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Reject the KEX requests a client never receives #1222

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Merged
ejohnstown merged 1 commit into from
Sep 2, 2026
Merged

Reject the KEX requests a client never receives #1222

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7 changes: 5 additions & 2 deletions src/internal.c
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Original file line number Diff line number Diff line change
Expand Up @@ -703,7 +703,7 @@ INLINE static int IsMessageAllowedServer(WOLFSSH *ssh, byte msg)
{
/* Only the server should send these messages, never receive. */
if (msg == MSGID_SERVICE_ACCEPT ||
msg == MSGID_KEXDH_REPLY || /* aliases MSGID_KEXDH_GEX_GROUP */
msg == MSGID_KEXDH_REPLY || /* 31: also ECDH, KEM, GEX_GROUP */
msg == MSGID_KEXDH_GEX_REPLY) {
WLOG(WS_LOG_DEBUG, "Message ID %u not allowed by %s %s",
msg, "server", "ever");
Expand Down Expand Up @@ -826,7 +826,10 @@ INLINE static int IsMessageAllowedServer(WOLFSSH *ssh, byte msg)
INLINE static int IsMessageAllowedClient(WOLFSSH *ssh, byte msg)
{
/* Only the client should send these messages, never receive. */
if (msg == MSGID_SERVICE_REQUEST || msg == MSGID_USERAUTH_REQUEST) {
if (msg == MSGID_SERVICE_REQUEST || msg == MSGID_USERAUTH_REQUEST ||
msg == MSGID_KEXDH_INIT || /* 30: also ECDH, KEM, GEX_REQ_OLD */
msg == MSGID_KEXDH_GEX_INIT ||
msg == MSGID_KEXDH_GEX_REQUEST) {
WLOG(WS_LOG_DEBUG, "Message ID %u not allowed by %s %s",
msg, "client", "ever");
ssh->error = WS_MSGID_NOT_ALLOWED_E;
Expand Down
60 changes: 60 additions & 0 deletions tests/regress.c
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Original file line number Diff line number Diff line change
Expand Up @@ -2263,6 +2263,55 @@ static void TestChannelAllowedAfterAuth(WOLFSSH* ssh)
}


/* Reject the key exchange messages that only the client sends. */
static void TestClientOnlyKexMsgsBlocked(WOLFSSH* ssh)
{
int allowed;

ResetSession(ssh);
/* Our KEXINIT is out, the server's group has not arrived. */
ssh->connectState = CONNECT_CLIENT_KEXINIT_SENT;
ssh->isKeying = WOLFSSH_PEER_IS_KEYING;
ssh->handshake = AllocHandshake(ssh);
ssh->handshake->kexId = ID_DH_GEX_SHA256;

allowed = wolfSSH_TestIsMessageAllowed(ssh, MSGID_KEXDH_INIT,
WS_MSG_RECV);
AssertFalse(allowed);
AssertIntEQ(ssh->error, WS_MSGID_NOT_ALLOWED_E);

ssh->error = 0;
allowed = wolfSSH_TestIsMessageAllowed(ssh, MSGID_KEXDH_GEX_INIT,
WS_MSG_RECV);
AssertFalse(allowed);
AssertIntEQ(ssh->error, WS_MSGID_NOT_ALLOWED_E);

ssh->error = 0;
allowed = wolfSSH_TestIsMessageAllowed(ssh, MSGID_KEXDH_GEX_REQUEST,
WS_MSG_RECV);
AssertFalse(allowed);
AssertIntEQ(ssh->error, WS_MSGID_NOT_ALLOWED_E);

/* 31 is the server's answer to a group request, so it stays allowed
* where the client expects it. */
ssh->error = 0;
ssh->handshake->expectMsgId = MSGID_KEXDH_GEX_GROUP;
allowed = wolfSSH_TestIsMessageAllowed(ssh, MSGID_KEXDH_GEX_GROUP,
WS_MSG_RECV);
AssertTrue(allowed);
AssertIntEQ(ssh->handshake->expectMsgId, MSGID_NONE);

/* Same answer during a rekey on an established session. */
ssh->error = 0;
ssh->connectState = CONNECT_DONE;

allowed = wolfSSH_TestIsMessageAllowed(ssh, MSGID_KEXDH_GEX_REQUEST,
WS_MSG_RECV);
AssertFalse(allowed);
AssertIntEQ(ssh->error, WS_MSGID_NOT_ALLOWED_E);
}


/* A service accept is allowed only when no key exchange is in flight.
* connectState does not see a rekey, so isKeying is checked too. */
static void TestClientServiceAcceptBlockedDuringKeying(WOLFSSH* ssh)
Expand Down Expand Up @@ -2561,6 +2610,16 @@ static void TestServerOnlyKexMsgsBlocked(WOLFSSH* ssh)
AssertTrue(allowed);
AssertIntEQ(ssh->error, WS_SUCCESS);

/* 32 sits between the two blocked ids and has to stay allowed. Assert
* it where the server actually expects it, once it has sent the
* group. */
ssh->error = 0;
ssh->handshake->expectMsgId = MSGID_KEXDH_GEX_INIT;
allowed = wolfSSH_TestIsMessageAllowed(ssh, MSGID_KEXDH_GEX_INIT,
WS_MSG_RECV);
AssertTrue(allowed);
AssertIntEQ(ssh->handshake->expectMsgId, MSGID_NONE);

/* Same answer during a rekey on an established session. The pre-keyed
* range check does not run this far along, so a check placed there
* would leave this window open. */
Expand Down Expand Up @@ -11332,6 +11391,7 @@ int main(int argc, char** argv)
TestChannelBlockedBeforeAuth(ssh);
TestChannelBlockedEveryPreAuthState(ssh);
TestChannelAllowedAfterAuth(ssh);
TestClientOnlyKexMsgsBlocked(ssh);
TestClientServiceAcceptBlockedDuringKeying(ssh);
TestChannelOpenRejectedBeforeKex(CONNECT_CLIENT_KEXINIT_SENT);
TestChannelOpenRejectedBeforeKex(CONNECT_CLIENT_KEXDH_INIT_SENT);
Expand Down
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