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From 4cef5e965a46e9271aed62631b152e4bd23c1e3c Mon Sep 17 00:00:00 2001
From: Aris Adamantiadis <[email protected]>
Date: Tue, 12 Dec 2023 23:09:57 +0100
Subject: [PATCH 11/20] CVE-2023-48795: client side mitigation
Signed-off-by: Aris Adamantiadis <[email protected]>
Signed-off-by: Jakub Jelen <[email protected]>
Reviewed-by: Andreas Schneider <[email protected]>
---
include/libssh/packet.h | 1 +
include/libssh/session.h | 6 +++++
src/curve25519.c | 19 +++----------
src/dh-gex.c | 7 +----
src/dh.c | 17 +++---------
src/ecdh.c | 8 +-----
src/ecdh_crypto.c | 12 +++------
src/ecdh_gcrypt.c | 10 +++----
src/ecdh_mbedcrypto.c | 11 +++-----
src/kex.c | 34 +++++++++++++++++++----
src/packet.c | 58 ++++++++++++++++++++++++++++++++++++++++
src/packet_cb.c | 12 +++++++++
12 files changed, 126 insertions(+), 69 deletions(-)
diff --git a/include/libssh/packet.h b/include/libssh/packet.h
index 7f10a709..f0c8cb20 100644
--- a/include/libssh/packet.h
+++ b/include/libssh/packet.h
@@ -67,6 +67,7 @@ SSH_PACKET_CALLBACK(ssh_packet_ext_info);
SSH_PACKET_CALLBACK(ssh_packet_kexdh_init);
#endif
+int ssh_packet_send_newkeys(ssh_session session);
int ssh_packet_send_unimplemented(ssh_session session, uint32_t seqnum);
int ssh_packet_parse_type(ssh_session session);
//int packet_flush(ssh_session session, int enforce_blocking);
diff --git a/include/libssh/session.h b/include/libssh/session.h
index eb14e97a..97936195 100644
--- a/include/libssh/session.h
+++ b/include/libssh/session.h
@@ -81,6 +81,12 @@ enum ssh_pending_call_e {
* sending it twice during key exchange to simplify the state machine. */
#define SSH_SESSION_FLAG_KEXINIT_SENT 4
+/* The current SSH2 session implements the "strict KEX" feature and should behave
+ * differently on SSH2_MSG_NEWKEYS. */
+#define SSH_SESSION_FLAG_KEX_STRICT 0x0010
+/* Unexpected packets have been sent while the session was still unencrypted */
+#define SSH_SESSION_FLAG_KEX_TAINTED 0x0020
+
/* codes to use with ssh_handle_packets*() */
/* Infinite timeout */
#define SSH_TIMEOUT_INFINITE -1
diff --git a/src/curve25519.c b/src/curve25519.c
index 66291b5f..4aeb4756 100644
--- a/src/curve25519.c
+++ b/src/curve25519.c
@@ -335,16 +335,10 @@ static SSH_PACKET_CALLBACK(ssh_packet_client_curve25519_reply){
}
/* Send the MSG_NEWKEYS */
- if (ssh_buffer_add_u8(session->out_buffer, SSH2_MSG_NEWKEYS) < 0) {
- goto error;
- }
-
- rc=ssh_packet_send(session);
+ rc = ssh_packet_send_newkeys(session);
if (rc == SSH_ERROR) {
goto error;
}
-
- SSH_LOG(SSH_LOG_PROTOCOL, "SSH_MSG_NEWKEYS sent");
session->dh_handshake_state = DH_STATE_NEWKEYS_SENT;
return SSH_PACKET_USED;
