diff --git a/lib/protocol/Protocol.js b/lib/protocol/Protocol.js
index 85e1249a..73024881 100644
--- a/lib/protocol/Protocol.js
+++ b/lib/protocol/Protocol.js
@@ -218,11 +218,18 @@ class Protocol {
     if (typeof offer !== 'object' || offer === null) {
       offer = (this._server ? DEFAULT_KEXINIT_SERVER : DEFAULT_KEXINIT_CLIENT);
     } else if (offer.constructor !== KexInit) {
-      if (!this._server)
-        offer.kex = offer.kex.concat(['ext-info-c']);
+      if (this._server) {
+        offer.kex = offer.kex.concat(['kex-strict-s-v00@openssh.com']);
+      } else {
+        offer.kex = offer.kex.concat([
+          'ext-info-c',
+          'kex-strict-c-v00@openssh.com',
+        ]);
+      }
       offer = new KexInit(offer);
     }
     this._kex = undefined;
+    this._strictMode = undefined;
     this._kexinit = undefined;
     this._offer = offer;
     this._cipher = new NullCipher(0, this._onWrite);
diff --git a/lib/protocol/kex.js b/lib/protocol/kex.js
index 1724ff1e..811e631b 100644
--- a/lib/protocol/kex.js
+++ b/lib/protocol/kex.js
@@ -232,13 +232,37 @@ function handleKexInit(self, payload) {
     clientList = localKex;
     remoteExtInfoEnabled = (serverList.indexOf('ext-info-s') !== -1);
   }
+  if (self._strictMode === undefined) {
+    if (self._server) {
+      self._strictMode =
+        (clientList.indexOf('kex-strict-c-v00@openssh.com') !== -1);
+    } else {
+      self._strictMode =
+        (serverList.indexOf('kex-strict-s-v00@openssh.com') !== -1);
+    }
+    // Note: We check for seqno of 1 instead of 0 since we increment before
+    //       calling the packet handler
+    if (self._strictMode) {
+      debug && debug('Handshake: strict KEX mode enabled');
+      if (self._decipher.inSeqno !== 1) {
+        if (debug)
+          debug('Handshake: KEXINIT not first packet in strict KEX mode');
+        return doFatalError(
+          self,
+          'Handshake failed: KEXINIT not first packet in strict KEX mode',
+          'handshake',
+          DISCONNECT_REASON.KEY_EXCHANGE_FAILED
+        );
+      }
+    }
+  }
   // Check for agreeable key exchange algorithm
   for (i = 0;
        i < clientList.length && serverList.indexOf(clientList[i]) === -1;
        ++i);
   if (i === clientList.length) {
     // No suitable match found!
-    debug && debug('Handshake: No matching key exchange algorithm');
+    debug && debug('Handshake: no matching key exchange algorithm');
     return doFatalError(
       self,
       'Handshake failed: no matching key exchange algorithm',
@@ -1236,6 +1260,8 @@ const createKeyExchange = (() => {
             'Inbound: NEWKEYS'
           );
           this._receivedNEWKEYS = true;
+          if (this._protocol._strictMode)
+            this._protocol._decipher.inSeqno = 0;
           ++this._step;
 
           return this.finish(!this._protocol._server && !this._hostVerified);
@@ -1756,11 +1782,20 @@ function onKEXPayload(state, payload) {
   payload = this._packetRW.read.read(payload);
 
   const type = payload[0];
+
+  if (!this._strictMode) {
+    switch (type) {
+      case MESSAGE.IGNORE:
+      case MESSAGE.UNIMPLEMENTED:
+      case MESSAGE.DEBUG:
+        if (!MESSAGE_HANDLERS)
+          MESSAGE_HANDLERS = require('./handlers.js');
+        return MESSAGE_HANDLERS[type](this, payload);
+    }
+  }
+
   switch (type) {
     case MESSAGE.DISCONNECT:
-    case MESSAGE.IGNORE:
-    case MESSAGE.UNIMPLEMENTED:
-    case MESSAGE.DEBUG:
       if (!MESSAGE_HANDLERS)
         MESSAGE_HANDLERS = require('./handlers.js');
       return MESSAGE_HANDLERS[type](this, payload);
@@ -1776,6 +1811,8 @@ function onKEXPayload(state, payload) {
       state.firstPacket = false;
       return handleKexInit(this, payload);
     default:
+      // Ensure packet is either an algorithm negotiation or KEX
+      // algorithm-specific packet
       if (type < 20 || type > 49) {
         return doFatalError(
           this,
@@ -1824,6 +1861,8 @@ function trySendNEWKEYS(kex) {
       kex._protocol._packetRW.write.finalize(packet, true)
     );
     kex._sentNEWKEYS = true;
+    if (kex._protocol._strictMode)
+      kex._protocol._cipher.outSeqno = 0;
   }
 }
 
@@ -1832,7 +1871,7 @@ module.exports = {
   kexinit,
   onKEXPayload,
   DEFAULT_KEXINIT_CLIENT: new KexInit({
-    kex: DEFAULT_KEX.concat(['ext-info-c']),
+    kex: DEFAULT_KEX.concat(['ext-info-c', 'kex-strict-c-v00@openssh.com']),
     serverHostKey: DEFAULT_SERVER_HOST_KEY,
     cs: {
       cipher: DEFAULT_CIPHER,
@@ -1848,7 +1887,7 @@ module.exports = {
     },
   }),
   DEFAULT_KEXINIT_SERVER: new KexInit({
-    kex: DEFAULT_KEX,
+    kex: DEFAULT_KEX.concat(['kex-strict-s-v00@openssh.com']),
     serverHostKey: DEFAULT_SERVER_HOST_KEY,
     cs: {
       cipher: DEFAULT_CIPHER,
diff --git a/lib/server.js b/lib/server.js
index c10c42ed..306d6584 100644
--- a/lib/server.js
+++ b/lib/server.js
@@ -294,7 +294,11 @@ class Server extends EventEmitter {
     }
 
     const algorithms = {
-      kex: generateAlgorithmList(cfgAlgos.kex, DEFAULT_KEX, SUPPORTED_KEX),
+      kex: generateAlgorithmList(
+        cfgAlgos.kex,
+        DEFAULT_KEX,
+        SUPPORTED_KEX
+      ).concat(['kex-strict-s-v00@openssh.com']),
       serverHostKey: hostKeyAlgoOrder,
       cs: {
         cipher: generateAlgorithmList(