diff --git a/client/components/NetworkForm.vue b/client/components/NetworkForm.vue
index 2c893314..1f295e0c 100644
--- a/client/components/NetworkForm.vue
+++ b/client/components/NetworkForm.vue
@@ -65,10 +65,18 @@
 								type="checkbox"
 								name="tls"
 								:checked="defaults.tls ? true : false"
-								:disabled="config.lockNetwork ? true : false"
+								:disabled="
+									config.lockNetwork || defaults.hasSTSPolicy ? true : false
+								"
 								@change="onSecureChanged"
 							/>
 							Use secure connection (TLS)
+							<span
+								v-if="defaults.hasSTSPolicy"
+								class="tooltipped tooltipped-n tooltipped-no-delay"
+								aria-label="This network has a strict transport security policy, you will be unable to disable TLS"
+								>🔒 STS</span
+							>
 						</label>
 						<label class="tls">
 							<input
diff --git a/src/client.js b/src/client.js
index c126aba7..b188fafa 100644
--- a/src/client.js
+++ b/src/client.js
@@ -22,6 +22,7 @@ module.exports = Client;
 
 const events = [
 	"away",
+	"cap",
 	"connection",
 	"unhandled",
 	"ctcp",
@@ -616,10 +617,7 @@ Client.prototype.quit = function(signOut) {
 	}
 
 	this.networks.forEach((network) => {
-		if (network.irc) {
-			network.irc.quit(Helper.config.leaveMessage);
-		}
-
+		network.quit(Helper.config.leaveMessage);
 		network.destroy();
 	});
 
diff --git a/src/models/network.js b/src/models/network.js
index 41cab75b..07fbf4ba 100644
--- a/src/models/network.js
+++ b/src/models/network.js
@@ -6,6 +6,7 @@ const IrcFramework = require("irc-framework");
 const Chan = require("./chan");
 const Msg = require("./msg");
 const Helper = require("../helper");
+const STSPolicies = require("../plugins/sts");
 
 module.exports = Network;
 
@@ -78,7 +79,7 @@ Network.prototype.validate = function(client) {
 	this.username = cleanString(this.username) || "thelounge";
 	this.realname = cleanString(this.realname) || "The Lounge User";
 	this.password = cleanString(this.password);
-	this.host = cleanString(this.host);
+	this.host = cleanString(this.host).toLowerCase();
 	this.name = cleanString(this.name);
 
 	if (!this.port) {
@@ -124,6 +125,23 @@ Network.prototype.validate = function(client) {
 		return false;
 	}
 
+	const stsPolicy = STSPolicies.get(this.host);
+
+	if (stsPolicy && !this.tls) {
+		this.channels[0].pushMessage(
+			client,
+			new Msg({
+				type: Msg.Type.ERROR,
+				text: `${this.host} has an active strict transport security policy, will connect to port ${stsPolicy.port} over a secure connection.`,
+			}),
+			true
+		);
+
+		this.port = stsPolicy.port;
+		this.tls = true;
+		this.rejectUnauthorized = true;
+	}
+
 	return true;
 };
 
@@ -355,6 +373,17 @@ Network.prototype.addChannel = function(newChan) {
 	return index;
 };
 
+Network.prototype.quit = function(quitMessage) {
+	if (!this.irc) {
+		return;
+	}
+
+	// https://ircv3.net/specs/extensions/sts#rescheduling-expiry-on-disconnect
+	STSPolicies.refreshExpiration(this.host);
+
+	this.irc.quit(quitMessage || Helper.config.leaveMessage);
+};
+
 Network.prototype.exportForEdit = function() {
 	let fieldsToReturn;
 
@@ -378,7 +407,11 @@ Network.prototype.exportForEdit = function() {
 		fieldsToReturn = ["name", "nick", "username", "password", "realname"];
 	}
 
-	return _.pick(this, fieldsToReturn);
+	const data = _.pick(this, fieldsToReturn);
+
+	data.hasSTSPolicy = !!STSPolicies.get(this.host);
+
+	return data;
 };
 
 Network.prototype.export = function() {
diff --git a/src/plugins/inputs/disconnect.js b/src/plugins/inputs/disconnect.js
index db733a44..3fe2aa30 100644
--- a/src/plugins/inputs/disconnect.js
+++ b/src/plugins/inputs/disconnect.js
@@ -1,19 +1,13 @@
 "use strict";
 
-const Helper = require("../../helper");
-
 exports.commands = ["disconnect"];
 exports.allowDisconnected = true;
 
 exports.input = function(network, chan, cmd, args) {
-	const quitMessage = args[0] ? args.join(" ") : Helper.config.leaveMessage;
-
-	// Even if we are disconnected, but there is an internal connection object
-	// pass the quit/end to it, so the reconnection timer stops
-	if (network.irc && network.irc.connection) {
-		network.irc.quit(quitMessage);
-	}
+	const quitMessage = args[0] ? args.join(" ") : null;
 
+	network.quit(quitMessage);
 	network.userDisconnected = true;
+
 	this.save();
 };
diff --git a/src/plugins/inputs/quit.js b/src/plugins/inputs/quit.js
index 252f050b..0bce3fa3 100644
--- a/src/plugins/inputs/quit.js
+++ b/src/plugins/inputs/quit.js
@@ -1,7 +1,6 @@
 "use strict";
 
 const _ = require("lodash");
-const Helper = require("../../helper");
 
 exports.commands = ["quit"];
 exports.allowDisconnected = true;
@@ -16,10 +15,8 @@ exports.input = function(network, chan, cmd, args) {
 		network: network.uuid,
 	});
 