@@ -502,18 +496,13 @@ static SSH_PACKET_CALLBACK(ssh_packet_server_curve25519_init){
return SSH_ERROR;
}
- /* Send the MSG_NEWKEYS */
- rc = ssh_buffer_add_u8(session->out_buffer, SSH2_MSG_NEWKEYS);
- if (rc < 0) {
- goto error;
- }
-
session->dh_handshake_state = DH_STATE_NEWKEYS_SENT;
- rc = ssh_packet_send(session);
+
+ /* Send the MSG_NEWKEYS */
+ rc = ssh_packet_send_newkeys(session);
if (rc == SSH_ERROR) {
goto error;
}
- SSH_LOG(SSH_LOG_PROTOCOL, "SSH_MSG_NEWKEYS sent");
return SSH_PACKET_USED;
error:
diff --git a/src/dh-gex.c b/src/dh-gex.c
index 91617081..642a88ae 100644
--- a/src/dh-gex.c
+++ b/src/dh-gex.c
@@ -297,15 +297,10 @@ static SSH_PACKET_CALLBACK(ssh_packet_client_dhgex_reply)
}
/* Send the MSG_NEWKEYS */
- if (ssh_buffer_add_u8(session->out_buffer, SSH2_MSG_NEWKEYS) < 0) {
- goto error;
- }
-
- rc = ssh_packet_send(session);
+ rc = ssh_packet_send_newkeys(session);
if (rc == SSH_ERROR) {
goto error;
}
- SSH_LOG(SSH_LOG_PROTOCOL, "SSH_MSG_NEWKEYS sent");
session->dh_handshake_state = DH_STATE_NEWKEYS_SENT;
return SSH_PACKET_USED;
diff --git a/src/dh.c b/src/dh.c
index 011d97b3..e19e43d1 100644
--- a/src/dh.c
+++ b/src/dh.c
@@ -398,16 +398,10 @@ SSH_PACKET_CALLBACK(ssh_packet_client_dh_reply){
}
/* Send the MSG_NEWKEYS */
- if (ssh_buffer_add_u8(session->out_buffer, SSH2_MSG_NEWKEYS) < 0) {
- goto error;
- }
-
- rc=ssh_packet_send(session);
+ rc = ssh_packet_send_newkeys(session);
if (rc == SSH_ERROR) {
goto error;
}
-
- SSH_LOG(SSH_LOG_PROTOCOL, "SSH_MSG_NEWKEYS sent");
session->dh_handshake_state = DH_STATE_NEWKEYS_SENT;
return SSH_PACKET_USED;
error:
@@ -551,15 +545,12 @@ int ssh_server_dh_process_init(ssh_session session, ssh_buffer packet)
}
SSH_LOG(SSH_LOG_DEBUG, "Sent KEX_DH_[GEX]_REPLY");
- if (ssh_buffer_add_u8(session->out_buffer, SSH2_MSG_NEWKEYS) < 0) {
- ssh_buffer_reinit(session->out_buffer);
- goto error;
- }
session->dh_handshake_state=DH_STATE_NEWKEYS_SENT;
- if (ssh_packet_send(session) == SSH_ERROR) {
+ /* Send the MSG_NEWKEYS */
+ rc = ssh_packet_send_newkeys(session);
+ if (rc == SSH_ERROR) {
goto error;
}
- SSH_LOG(SSH_LOG_PACKET, "SSH_MSG_NEWKEYS sent");
return SSH_OK;
error:
diff --git a/src/ecdh.c b/src/ecdh.c
index e5b11ba9..af80beec 100644
--- a/src/ecdh.c
+++ b/src/ecdh.c
@@ -93,16 +93,10 @@ SSH_PACKET_CALLBACK(ssh_packet_client_ecdh_reply){
}
/* Send the MSG_NEWKEYS */
- if (ssh_buffer_add_u8(session->out_buffer, SSH2_MSG_NEWKEYS) < 0) {
- goto error;
- }
-
- rc=ssh_packet_send(session);
+ rc = ssh_packet_send_newkeys(session);
if (rc == SSH_ERROR) {
goto error;
}
-
- SSH_LOG(SSH_LOG_PROTOCOL, "SSH_MSG_NEWKEYS sent");
session->dh_handshake_state = DH_STATE_NEWKEYS_SENT;
return SSH_PACKET_USED;
