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/TeraTermProject/teraterm
Language: C
Files changed: ttssh2/ttxssh/kex.c, ttssh2/ttxssh/ssh.c, ttssh2/ttxssh/ttxssh.h
Commit date: 2023-12-19 01:27:45 +0900
Commit message: Strict KEX 対応 (CVE-2023-48795)

とりあえず Strict KEX のネゴシエーションとシーケンス番号のリセットに対応。

Diff: diff --git a/ttssh2/ttxssh/kex.c b/ttssh2/ttxssh/kex.c
index d74eb23d0..536711a1b 100644
--- a/ttssh2/ttxssh/kex.c
+++ b/ttssh2/ttxssh/kex.c
@@ -144,12 +144,12 @@ void SSH2_update_kex_myproposal(PTInstVar pvar)
 	if (pvar->socket != INVALID_SOCKET) {
 		if (pvar->kex_status & KEX_FLAG_REKEYING) {
 			// L[Äì¬ÌêÉÍAÚ±É pvar->settings ©çgÝ§Äçê½ myproposal ð«·¦éB
 			//   pvar->settings ª Ú±É myproposal ðì¬µ½Æ«Ìl©çÏíÁÄ¢È¢ÛØªÈ¢B
 			//   ÄxgÝ§ÄéÌÅÍÈ­ù¶Ì myproposal ð«·¦é±ÆÉµ½B
-			int pos = strlen(myproposal[PROPOSAL_KEX_ALGS]) - strlen(",ext-info-c");
-			if (strcmp(myproposal[PROPOSAL_KEX_ALGS] + pos, ",ext-info-c") == 0) {
+			int pos = strlen(myproposal[PROPOSAL_KEX_ALGS]) - strlen(",ext-info-c,kex-strict-c-v00@openssh.com");
+			if (strcmp(myproposal[PROPOSAL_KEX_ALGS] + pos, ",ext-info-c,kex-strict-c-v00@openssh.com") == 0) {
 				myproposal[PROPOSAL_KEX_ALGS][pos] = '\0';
 			}
 		}
 		return;
 	}
@@ -161,12 +161,12 @@ void SSH2_update_kex_myproposal(PTInstVar pvar)
 			break;
 		strncat_s(buf, sizeof(buf), get_kex_algorithm_name(index), _TRUNCATE);
 		strncat_s(buf, sizeof(buf), ",", _TRUNCATE);
 	}
 
-	// RFC 8308 Extension Negotiation
-	strncat_s(buf, sizeof(buf), "ext-info-c", _TRUNCATE);
+	// Enables RFC 8308 Extension Negotiation & Strict KEX mode (for CVE-2023-48795)
+	strncat_s(buf, sizeof(buf), "ext-info-c,kex-strict-c-v00@openssh.com", _TRUNCATE);
 
 	myproposal[PROPOSAL_KEX_ALGS] = buf; 
 }
 
 
diff --git a/ttssh2/ttxssh/ssh.c b/ttssh2/ttxssh/ssh.c
index e52b9e696..4ed4cf27d 100644
--- a/ttssh2/ttxssh/ssh.c
+++ b/ttssh2/ttxssh/ssh.c
@@ -2995,10 +2995,11 @@ void SSH_init(PTInstVar pvar)
 	pvar->tryed_ssh2_authlist = FALSE;
 	pvar->agentfwd_enable = FALSE;
 	pvar->use_subsystem = FALSE;
 	pvar->nosession = FALSE;
 	pvar->server_sig_algs = NULL;
+	pvar->server_strict_kex = FALSE;
 
 }
 
 void SSH_open(PTInstVar pvar)
 {
@@ -4811,11 +4812,11 @@ static BOOL handle_SSH2_kexinit(PTInstVar pvar)
 	// ·ÅÉL[ð·ªIíÁÄ¢éÉàÖíç¸AT[o©ç SSH2_MSG_KEXINIT ª
 	// çêÄ­éêÍAL[Äì¬ðs¤B(2004.10.24 yutaka)
 	if (pvar->kex_status == KEX_FLAG_KEXDONE) {
 		pvar->kex_status = KEX_FLAG_REKEYING;
 
-		// L[Äì¬Í myproposal ©ç ",ext-info-c" ðí·é
+		// L[Äì¬Í myproposal ©ç ",ext-info-c,kex-strict-c-v00@openssh.com" ðí·é
 		// XV·éÌÍ KEX ÌÝÅæ¢
 		SSH2_update_kex_myproposal(pvar);
 
 		// T[oÖSSH2_MSG_KEXINIT ðé
 		SSH2_send_kexinit(pvar);
@@ -4876,10 +4877,17 @@ static BOOL handle_SSH2_kexinit(PTInstVar pvar)
 		strncat_s(tmp, sizeof(tmp), buf, _TRUNCATE);
 		msg = tmp;
 		goto error;
 	}
 
