diff --git a/chess-server/Sources/App/Middleware/SecurityHeadersMiddleware.swift b/chess-server/Sources/App/Middleware/SecurityHeadersMiddleware.swift
new file mode 100644
index 0000000..2415c26
--- /dev/null
+++ b/chess-server/Sources/App/Middleware/SecurityHeadersMiddleware.swift
@@ -0,0 +1,30 @@
+import Vapor
+
+/// Adds baseline hardening headers to every response. This is a JSON API
+/// with no HTML surface, so these mitigate second-order risk (a browser ever
+/// rendering a response body, e.g. an error page or a misconfigured proxy)
+/// rather than a direct attack this server itself is exposed to.
+struct SecurityHeadersMiddleware: AsyncMiddleware {
+    func respond(to request: Request, chainingTo next: AsyncResponder) async throws -> Response {
+        let response = try await next.respond(to: request)
+        // A JSON API is never a plausible framing target, but this costs
+        // nothing and forecloses it outright.
+        response.headers.replaceOrAdd(name: .init("X-Frame-Options"), value: "DENY")
+        // Stops a browser from MIME-sniffing a response into something more
+        // dangerous than the declared Content-Type (e.g. text/html).
+        response.headers.replaceOrAdd(name: .init("X-Content-Type-Options"), value: "nosniff")
+        // No page on this origin should ever leak the request URL (which can
+        // carry a bearer-adjacent path segment, e.g. /players/:id) via the
+        // Referer header on an outbound link.
+        response.headers.replaceOrAdd(name: .init("Referrer-Policy"), value: "no-referrer")
+        // Fly's edge terminates TLS and forwards over the same TLS
+        // connection to the client, so this reaches browsers over HTTPS even
+        // though Vapor itself only ever sees plain HTTP behind the proxy.
+        // Harmless for the iOS client, which ignores it.
+        response.headers.replaceOrAdd(
+            name: .init("Strict-Transport-Security"),
+            value: "max-age=31536000; includeSubDomains"
+        )
+        return response
+    }
+}
diff --git a/chess-server/Sources/App/configure.swift b/chess-server/Sources/App/configure.swift
index e647f9a..2cf0c45 100644
--- a/chess-server/Sources/App/configure.swift
+++ b/chess-server/Sources/App/configure.swift
@@ -79,5 +79,14 @@ public func configure(_ app: Application) async throws {
 
     app.gameCoordinator = GameCoordinator(app: app)
 
+    // MARK: Response hardening
+
+    // `.beginning` so this wraps Vapor's default ErrorMiddleware rather than
+    // being wrapped by it: at `.end` (the default position), an error thrown
+    // by routing or a handler unwinds straight past this middleware without
+    // running its header-setting code, so 404s/401s/429s/500s would ship
+    // without these headers — exactly the responses where they matter most.
+    app.middleware.use(SecurityHeadersMiddleware(), at: .beginning)
+
     try routes(app)
 }
diff --git a/chess-server/Tests/AppTests/SecurityHeadersMiddlewareTests.swift b/chess-server/Tests/AppTests/SecurityHeadersMiddlewareTests.swift
new file mode 100644
index 0000000..b42420b
--- /dev/null
+++ b/chess-server/Tests/AppTests/SecurityHeadersMiddlewareTests.swift
@@ -0,0 +1,32 @@
+@testable import App
+import XCTVapor
+
+final class SecurityHeadersMiddlewareTests: XCTestCase {
+    var app: Application!
+
+    override func setUp() async throws {
+        app = try await Application.make(.testing)
+        try await configure(app)
+    }
+
+    override func tearDown() async throws {
+        try await app.asyncShutdown()
+    }
+
+    func testResponsesCarryBaselineSecurityHeaders() async throws {
+        try await app.test(.GET, "health", afterResponse: { res async in
+            XCTAssertEqual(res.headers.first(name: "X-Frame-Options"), "DENY")
+            XCTAssertEqual(res.headers.first(name: "X-Content-Type-Options"), "nosniff")
+            XCTAssertEqual(res.headers.first(name: "Referrer-Policy"), "no-referrer")
+            XCTAssertEqual(res.headers.first(name: "Strict-Transport-Security"), "max-age=31536000; includeSubDomains")
+        })
+    }
+
+    func testHeadersAreAlsoPresentOnErrorResponses() async throws {
+        // The middleware wraps every response, including ones the default
+        // error handler generates further down the chain (a 404 here).
+        try await app.test(.GET, "no-such-route", afterResponse: { res async in
+            XCTAssertEqual(res.headers.first(name: "X-Content-Type-Options"), "nosniff")
+        })
+    }
+}