diff --git a/src/ecdh_crypto.c b/src/ecdh_crypto.c
index 51084b7a..069b1372 100644
--- a/src/ecdh_crypto.c
+++ b/src/ecdh_crypto.c
@@ -619,18 +619,12 @@ SSH_PACKET_CALLBACK(ssh_packet_server_ecdh_init){
goto error;
}
- /* Send the MSG_NEWKEYS */
- rc = ssh_buffer_add_u8(session->out_buffer, SSH2_MSG_NEWKEYS);
- if (rc < 0) {
- goto error;
- }
-
session->dh_handshake_state = DH_STATE_NEWKEYS_SENT;
- rc = ssh_packet_send(session);
- if (rc == SSH_ERROR){
+ /* Send the MSG_NEWKEYS */
+ rc = ssh_packet_send_newkeys(session);
+ if (rc == SSH_ERROR) {
goto error;
}
- SSH_LOG(SSH_LOG_PROTOCOL, "SSH_MSG_NEWKEYS sent");
return SSH_PACKET_USED;
error:
diff --git a/src/ecdh_gcrypt.c b/src/ecdh_gcrypt.c
index 235f2904..3d9d426f 100644
--- a/src/ecdh_gcrypt.c
+++ b/src/ecdh_gcrypt.c
@@ -372,17 +372,13 @@ SSH_PACKET_CALLBACK(ssh_packet_server_ecdh_init){
goto out;
}
-
+ session->dh_handshake_state = DH_STATE_NEWKEYS_SENT;
/* Send the MSG_NEWKEYS */
- rc = ssh_buffer_add_u8(session->out_buffer, SSH2_MSG_NEWKEYS);
- if (rc != SSH_OK) {
+ rc = ssh_packet_send_newkeys(session);
+ if (rc == SSH_ERROR) {
goto out;
}
- session->dh_handshake_state = DH_STATE_NEWKEYS_SENT;
- rc = ssh_packet_send(session);
- SSH_LOG(SSH_LOG_PROTOCOL, "SSH_MSG_NEWKEYS sent");
-
out:
gcry_sexp_release(param);
gcry_sexp_release(key);
diff --git a/src/ecdh_mbedcrypto.c b/src/ecdh_mbedcrypto.c
index cfe017a0..dda73922 100644
--- a/src/ecdh_mbedcrypto.c
+++ b/src/ecdh_mbedcrypto.c
@@ -318,16 +318,13 @@ SSH_PACKET_CALLBACK(ssh_packet_server_ecdh_init){
goto out;
}
- rc = ssh_buffer_add_u8(session->out_buffer, SSH2_MSG_NEWKEYS);
- if (rc < 0) {
- rc = SSH_ERROR;
+ session->dh_handshake_state = DH_STATE_NEWKEYS_SENT;
+ /* Send the MSG_NEWKEYS */
+ rc = ssh_packet_send_newkeys(session);
+ if (rc == SSH_ERROR) {
goto out;
}
- session->dh_handshake_state = DH_STATE_NEWKEYS_SENT;
- rc = ssh_packet_send(session);
- SSH_LOG(SSH_LOG_PROTOCOL, "SSH_MSG_NEWKEYS sent");
-
out:
mbedtls_ecp_group_free(&grp);
if (rc == SSH_ERROR) {
diff --git a/src/kex.c b/src/kex.c
index b9455d2d..3818297b 100644
--- a/src/kex.c
+++ b/src/kex.c
@@ -188,6 +188,9 @@
/* RFC 8308 */
#define KEX_EXTENSION_CLIENT "ext-info-c"
+/* Strict kex mitigation against CVE-2023-48795 */
+#define KEX_STRICT_CLIENT "[email protected]"
+#define KEX_STRICT_SERVER "[email protected]"
/* Allowed algorithms in FIPS mode */
#define FIPS_ALLOWED_CIPHERS "[email protected],"\
@@ -516,6 +519,27 @@ SSH_PACKET_CALLBACK(ssh_packet_kexinit)
session->first_kex_follows_guess_wrong ? "wrong" : "right");
}
+ /*
+ * handle the "strict KEX" feature. If supported by peer, then set up the
+ * flag and verify packet sequence numbers.