+	// T[o[¤ªStrict KEXÉÎµÄ¢é©ÌmF
+	choose_SSH2_proposal(buf, "kex-strict-s-v00@openssh.com", tmp, sizeof(tmp));
+	if (tmp[0] != '\0') {
+		pvar->server_strict_kex = TRUE;
+		logprintf(LOG_LEVEL_INFO, "Server supports strict kex. Strict kex will be enabled.");
+	}
+
 	// zXg®ASY
 	switch (get_namelist_from_payload(pvar, buf, sizeof(buf), &size)) {
 	case GetPayloadError:
 		_snprintf_s(tmp, sizeof(tmp), _TRUNCATE,
 					"%s: truncated packet (hostkey algorithms)", __FUNCTION__);
@@ -5642,10 +5650,14 @@ static void ssh2_send_newkeys(PTInstVar pvar)
 	pvar->ssh2_keys[MODE_OUT].comp.enabled = 1;
 	enable_send_compression(pvar);
 
 	pvar->kex_status |= KEX_FLAG_NEWKEYS_SENT;
 
+	if (pvar->server_strict_kex) {
+		pvar->ssh_state.sender_sequence_number = 0;
+	}
+
 	// SSH2_MSG_NEWKEYS ðùÉó¯æÁÄ¢½çKEXÍ®¹BÌÉÚéB
 	if (pvar->kex_status & KEX_FLAG_NEWKEYS_RECEIVED) {
 		if ((pvar->kex_status & KEX_FLAG_REKEYING)) {
 			do_SSH2_dispatch_setup_for_transfer(pvar);
 		}
@@ -6236,10 +6248,14 @@ static BOOL handle_SSH2_newkeys(PTInstVar pvar)
 	// ¯lÉAMAC¨æÑ³kðLøÉ·éB
 	pvar->ssh2_keys[MODE_IN].mac.enabled = 1;
 	pvar->ssh2_keys[MODE_IN].comp.enabled = 1;
 	enable_recv_compression(pvar);
 
+	if (pvar->server_strict_kex) {
+		pvar->ssh_state.receiver_sequence_number = 0;
+	}
+
 	SSH2_dispatch_add_message(SSH2_MSG_EXT_INFO);
 
 	// SSH2_MSG_NEWKEYS ðùÉÁÄ¢½çKEXÍ®¹BÌÉÚéB
 	if (pvar->kex_status & KEX_FLAG_NEWKEYS_SENT) {
 		if (pvar->kex_status & KEX_FLAG_REKEYING) {
diff --git a/ttssh2/ttxssh/ttxssh.h b/ttssh2/ttxssh/ttxssh.h
index 135db6392..d52e4ecc6 100644
--- a/ttssh2/ttxssh/ttxssh.h
+++ b/ttssh2/ttxssh/ttxssh.h
@@ -361,10 +361,11 @@ typedef struct _TInstVar {
 		BOOL data_finished;	// TRUEÌÆ«,f[^ÌóMÍ®¹µ½
 		BOOL close_request;
 	} recv;
 
 	char *server_sig_algs;
+	BOOL server_strict_kex;
 
 	char UIMsg[MAX_UIMSG];
 } TInstVar;
 
 // o[WÉí¹Ä©®ÏX³êéB á: TTSSH_2-81_TS_data

Vulnerable code: 			int pos = strlen(myproposal[PROPOSAL_KEX_ALGS]) - strlen(",ext-info-c");
			if (strcmp(myproposal[PROPOSAL_KEX_ALGS] + pos, ",ext-info-c") == 0) {
	// RFC 8308 Extension Negotiation
	strncat_s(buf, sizeof(buf), "ext-info-c", _TRUNCATE);
		// L[Äì¬Í myproposal ©ç ",ext-info-c" ðí·é
Fixed code: 			int pos = strlen(myproposal[PROPOSAL_KEX_ALGS]) - strlen(",ext-info-c,kex-strict-c-v00@openssh.com");
			if (strcmp(myproposal[PROPOSAL_KEX_ALGS] + pos, ",ext-info-c,kex-strict-c-v00@openssh.com") == 0) {
	// Enables RFC 8308 Extension Negotiation & Strict KEX mode (for CVE-2023-48795)
	strncat_s(buf, sizeof(buf), "ext-info-c,kex-strict-c-v00@openssh.com", _TRUNCATE);
	pvar->server_strict_kex = FALSE;
		// L[Äì¬Í myproposal ©ç ",ext-info-c,kex-strict-c-v00@openssh.com" ðí·é
	// T[o[¤ªStrict KEXÉÎµÄ¢é©ÌmF
	choose_SSH2_proposal(buf, "kex-strict-s-v00@openssh.com", tmp, sizeof(tmp));
	if (tmp[0] != '\0') {
		pvar->server_strict_kex = TRUE;
		logprintf(LOG_LEVEL_INFO, "Server supports strict kex. Strict kex will be enabled.");
	}

	if (pvar->server_strict_kex) {
		pvar->ssh_state.sender_sequence_number = 0;
	}

	if (pvar->server_strict_kex) {
		pvar->ssh_state.receiver_sequence_number = 0;
	}

	BOOL server_strict_kex;