diff --git a/lib/Server.js b/lib/Server.js
index 9f78cb2042..9cf304e8c3 100644
--- a/lib/Server.js
+++ b/lib/Server.js
@@ -3343,10 +3343,20 @@ class Server {
       return true;
     }
 
+    const loopbacks = new Set(["localhost", "127.0.0.1", "::1"]);
+
+    // url.parse cannot handle bare IPv6 like "::1", need to bracket it first
+    const hostForParsing =
+      loopbacks.has(hostHeader) && hostHeader.includes(":")
+        ? `[${hostHeader}]`
+        : hostHeader;
+
     // eslint-disable-next-line n/no-deprecated-api
     const host = url.parse(
       // if hostHeader doesn't have scheme, add // for parsing.
-      /^(.+:)?\/\//.test(hostHeader) ? hostHeader : `//${hostHeader}`,
+      /^(.+:)?\/\//.test(hostForParsing)
+        ? hostForParsing
+        : `//${hostForParsing}`,
       false,
       true,
     ).hostname;
@@ -3359,6 +3369,12 @@ class Server {
       return true;
     }
 
+    // Treat all loopback aliases as equivalent, localhost may resolve to
+    // 127.0.0.1 or ::1 depending on the OS, causing a false mismatch.
+    if (loopbacks.has(origin) && loopbacks.has(host)) {
+      return true;
+    }
+
     return origin === host;
   }
 
diff --git a/test/e2e/__snapshots__/allowed-hosts.test.js.snap.webpack5 b/test/e2e/__snapshots__/allowed-hosts.test.js.snap.webpack5
index edd5d77571..41bada9913 100644
--- a/test/e2e/__snapshots__/allowed-hosts.test.js.snap.webpack5
+++ b/test/e2e/__snapshots__/allowed-hosts.test.js.snap.webpack5
@@ -1,5 +1,11 @@
 // Jest Snapshot v1, https://jestjs.io/docs/snapshot-testing
 
+exports[`allowed hosts check host headers should NOT allow websocket connection when origin is a non-loopback address mismatching host (loopback fix must not widen trust): console messages 1`] = `[]`;
+
+exports[`allowed hosts check host headers should NOT allow websocket connection when origin is a non-loopback address mismatching host (loopback fix must not widen trust): page errors 1`] = `[]`;
+
+exports[`allowed hosts check host headers should NOT allow websocket connection when origin is a non-loopback address mismatching host (loopback fix must not widen trust): response status 1`] = `200`;
+
 exports[`allowed hosts check host headers should allow hosts in allowedHosts: console messages 1`] = `[]`;
 
 exports[`allowed hosts check host headers should allow hosts in allowedHosts: page errors 1`] = `[]`;
@@ -12,6 +18,24 @@ exports[`allowed hosts check host headers should allow hosts that pass a wildcar
 
 exports[`allowed hosts check host headers should allow hosts that pass a wildcard in allowedHosts: response status 1`] = `200`;
 
+exports[`allowed hosts check host headers should allow websocket connection when host is 'localhost' but resolves to '::1' (loopback alias mismatch): console messages 1`] = `[]`;
+
+exports[`allowed hosts check host headers should allow websocket connection when host is 'localhost' but resolves to '::1' (loopback alias mismatch): page errors 1`] = `[]`;
+
+exports[`allowed hosts check host headers should allow websocket connection when host is 'localhost' but resolves to '::1' (loopback alias mismatch): response status 1`] = `200`;
+
+exports[`allowed hosts check host headers should allow websocket connection when host is 'localhost' but resolves to '127.0.0.1' (loopback alias mismatch): console messages 1`] = `[]`;
+
+exports[`allowed hosts check host headers should allow websocket connection when host is 'localhost' but resolves to '127.0.0.1' (loopback alias mismatch): page errors 1`] = `[]`;
+
+exports[`allowed hosts check host headers should allow websocket connection when host is 'localhost' but resolves to '127.0.0.1' (loopback alias mismatch): response status 1`] = `200`;
+
+exports[`allowed hosts check host headers should allow websocket connection when origin is '127.0.0.1' but host is 'localhost' (reverse loopback alias mismatch): console messages 1`] = `[]`;
+
+exports[`allowed hosts check host headers should allow websocket connection when origin is '127.0.0.1' but host is 'localhost' (reverse loopback alias mismatch): page errors 1`] = `[]`;
+
+exports[`allowed hosts check host headers should allow websocket connection when origin is '127.0.0.1' but host is 'localhost' (reverse loopback alias mismatch): response status 1`] = `200`;
+
 exports[`allowed hosts check host headers should always allow \`localhost\` if options.allowedHosts is auto: console messages 1`] = `[]`;
 
 exports[`allowed hosts check host headers should always allow \`localhost\` if options.allowedHosts is auto: page errors 1`] = `[]`;
diff --git a/test/e2e/allowed-hosts.test.js b/test/e2e/allowed-hosts.test.js
index 163a24f265..ba1cadf726 100644
--- a/test/e2e/allowed-hosts.test.js
+++ b/test/e2e/allowed-hosts.test.js
@@ -1874,5 +1874,184 @@ describe("allowed hosts", () => {
 