+ */
+ if (server_kex) {
+ ok = ssh_match_group(crypto->client_kex.methods[SSH_KEX],
+ KEX_STRICT_CLIENT);
+ if (ok) {
+ SSH_LOG(SSH_LOG_DEBUG, "Client supports strict kex, enabling.");
+ session->flags |= SSH_SESSION_FLAG_KEX_STRICT;
+ }
+ } else {
+ /* client kex */
+ ok = ssh_match_group(crypto->server_kex.methods[SSH_KEX],
+ KEX_STRICT_SERVER);
+ if (ok) {
+ SSH_LOG(SSH_LOG_DEBUG, "Server supports strict kex, enabling.");
+ session->flags |= SSH_SESSION_FLAG_KEX_STRICT;
+ }
+ }
+
if (server_kex) {
/*
* If client sent a ext-info-c message in the kex list, it supports
@@ -792,21 +816,21 @@ int ssh_set_client_kex(ssh_session session)
return SSH_OK;
}
- /* Here we append ext-info-c to the list of kex algorithms */
+ /* Here we append ext-info-c and [email protected] to the list of kex algorithms */
kex = client->methods[SSH_KEX];
len = strlen(kex);
- if (len + strlen(KEX_EXTENSION_CLIENT) + 2 < len) {
+ /* Comma, comma, nul byte */
+ kex_len = len + 1 + strlen(KEX_EXTENSION_CLIENT) + 1 + strlen(KEX_STRICT_CLIENT ) + 1;
+ if (kex_len >= MAX_PACKET_LEN) {
/* Overflow */
return SSH_ERROR;
}
- kex_len = len + strlen(KEX_EXTENSION_CLIENT) + 2; /* comma, NULL */
kex_tmp = realloc(kex, kex_len);
if (kex_tmp == NULL) {
- free(kex);
ssh_set_error_oom(session);
return SSH_ERROR;
}
- snprintf(kex_tmp + len, kex_len - len, ",%s", KEX_EXTENSION_CLIENT);
+ snprintf(kex_tmp + len, kex_len - len, ",%s,%s", KEX_EXTENSION_CLIENT, KEX_STRICT_CLIENT);
client->methods[SSH_KEX] = kex_tmp;
return SSH_OK;
diff --git a/src/packet.c b/src/packet.c
index eb7eb42a..ea73f9ad 100644
--- a/src/packet.c
+++ b/src/packet.c
@@ -1314,6 +1314,19 @@ ssh_packet_socket_callback(const void *data, size_t receivedlen, void *user)
}
#endif /* WITH_ZLIB */
payloadsize = ssh_buffer_get_len(session->in_buffer);
+ if (session->recv_seq == UINT32_MAX) {
+ /* Overflowing sequence numbers is always fishy */
+ if (crypto == NULL) {
+ /* don't allow sequence number overflow when unencrypted */
+ ssh_set_error(session,
+ SSH_FATAL,
+ "Incoming sequence number overflow");
+ goto error;
+ } else {
+ SSH_LOG(SSH_LOG_WARNING,
+ "Incoming sequence number overflow");
+ }
+ }
session->recv_seq++;
if (crypto != NULL) {
struct ssh_cipher_struct *cipher = NULL;
@@ -1338,7 +1351,19 @@ ssh_packet_socket_callback(const void *data, size_t receivedlen, void *user)
"comp=%" PRIu32 ",payload=%" PRIu32 "]",
session->in_packet.type, packet_len, padding, compsize,
payloadsize);
+ if (crypto == NULL) {
+ /* In strict kex, only a few packets are allowed. Taint the session
+ * if we received packets that are normally allowed but to be
+ * refused if we are in strict kex when KEX is over.
