username || parsed.password || parsed.pathname !== "/" || parsed.search || parsed.hash) {
+      return null;
+    }
+    const hostname = normalizeHostname(parsed.hostname);
+    return parsed.port ? `${hostname}:${parsed.port}` : hostname;
+  } catch {
+    return null;
+  }
+}
+
 function normalizeConfiguredHostname(value: string | undefined): string | null {
   const trimmed = value?.trim();
   if (!trimmed) return null;
@@ -87,6 +101,30 @@ export function isApiRequestHostAllowed(
   );
 }
 
+/**
+ * A relay can report the external scheme in `x-forwarded-proto` while rewriting
+ * `Origin` onto the backend authority, so the two disagree on the scheme alone
+ * for a request that really is same-origin (Azure Dev Tunnels does this). Accept
+ * that pairing only when the Origin's authority still equals the Host header,
+ * which a cross-origin page cannot forge, and only when a proxy is in front.
+ */
+function isProxyRewrittenSameOrigin(request: Request, origin: string): boolean {
+  if (!request.headers.get("x-forwarded-proto")) return false;
+
+  const host = request.headers.get("host");
+  if (!host) return false;
+
+  let originHost: string;
+  try {
+    originHost = new URL(origin).host;
+  } catch {
+    return false;
+  }
+
+  const originAuthority = normalizeAuthority(originHost);
+  return originAuthority !== null && originAuthority === normalizeAuthority(host);
+}
+
 /** Reject browser cross-site API requests while preserving non-browser clients. */
 export function isApiRequestOriginAllowed(request: Request): boolean {
   const origin = request.headers.get("origin");
@@ -95,7 +133,9 @@ export function isApiRequestOriginAllowed(request: Request): boolean {
   if (!origin) return true;
 
   const requestOrigin = getRequestOrigin(request);
-  return requestOrigin !== null && canonicalOrigin(origin) === requestOrigin;
+  if (requestOrigin !== null && canonicalOrigin(origin) === requestOrigin) return true;
+
+  return isProxyRewrittenSameOrigin(request, origin);
 }
 
 export function shouldCheckApiRequestOrigin(request: Request): boolean {