       expect(pageErrors).toMatchSnapshot("page errors");
     });
+
+    it("should allow websocket connection when host is 'localhost' but resolves to '127.0.0.1' (loopback alias mismatch)", async () => {
+      const options = {
+        allowedHosts: "auto",
+        host: "localhost",
+        port: port1,
+      };
+
+      server = new Server(options, compiler);
+
+      await server.start();
+
+      ({ page, browser } = await runBrowser());
+
+      page
+        .on("console", (message) => {
+          consoleMessages.push(message);
+        })
+        .on("pageerror", (error) => {
+          pageErrors.push(error);
+        });
+
+      // Simulate: browser opens from localhost, but OS resolved
+      // 'localhost' to '127.0.0.1' so host header is the IP
+      const headersLocalhostOriginIPv4Host = {
+        host: "127.0.0.1",
+        origin: "http://localhost",
+      };
+
+      if (!server.isSameOrigin(headersLocalhostOriginIPv4Host)) {
+        throw new Error(
+          "isSameOrigin should treat localhost and 127.0.0.1 as equivalent loopback addresses",
+        );
+      }
+
+      const response = await page.goto(`http://localhost:${port1}/main.js`, {
+        waitUntil: "networkidle0",
+      });
+
+      expect(response.status()).toMatchSnapshot("response status");
+      expect(consoleMessages.map((message) => message.text())).toMatchSnapshot(
+        "console messages",
+      );
+      expect(pageErrors).toMatchSnapshot("page errors");
+    });
+
+    it("should allow websocket connection when host is 'localhost' but resolves to '::1' (loopback alias mismatch)", async () => {
+      const options = {
+        allowedHosts: "auto",
+        host: "localhost",
+        port: port1,
+      };
+
+      server = new Server(options, compiler);
+
+      await server.start();
+
+      ({ page, browser } = await runBrowser());
+
+      page
+        .on("console", (message) => {
+          consoleMessages.push(message);
+        })
+        .on("pageerror", (error) => {
+          pageErrors.push(error);
+        });
+
+      // Simulate: browser opens from localhost, but OS resolved
+      // 'localhost' to '::1' (IPv6) so host header is the IPv6 address
+      const headersLocalhostOriginIPv6Host = {
+        host: "::1",
+        origin: "http://localhost",
+      };
+
+      if (!server.isSameOrigin(headersLocalhostOriginIPv6Host)) {
+        throw new Error(
+          "isSameOrigin should treat localhost and ::1 as equivalent loopback addresses",
+        );
+      }
+
+      const response = await page.goto(`http://localhost:${port1}/main.js`, {
+        waitUntil: "networkidle0",
+      });
+
+      expect(response.status()).toMatchSnapshot("response status");
+      expect(consoleMessages.map((message) => message.text())).toMatchSnapshot(
+        "console messages",
+      );
+      expect(pageErrors).toMatchSnapshot("page errors");
+    });
+
+    it("should allow websocket connection when origin is '127.0.0.1' but host is 'localhost' (reverse loopback alias mismatch)", async () => {
+      const options = {
+        allowedHosts: "auto",
+        host: "127.0.0.1",
+        port: port1,
+      };
+
+      server = new Server(options, compiler);
+
+      await server.start();
+
+      ({ page, browser } = await runBrowser());
+
+      page
+        .on("console", (message) => {
+          consoleMessages.push(message);
+        })
+        .on("pageerror", (error) => {
+          pageErrors.push(error);
+        });
+
+      // Reverse of above: server bound to 127.0.0.1, but browser
+      // sent origin header using 'localhost' name
+      const headersIPv4OriginLocalhostHost = {
+        host: "localhost",
+        origin: "http://127.0.0.1",
+      };
+
+      if (!server.isSameOrigin(headersIPv4OriginLocalhostHost)) {
+        throw new Error(
+          "isSameOrigin should treat 127.0.0.1 and localhost as equivalent loopback addresses",
+        );
+      }
+
+      const response = await page.goto(`http://127.0.0.1:${port1}/main.js`, {
+        waitUntil: "networkidle0",
+      });
+
+      expect(response.status()).toMatchSnapshot("response status");
+      expect(consoleMessages.map((message) => message.text())).toMatchSnapshot(
+        "console messages",
+      );
+      expect(pageErrors).toMatchSnapshot("page errors");
+    });
+
+    it("should NOT allow websocket connection when origin is a non-loopback address mismatching host (loopback fix must not widen trust)", async () => {
+      const options = {
+        allowedHosts: "auto",
+        host: "localhost",
+        port: port1,
+      };
+
+      server = new Server(options, compiler);
+
+      await server.start();
+
+      ({ page, browser } = await runBrowser());
+
+      page
+        .on("console", (message) => {
+          consoleMessages.push(message);
+        })
+        .on("pageerror", (error) => {
+          pageErrors.push(error);
+        });
+
+      // A real external origin must never pass as loopback equivalent.
+      const headersExternalOrigin = {
+        host: "localhost",
+        origin: "http://evil.example.com",
+      };
+
+      if (server.isSameOrigin(headersExternalOrigin)) {
+        throw new Error(
+          "isSameOrigin must NOT allow external origins to match loopback host",
+        );
+      }
+
+      const response = await page.goto(`http://localhost:${port1}/main.js`, {
+        waitUntil: "networkidle0",
+      });
+
+      expect(response.status()).toMatchSnapshot("response status");
+      expect(consoleMessages.map((message) => message.text())).toMatchSnapshot(
+        "console messages",
+      );
+      expect(pageErrors).toMatchSnapshot("page errors");
+    });
   });
 });