diff --git a/src/api/notifications.rs b/src/api/notifications.rs
index 8bfcd518..32204916 100644
--- a/src/api/notifications.rs
+++ b/src/api/notifications.rs
@@ -1,7 +1,7 @@
 use std::{
     net::IpAddr,
     sync::{Arc, LazyLock},
-    time::Duration,
+    time::{Duration, Instant},
 };
 
 use chrono::{NaiveDateTime, Utc};
@@ -41,6 +41,14 @@ pub static WS_ANONYMOUS_SUBSCRIPTIONS: LazyLock<Arc<AnonymousWebSocketSubscripti
 /// One connection is needed per pending login request, several at once are only expected behind NAT.
 const MAX_ANONYMOUS_CONNECTIONS_PER_IP: u32 = 25;
 
+/// How often a Ping is sent to the client.
+const WS_PING_INTERVAL: Duration = Duration::from_secs(15);
+
+/// Close the connection if nothing, not even a Pong, was received from the client for this long.
+/// Otherwise half-open connections (client gone without a FIN, e.g. behind a proxy or NAT) are kept forever.
+/// Same as the default `ClientTimeoutInterval` of ASP.NET Core SignalR, which the official server uses.
+const WS_CLIENT_TIMEOUT: Duration = Duration::from_secs(30);
+
 static NOTIFICATIONS_DISABLED: LazyLock<bool> = LazyLock::new(|| !CONFIG.enable_websocket() && !CONFIG.push_enabled());
 
 pub fn routes() -> Vec<Route> {
@@ -156,12 +164,16 @@ fn websockets_hub<'r>(
         rocket_ws::Stream! { ws => {
             let mut ws = ws;
             let _guard = guard;
-            let mut interval = tokio::time::interval(Duration::from_secs(15));
+            let mut interval = tokio::time::interval(WS_PING_INTERVAL);
+            let mut last_received = Instant::now();
             loop {
                 tokio::select! {
                     res = ws.next() =>  {
                         match res {
                             Some(Ok(message)) => {
+                                // Any message, including a Pong, means the client is still there
+                                last_received = Instant::now();
+
                                 match message {
                                     // Respond to any pings
                                     Message::Ping(ping) => yield Message::Pong(ping),
@@ -195,7 +207,13 @@ fn websockets_hub<'r>(
                         }
                     }
 
-                    _ = interval.tick() => yield Message::Ping(create_ping())
+                    _ = interval.tick() => {
+                        // The client stopped responding without closing the connection, drop it
+                        if last_received.elapsed() > WS_CLIENT_TIMEOUT {
+                            break;
+                        }
+                        yield Message::Ping(create_ping());
+                    }
                 }
             }
         }}
@@ -229,12 +247,16 @@ fn anonymous_websockets_hub<'r>(ws: WebSocket, token: String, ip: ClientIp) -> R
         rocket_ws::Stream! { ws => {
             let mut ws = ws;
             let _guard = guard;
-            let mut interval = tokio::time::interval(Duration::from_secs(15));
+            let mut interval = tokio::time::interval(WS_PING_INTERVAL);
+            let mut last_received = Instant::now();
             loop {
                 tokio::select! {
                     res = ws.next() =>  {
                         match res {
                             Some(Ok(message)) => {
+                                // Any message, including a Pong, means the client is still there
+                                last_received = Instant::now();
+
                                 match message {
                                     // Respond to any pings
                                     Message::Ping(ping) => yield Message::Pong(ping),
@@ -268,7 +290,13 @@ fn anonymous_websockets_hub<'r>(ws: WebSocket, token: String, ip: ClientIp) -> R
                         }
                     }
 
-                    _ = interval.tick() => yield Message::Ping(create_ping())
+                    _ = interval.tick() => {
+                        // The client stopped responding without closing the connection, drop it
+                        if last_received.elapsed() > WS_CLIENT_TIMEOUT {
+                            break;
+                        }
+                        yield Message::Ping(create_ping());
+                    }
                 }
             }
         }}