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Reject the KEX replies a server never receives #1221

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

Reject the KEX replies a server never receives #1221

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4 changes: 3 additions & 1 deletion src/internal.c
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Original file line number Diff line number Diff line change
Expand Up @@ -702,7 +702,9 @@ static void HandshakeInfoFree(HandshakeInfo* hs, void* heap)
INLINE static int IsMessageAllowedServer(WOLFSSH *ssh, byte msg)
{
/* Only the server should send these messages, never receive. */
if (msg == MSGID_SERVICE_ACCEPT) {
if (msg == MSGID_SERVICE_ACCEPT ||
msg == MSGID_KEXDH_REPLY || /* aliases MSGID_KEXDH_GEX_GROUP */
msg == MSGID_KEXDH_GEX_REPLY) {
WLOG(WS_LOG_DEBUG, "Message ID %u not allowed by %s %s",
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msg, "server", "ever");
ssh->error = WS_MSGID_NOT_ALLOWED_E;
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53 changes: 53 additions & 0 deletions tests/regress.c
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Expand Up @@ -2528,6 +2528,58 @@ static void TestServerOnlyUserauthMsgsBlocked(WOLFSSH* ssh)
}


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

ResetSession(ssh);
/* The peer's KEXINIT has landed, its GEX request has not. */
ssh->acceptState = ACCEPT_SERVER_KEXINIT_SENT;
ssh->isKeying = WOLFSSH_PEER_IS_KEYING;
ssh->handshake = AllocHandshake(ssh);
ssh->handshake->kexId = ID_DH_GEX_SHA256;
/* The server expects no particular message yet, so the expectMsgId
* check cannot catch these, the role check has to. */
AssertIntEQ(ssh->handshake->expectMsgId, MSGID_NONE);

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

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

/* The message a conformant client sends in this state is unaffected. */
ssh->error = 0;
allowed = wolfSSH_TestIsMessageAllowed(ssh, MSGID_KEXDH_GEX_REQUEST,
WS_MSG_RECV);
AssertTrue(allowed);
AssertIntEQ(ssh->error, WS_SUCCESS);

/* 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. */
ssh->error = 0;
ssh->acceptState = ACCEPT_CLIENT_SESSION_ESTABLISHED;

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

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


/* The server accepts a service request only once keyed, and never accepts
* the service accept that only it sends. */
static void TestServerServiceRequestStateGated(WOLFSSH* ssh)
Expand Down Expand Up @@ -11024,6 +11076,7 @@ int main(int argc, char** argv)
TestServerKnownAuthMsgIdBeforeAuthDisconnects();
TestServerUnknownHighMsgIdBeforeAuthDisconnects();
TestServerOnlyUserauthMsgsBlocked(serverSsh);
TestServerOnlyKexMsgsBlocked(serverSsh);
TestServerServiceRequestStateGated(serverSsh);
TestServerServiceRequestRejectedDuringKeying();
TestFailedSendClearsPendingPlaintext();
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