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fix: send SSH_MSG_DISCONNECT on KEX failure #1091

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Merged
JacobBarthelmeh merged 1 commit into from
Jul 24, 2026
Merged

fix: send SSH_MSG_DISCONNECT on KEX failure #1091

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17 changes: 16 additions & 1 deletion src/internal.c
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Original file line number Diff line number Diff line change
Expand Up @@ -5077,6 +5077,14 @@ static void SetPeerAlgoIds(HandshakeInfo* hs,
}


/* The KEXINIT negotiation failures DoKexInit() answers with a disconnect. */
static int IsKexMatchError(int ret)
{
return ret == WS_MATCH_KEX_ALGO_E || ret == WS_MATCH_KEY_ALGO_E ||
ret == WS_MATCH_ENC_ALGO_E || ret == WS_MATCH_MAC_ALGO_E;
}


static int DoKexInit(WOLFSSH* ssh, byte* buf, word32 len, word32* idx)
{
int ret = WS_SUCCESS;
Expand Down Expand Up @@ -5473,6 +5481,10 @@ static int DoKexInit(WOLFSSH* ssh, byte* buf, word32 len, word32* idx)
ret = ssh->error;
}
}
/* RFC 4253 7.1: no common algorithm means both sides disconnect. */
if (IsKexMatchError(ret)) {
(void)SendDisconnect(ssh, WOLFSSH_DISCONNECT_KEY_EXCHANGE_FAILED);
}
WLOG(WS_LOG_DEBUG, "Leaving DoKexInit(), ret = %d", ret);
return ret;
}
Expand Down Expand Up @@ -11674,7 +11686,10 @@ static int DoPacket(WOLFSSH* ssh, byte* bufferConsumed)
case MSGID_KEXINIT:
WLOG(WS_LOG_DEBUG, "Decoding MSGID_KEXINIT");
ret = DoKexInit(ssh, buf + idx, payloadSz, &payloadIdx);
if (ssh->isKeying &&
/* Don't answer a KEXINIT that failed to negotiate; DoKexInit()
* has already disconnected. A healthy rekey lands here as
* WS_REKEYING, so this cannot test for WS_SUCCESS. */
if (!IsKexMatchError(ret) && ssh->isKeying &&
ssh->connectState == CONNECT_SERVER_CHANNEL_REQUEST_DONE) {
if (ssh->handshake->kexId == ID_DH_GEX_SHA256) {
#if !defined(WOLFSSH_NO_DH) && !defined(WOLFSSH_NO_DH_GEX_SHA256)
Expand Down
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