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From d34d9258b8420b19ec3f97b4cc5bf7aa7d98e35a Mon Sep 17 00:00:00 2001
From: Michael Buckley <michael@buckleyisms.com>
Date: Thu, 30 Nov 2023 15:08:02 -0800
Subject: [PATCH] src: add 'strict KEX' to fix CVE-2023-48795 "Terrapin
Attack"
Refs:
https://terrapin-attack.com/
https://seclists.org/oss-sec/2023/q4/292
https://osv.dev/list?ecosystem=&q=CVE-2023-48795
https://github.com/advisories/GHSA-45x7-px36-x8w8
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2023-48795
Fixes #1290
Closes #1291
Reference:https://github.com/libssh2/libssh2/commit/d34d9258b8420b19ec3f97b4cc5bf7aa7d98e35a
---
src/kex.c | 63 +++++++++++++++++++++++------------
src/libssh2_priv.h | 18 +++++++---
src/packet.c | 83 +++++++++++++++++++++++++++++++++++++++++++---
src/packet.h | 2 +-
src/session.c | 3 ++
src/transport.c | 12 ++++++-
6 files changed, 149 insertions(+), 32 deletions(-)
diff --git a/src/kex.c b/src/kex.c
index d4034a0..b4b748c 100644
--- a/src/kex.c
+++ b/src/kex.c
@@ -3037,6 +3037,13 @@ kex_method_extension_negotiation = {
0,
};
+static const LIBSSH2_KEX_METHOD
+kex_method_strict_client_extension = {
+ "kex-strict-c-v00@openssh.com",
+ NULL,
+ 0,
+};
+
static const LIBSSH2_KEX_METHOD *libssh2_kex_methods[] = {
#if LIBSSH2_ED25519
&kex_method_ssh_curve25519_sha256,
@@ -3055,6 +3062,7 @@ static const LIBSSH2_KEX_METHOD *libssh2_kex_methods[] = {
&kex_method_diffie_helman_group1_sha1,
&kex_method_diffie_helman_group_exchange_sha1,
&kex_method_extension_negotiation,
+ &kex_method_strict_client_extension,
NULL
};
@@ -3307,13 +3315,13 @@ static int kexinit(LIBSSH2_SESSION * session)
return 0;
}
-/* kex_agree_instr
+/* _libssh2_kex_agree_instr
* Kex specific variant of strstr()
* Needle must be preceded by BOL or ',', and followed by ',' or EOL
*/
-static unsigned char *
-kex_agree_instr(unsigned char *haystack, size_t haystack_len,
- const unsigned char *needle, size_t needle_len)
+unsigned char *
+_libssh2_kex_agree_instr(unsigned char *haystack, size_t haystack_len,
+ const unsigned char *needle, size_t needle_len)
{
unsigned char *s;
unsigned char *end_haystack;
@@ -3398,7 +3406,7 @@ static int kex_agree_hostkey(LIBSSH2_SESSION * session,
while(s && *s) {
unsigned char *p = (unsigned char *) strchr((char *) s, ',');
size_t method_len = (p ? (size_t)(p - s) : strlen((char *) s));
- if(kex_agree_instr(hostkey, hostkey_len, s, method_len)) {
+ if(_libssh2_kex_agree_instr(hostkey, hostkey_len, s, method_len)) {
const LIBSSH2_HOSTKEY_METHOD *method =
(const LIBSSH2_HOSTKEY_METHOD *)
kex_get_method_by_name((char *) s, method_len,
@@ -3432,9 +3440,9 @@ static int kex_agree_hostkey(LIBSSH2_SESSION * session,
}
while(hostkeyp && (*hostkeyp) && (*hostkeyp)->name) {
- s = kex_agree_instr(hostkey, hostkey_len,
- (unsigned char *) (*hostkeyp)->name,
- strlen((*hostkeyp)->name));
+ s = _libssh2_kex_agree_instr(hostkey, hostkey_len,
+ (unsigned char *) (*hostkeyp)->name,
+ strlen((*hostkeyp)->name));
if(s) {
/* So far so good, but does it suit our purposes? (Encrypting vs
Signing) */
@@ -3468,6 +3476,12 @@ static int kex_agree_kex_hostkey(LIBSSH2_SESSION * session, unsigned char *kex,
{
const LIBSSH2_KEX_METHOD **kexp = libssh2_kex_methods;
unsigned char *s;
+ const unsigned char *strict =
+ (unsigned char *)"kex-strict-s-v00@openssh.com";
+
+ if(_libssh2_kex_agree_instr(kex, kex_len, strict, 28)) {
+ session->kex_strict = 1;
+ }
if(session->kex_prefs) {
s = (unsigned char *) session->kex_prefs;
@@ -3475,7 +3489,7 @@ static int kex_agree_kex_hostkey(LIBSSH2_SESSION * session, unsigned char *kex,
while(s && *s) {
unsigned char *q, *p = (unsigned char *) strchr((char *) s, ',');
size_t method_len = (p ? (size_t)(p - s) : strlen((char *) s));
- q = kex_agree_instr(kex, kex_len, s, method_len);
+ q = _libssh2_kex_agree_instr(kex, kex_len, s, method_len);
if(q) {
const LIBSSH2_KEX_METHOD *method = (const LIBSSH2_KEX_METHOD *)
kex_get_method_by_name((char *) s, method_len,
@@ -3509,9 +3523,9 @@ static int kex_agree_kex_hostkey(LIBSSH2_SESSION * session, unsigned char *kex,
}
while(*kexp && (*kexp)->name) {
- s = kex_agree_instr(kex, kex_len,
- (unsigned char *) (*kexp)->name,
- strlen((*kexp)->name));
+ s = _libssh2_kex_agree_instr(kex, kex_len,
+ (unsigned char *) (*kexp)->name,
+ strlen((*kexp)->name));
if(s) {
/* We've agreed on a key exchange method,
* Can we agree on a hostkey that works with this kex?