+ */
+ uint8_t type = session->in_packet.type;
+ if (type != SSH2_MSG_KEXINIT && type != SSH2_MSG_NEWKEYS &&
+ (type < SSH2_MSG_KEXDH_INIT ||
+ type > SSH2_MSG_KEX_DH_GEX_REQUEST)) {
+ session->flags |= SSH_SESSION_FLAG_KEX_TAINTED;
+ }
+ }
/* Check if the packet is expected */
filter_result = ssh_packet_incoming_filter(session);
@@ -1354,6 +1379,9 @@ ssh_packet_socket_callback(const void *data, size_t receivedlen, void *user)
session->in_packet.type);
goto error;
case SSH_PACKET_UNKNOWN:
+ if (crypto == NULL) {
+ session->flags |= SSH_SESSION_FLAG_KEX_TAINTED;
+ }
ssh_packet_send_unimplemented(session, session->recv_seq - 1);
break;
}
@@ -1529,7 +1557,33 @@ void ssh_packet_process(ssh_session session, uint8_t type)
SSH_LOG(SSH_LOG_RARE, "Failed to send unimplemented: %s",
ssh_get_error(session));
}
+ if (session->current_crypto == NULL) {
+ session->flags |= SSH_SESSION_FLAG_KEX_TAINTED;
+ }
+ }
+}
+
+/** @internal
+ * @brief sends a SSH_MSG_NEWKEYS when enabling the new negotiated ciphers
+ * @param session the SSH session
+ * @return SSH_ERROR on error, else SSH_OK
+ */
+int ssh_packet_send_newkeys(ssh_session session)
+{
+ int rc;
+
+ /* Send the MSG_NEWKEYS */
+ rc = ssh_buffer_add_u8(session->out_buffer, SSH2_MSG_NEWKEYS);
+ if (rc < 0) {
+ return rc;
}
+
+ rc = ssh_packet_send(session);
+ if (rc == SSH_ERROR) {
+ return rc;
+ }
+ SSH_LOG(SSH_LOG_DEBUG, "SSH_MSG_NEWKEYS sent");
+ return rc;
}
/** @internal
@@ -1842,6 +1896,10 @@ int ssh_packet_send(ssh_session session)
if (rc == SSH_OK && type == SSH2_MSG_NEWKEYS) {
struct ssh_iterator *it;
+ if (session->flags & SSH_SESSION_FLAG_KEX_STRICT) {
+ /* reset packet sequence number when running in strict kex mode */
+ session->send_seq = 0;
+ }
for (it = ssh_list_get_iterator(session->out_queue);
it != NULL;
it = ssh_list_get_iterator(session->out_queue)) {
diff --git a/src/packet_cb.c b/src/packet_cb.c
index 0ecf8771..2f364c26 100644
--- a/src/packet_cb.c
+++ b/src/packet_cb.c
@@ -115,6 +115,18 @@ SSH_PACKET_CALLBACK(ssh_packet_newkeys){
goto error;
}
+ if (session->flags & SSH_SESSION_FLAG_KEX_STRICT) {
+ /* reset packet sequence number when running in strict kex mode */
+ session->recv_seq = 0;
+ /* Check that we aren't tainted */
+ if (session->flags & SSH_SESSION_FLAG_KEX_TAINTED) {
+ ssh_set_error(session,
+ SSH_FATAL,
+ "Received unexpected packets in strict KEX mode.");
+ goto error;
+ }
+ }
+
if(session->server){
/* server things are done in server.c */
session->dh_handshake_state=DH_STATE_FINISHED;
--
2.33.0
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