CVE-2023-48795: Improper Validation of Integrity Check Value (CWE-354)
value: The SSH transport protocol with certain OpenSSH extensions, found in OpenSSH before 9.6 and other products, allows remote attackers to bypass integrity checks such that some packets are omitted (from the extension negotiation message), and a client and server may consequently end up with a connection for which some security features have been downgraded or disabled, aka a Terrapin attack. This occurs because the SSH Binary Packet Protocol (BPP), implemented by these extensions, mishandles the handshake phase and mishandles use of sequence numbers. For example, there is an effective attack against SSH's use of ChaCha20-Poly1305 (and CBC with Encrypt-then-MAC). The bypass occurs in chacha20-poly1305@openssh.com and (if CBC is used) the -etm@openssh.com MAC algorithms. This also affects Maverick Synergy Java SSH API before 3.1.0-SNAPSHOT, Dropbear through 2022.83, Ssh before 5.1.1 in Erlang/OTP, PuTTY before 0.80, AsyncSSH before 2.14.2, golang.org/x/crypto before 0.17.0, libssh before 0.10.6, libssh2 through 1.11.0, Thorn Tech SFTP Gateway before 3.4.6, Tera Term before 5.1, Paramiko before 3.4.0, jsch before 0.2.15, SFTPGo before 2.5.6, Netgate pfSense Plus through 23.09.1, Netgate pfSense CE through 2.7.2, HPN-SSH through 18.2.0, ProFTPD before 1.3.8b (and before 1.3.9rc2), ORYX CycloneSSH before 2.3.4, NetSarang XShell 7 before Build 0144, CrushFTP before 10.6.0, ConnectBot SSH library before 2.2.22, Apache MINA sshd through 2.11.0, sshj through 0.37.0, TinySSH through 20230101, trilead-ssh2 6401, LANCOM LCOS and LANconfig, FileZilla before 3.66.4, Nova before 11.8, PKIX-SSH before 14.4, SecureCRT before 9.4.3, Transmit5 before 5.10.4, Win32-OpenSSH before 9.5.0.0p1-Beta, WinSCP before 6.2.2, Bitvise SSH Server before 9.32, Bitvise SSH Client before 9.33, KiTTY through 0.76.1.13, the net-ssh gem 7.2.0 for Ruby, the mscdex ssh2 module before 1.15.0 for Node.js, the thrussh library before 0.35.1 for Rust, and the Russh crate before 0.40.2 for Rust.
CWE description: The product does not validate or incorrectly validates the integrity check values or "checksums" of a message. This may prevent it from detecting if the data has been modified or corrupted in transmission.
CVSS v3 base score: 5.9
Published: 2023-12-18T16:15Z
Repository: https://github.com/mscdex/ssh2
Language: JavaScript
Files changed: lib/protocol/Protocol.js, lib/protocol/kex.js, lib/server.js
Commit date: 2023-12-18 14:42:10 -0500
Commit message: lib: add strict key exchange mode support

Diff: diff --git a/lib/protocol/Protocol.js b/lib/protocol/Protocol.js
index 85e1249..7302488 100644
--- a/lib/protocol/Protocol.js
+++ b/lib/protocol/Protocol.js
@@ -216,15 +216,22 @@ class Protocol {
 
     let offer = config.offer;
     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);
     this._decipher = undefined;
     this._skipNextInboundPacket = false;
diff --git a/lib/protocol/kex.js b/lib/protocol/kex.js
index 1724ff1..811e631 100644
--- a/lib/protocol/kex.js
+++ b/lib/protocol/kex.js
@@ -230,17 +230,41 @@ function handleKexInit(self, payload) {
   } else {
     serverList = remote.kex;
     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',
       'handshake',
       DISCONNECT_REASON.KEY_EXCHANGE_FAILED
@@ -1234,10 +1258,12 @@ const createKeyExchange = (() => {
           }
           this._protocol._debug && this._protocol._debug(
             'Inbound: NEWKEYS'
           );
           this._receivedNEWKEYS = true;
+          if (this._protocol._strictMode)
+            this._protocol._decipher.inSeqno = 0;
           ++this._step;
 
           return this.finish(!this._protocol._server && !this._hostVerified);
         default:
           return doFatalError(
@@ -1754,15 +1780,24 @@ 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);
     case MESSAGE.KEXINIT:
       if (!state.firstPacket) {
@@ -1774,10 +1809,12 @@ 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,
           `Received unexpected packet type ${type}`,
           'handshake',
@@ -1822,19 +1859,21 @@ function trySendNEWKEYS(kex) {
     packet[p] = MESSAGE.NEWKEYS;
     kex._protocol._cipher.encrypt(
       kex._protocol._packetRW.write.finalize(packet, true)
     );
     kex._sentNEWKEYS = true;
+    if (kex._protocol._strictMode)
+      kex._protocol._cipher.outSeqno = 0;
   }
 }
 
 module.exports = {
   KexInit,
   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,
       mac: DEFAULT_MAC,
       compress: DEFAULT_COMPRESSION,
@@ -1846,11 +1885,11 @@ module.exports = {
       compress: DEFAULT_COMPRESSION,
       lang: [],
     },
   }),
   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,
       mac: DEFAULT_MAC,
       compress: DEFAULT_COMPRESSION,
diff --git a/lib/server.js b/lib/server.js
index c10c42e..306d658 100644
--- a/lib/server.js
+++ b/lib/server.js
@@ -292,11 +292,15 @@ class Server extends EventEmitter {
         hostKeyAlgoOrder.push(privateKey.type);
       }
     }
 
     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(
                   cfgAlgos.cipher,
                   DEFAULT_CIPHER,

Vulnerable code:       if (!this._server)
        offer.kex = offer.kex.concat(['ext-info-c']);
    debug && debug('Handshake: No matching key exchange algorithm');
    case MESSAGE.IGNORE:
    case MESSAGE.UNIMPLEMENTED:
    case MESSAGE.DEBUG:
    kex: DEFAULT_KEX.concat(['ext-info-c']),
    kex: DEFAULT_KEX,
      kex: generateAlgorithmList(cfgAlgos.kex, DEFAULT_KEX, SUPPORTED_KEX),
Fixed code:       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',
        ]);
      }
    this._strictMode = undefined;
  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
        );
      }
    }
  }
    debug && debug('Handshake: no matching key exchange algorithm');
          if (this._protocol._strictMode)
            this._protocol._decipher.inSeqno = 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);
    }
  }

      // Ensure packet is either an algorithm negotiation or KEX
      // algorithm-specific packet
    if (kex._protocol._strictMode)
      kex._protocol._cipher.outSeqno = 0;
    kex: DEFAULT_KEX.concat(['ext-info-c', 'kex-strict-c-v00@openssh.com']),
    kex: DEFAULT_KEX.concat(['kex-strict-s-v00@openssh.com']),
      kex: generateAlgorithmList(
        cfgAlgos.kex,
        DEFAULT_KEX,
        SUPPORTED_KEX
      ).concat(['kex-strict-s-v00@openssh.com']),