@@ -3555,7 +3569,7 @@ static int kex_agree_crypt(LIBSSH2_SESSION * session,
unsigned char *p = (unsigned char *) strchr((char *) s, ',');
size_t method_len = (p ? (size_t)(p - s) : strlen((char *) s));
- if(kex_agree_instr(crypt, crypt_len, s, method_len)) {
+ if(_libssh2_kex_agree_instr(crypt, crypt_len, s, method_len)) {
const LIBSSH2_CRYPT_METHOD *method =
(const LIBSSH2_CRYPT_METHOD *)
kex_get_method_by_name((char *) s, method_len,
@@ -3577,9 +3591,9 @@ static int kex_agree_crypt(LIBSSH2_SESSION * session,
}
while(*cryptp && (*cryptp)->name) {
- s = kex_agree_instr(crypt, crypt_len,
- (unsigned char *) (*cryptp)->name,
- strlen((*cryptp)->name));
+ s = _libssh2_kex_agree_instr(crypt, crypt_len,
+ (unsigned char *) (*cryptp)->name,
+ strlen((*cryptp)->name));
if(s) {
endpoint->crypt = *cryptp;
return 0;
@@ -3619,7 +3633,7 @@ static int kex_agree_mac(LIBSSH2_SESSION * session,
unsigned char *p = (unsigned char *) strchr((char *) s, ',');
size_t method_len = (p ? (size_t)(p - s) : strlen((char *) s));
- if(kex_agree_instr(mac, mac_len, s, method_len)) {
+ if(_libssh2_kex_agree_instr(mac, mac_len, s, method_len)) {
const LIBSSH2_MAC_METHOD *method = (const LIBSSH2_MAC_METHOD *)
kex_get_method_by_name((char *) s, method_len,
(const LIBSSH2_COMMON_METHOD **)
@@ -3640,8 +3654,9 @@ static int kex_agree_mac(LIBSSH2_SESSION * session,
}
while(*macp && (*macp)->name) {
- s = kex_agree_instr(mac, mac_len, (unsigned char *) (*macp)->name,
- strlen((*macp)->name));
+ s = _libssh2_kex_agree_instr(mac, mac_len,
+ (unsigned char *) (*macp)->name,
+ strlen((*macp)->name));
if(s) {
endpoint->mac = *macp;
return 0;
@@ -3672,7 +3687,7 @@ static int kex_agree_comp(LIBSSH2_SESSION *session,
unsigned char *p = (unsigned char *) strchr((char *) s, ',');
size_t method_len = (p ? (size_t)(p - s) : strlen((char *) s));
- if(kex_agree_instr(comp, comp_len, s, method_len)) {
+ if(_libssh2_kex_agree_instr(comp, comp_len, s, method_len)) {
const LIBSSH2_COMP_METHOD *method =
(const LIBSSH2_COMP_METHOD *)
kex_get_method_by_name((char *) s, method_len,
@@ -3694,8 +3709,9 @@ static int kex_agree_comp(LIBSSH2_SESSION *session,
}
while(*compp && (*compp)->name) {
- s = kex_agree_instr(comp, comp_len, (unsigned char *) (*compp)->name,
- strlen((*compp)->name));
+ s = _libssh2_kex_agree_instr(comp, comp_len,
+ (unsigned char *) (*compp)->name,
+ strlen((*compp)->name));
if(s) {
endpoint->comp = *compp;
return 0;
@@ -3876,6 +3892,7 @@ _libssh2_kex_exchange(LIBSSH2_SESSION * session, int reexchange,
session->local.kexinit = key_state->oldlocal;
session->local.kexinit_len = key_state->oldlocal_len;
key_state->state = libssh2_NB_state_idle;
+ session->state &= ~LIBSSH2_STATE_INITIAL_KEX;
session->state &= ~LIBSSH2_STATE_KEX_ACTIVE;
session->state &= ~LIBSSH2_STATE_EXCHANGING_KEYS;
return -1;
@@ -3901,6 +3918,7 @@ _libssh2_kex_exchange(LIBSSH2_SESSION * session, int reexchange,
session->local.kexinit = key_state->oldlocal;
session->local.kexinit_len = key_state->oldlocal_len;
key_state->state = libssh2_NB_state_idle;