-	if (network.irc) {
-		const quitMessage = args[0] ? args.join(" ") : Helper.config.leaveMessage;
-		network.irc.quit(quitMessage);
-	}
+	const quitMessage = args[0] ? args.join(" ") : null;
+	network.quit(quitMessage);
 
 	return true;
 };
diff --git a/src/plugins/irc-events/cap.js b/src/plugins/irc-events/cap.js
new file mode 100644
index 00000000..da20d3cc
--- /dev/null
+++ b/src/plugins/irc-events/cap.js
@@ -0,0 +1,78 @@
+"use strict";
+
+const Msg = require("../../models/msg");
+const STSPolicies = require("../sts");
+
+module.exports = function(irc, network) {
+	const client = this;
+
+	irc.on("cap ls", (data) => {
+		handleSTS(data, true);
+	});
+
+	irc.on("cap new", (data) => {
+		handleSTS(data, false);
+	});
+
+	function handleSTS(data, shouldReconnect) {
+		if (!Object.prototype.hasOwnProperty.call(data.capabilities, "sts")) {
+			return;
+		}
+
+		const isSecure = irc.connection.transport.socket.encrypted;
+		const values = {};
+
+		data.capabilities.sts.split(",").map((value) => {
+			value = value.split("=", 2);
+			values[value[0]] = value[1];
+		});
+
+		if (isSecure) {
+			const duration = parseInt(values.duration, 10);
+
+			if (isNaN(duration)) {
+				return;
+			}
+
+			STSPolicies.update(network.host, network.port, duration);
+		} else {
+			const port = parseInt(values.port, 10);
+
+			if (isNaN(port)) {
+				return;
+			}
+
+			network.channels[0].pushMessage(
+				client,
+				new Msg({
+					text: `Server sent a strict transport security policy, reconnecting to ${network.host}:${port}…`,
+				}),
+				true
+			);
+
+			// Forcefully end the connection if STS is seen in CAP LS
+			// We will update the port and tls setting if we see CAP NEW,
+			// but will not force a reconnection
+			if (shouldReconnect) {
+				irc.connection.end();
+			}
+
+			// Update the port
+			network.port = port;
+			irc.options.port = port;
+
+			// Enable TLS
+			network.tls = true;
+			network.rejectUnauthorized = true;
+			irc.options.tls = true;
+			irc.options.rejectUnauthorized = true;
+
+			if (shouldReconnect) {
+				// Start a new connection
+				irc.connect();
+			}
+
+			client.save();
+		}
+	}
+};
diff --git a/src/plugins/sts.js b/src/plugins/sts.js
new file mode 100644
index 00000000..25164ee7
--- /dev/null
+++ b/src/plugins/sts.js
@@ -0,0 +1,95 @@
+"use strict";
+
+const _ = require("lodash");
+const fs = require("fs");
+const path = require("path");
+const log = require("../log");
+const Helper = require("../helper");
+
+class STSPolicies {
+	constructor() {
+		this.stsFile = path.join(Helper.getHomePath(), "sts-policies.json");
+		this.policies = new Map();
+		this.refresh = _.debounce(this.saveFile, 10000, {maxWait: 60000});
+
+		if (!fs.existsSync(this.stsFile)) {
+			return;
+		}
+
+		const storedPolicies = JSON.parse(fs.readFileSync(this.stsFile, "utf-8"));
+		const now = Date.now();
+
+		storedPolicies.forEach((value) => {
+			if (value.expires > now) {
+				this.policies.set(value.host, {
+					port: value.port,
+					duration: value.duration,
+					expires: value.expires,
+				});
+			}
+		});
+	}
+
+	get(host) {
+		const policy = this.policies.get(host);
+
+		if (typeof policy === "undefined") {
+			return null;
+		}
+
+		if (policy.expires <= Date.now()) {
+			this.policies.delete(host);
+			this.refresh();
+			return null;
+		}
+
+		return policy;
+	}
+
+	update(host, port, duration) {
+		if (duration > 0) {
+			this.policies.set(host, {
+				port: port,
+				duration: duration,
+				expires: Date.now() + duration * 1000,
+			});
+		} else {
+			this.policies.delete(host);
+		}
+
+		this.refresh();
+	}
+
+	refreshExpiration(host) {
+		const policy = this.policies.get(host);
+
+		if (typeof policy === "undefined") {
+			return null;
+		}
+
+		policy.expires = Date.now() + policy.duration * 1000;
+	}
+
+	saveFile() {
+		const policiesToStore = [];
+
+		this.policies.forEach((value, key) => {
+			policiesToStore.push({
+				host: key,
+				port: value.port,
+				duration: value.duration,
+				expires: value.expires,
+			});
+		});
+
+		const file = JSON.stringify(policiesToStore, null, "\t");
+
+		fs.writeFile(this.stsFile, file, {flag: "w+"}, (err) => {
+			if (err) {
+				log.error("Failed to update STS policies file!", err);
+			}
+		});
+	}
+}
+
+module.exports = new STSPolicies();