+ session->state &= ~LIBSSH2_STATE_INITIAL_KEX;
session->state &= ~LIBSSH2_STATE_KEX_ACTIVE;
session->state &= ~LIBSSH2_STATE_EXCHANGING_KEYS;
return -1;
@@ -3949,6 +3967,7 @@ _libssh2_kex_exchange(LIBSSH2_SESSION * session, int reexchange,
session->remote.kexinit = NULL;
}
+ session->state &= ~LIBSSH2_STATE_INITIAL_KEX;
session->state &= ~LIBSSH2_STATE_KEX_ACTIVE;
session->state &= ~LIBSSH2_STATE_EXCHANGING_KEYS;
diff --git a/src/libssh2_priv.h b/src/libssh2_priv.h
index 82c3afe..ee1d8b5 100644
--- a/src/libssh2_priv.h
+++ b/src/libssh2_priv.h
@@ -699,6 +699,9 @@ struct _LIBSSH2_SESSION
/* key signing algorithm preferences -- NULL yields server order */
char *sign_algo_prefs;
+ /* Whether to use the OpenSSH Strict KEX extension */
+ int kex_strict;
+
/* (remote as source of data -- packet_read ) */
libssh2_endpoint_data remote;
@@ -870,6 +873,7 @@ struct _LIBSSH2_SESSION
int fullpacket_macstate;
size_t fullpacket_payload_len;
int fullpacket_packet_type;
+ uint32_t fullpacket_required_type;
/* State variables used in libssh2_sftp_init() */
libssh2_nonblocking_states sftpInit_state;
@@ -910,10 +914,11 @@ struct _LIBSSH2_SESSION
};
/* session.state bits */
-#define LIBSSH2_STATE_EXCHANGING_KEYS 0x00000001
-#define LIBSSH2_STATE_NEWKEYS 0x00000002
-#define LIBSSH2_STATE_AUTHENTICATED 0x00000004
-#define LIBSSH2_STATE_KEX_ACTIVE 0x00000008
+#define LIBSSH2_STATE_INITIAL_KEX 0x00000001
+#define LIBSSH2_STATE_EXCHANGING_KEYS 0x00000002
+#define LIBSSH2_STATE_NEWKEYS 0x00000004
+#define LIBSSH2_STATE_AUTHENTICATED 0x00000008
+#define LIBSSH2_STATE_KEX_ACTIVE 0x00000010
/* session.flag helpers */
#ifdef MSG_NOSIGNAL
@@ -1144,6 +1149,11 @@ ssize_t _libssh2_send(libssh2_socket_t socket, const void *buffer,
int _libssh2_kex_exchange(LIBSSH2_SESSION * session, int reexchange,
key_exchange_state_t * state);
+unsigned char *_libssh2_kex_agree_instr(unsigned char *haystack,
+ size_t haystack_len,
+ const unsigned char *needle,
+ size_t needle_len);
+
/* Let crypt.c/hostkey.c expose their method structs */
const LIBSSH2_CRYPT_METHOD **libssh2_crypt_methods(void);
const LIBSSH2_HOSTKEY_METHOD **libssh2_hostkey_methods(void);
diff --git a/src/packet.c b/src/packet.c
index b5b4198..35d4d39 100644
--- a/src/packet.c
+++ b/src/packet.c
@@ -605,14 +605,13 @@ authagent_exit:
* layer when it has received a packet.
*
* The input pointer 'data' is pointing to allocated data that this function
- * is asked to deal with so on failure OR success, it must be freed fine.
- * The only exception is when the return code is LIBSSH2_ERROR_EAGAIN.
+ * will be freed unless return the code is LIBSSH2_ERROR_EAGAIN.
*
* This function will always be called with 'datalen' greater than zero.
*/
int
_libssh2_packet_add(LIBSSH2_SESSION * session, unsigned char *data,
- size_t datalen, int macstate)
+ size_t datalen, int macstate, uint32_t seq)
{
int rc = 0;
unsigned char *message = NULL;
@@ -657,6 +656,70 @@ _libssh2_packet_add(LIBSSH2_SESSION * session, unsigned char *data,
break;
}
+ if(session->state & LIBSSH2_STATE_INITIAL_KEX) {
+ if(msg == SSH_MSG_KEXINIT) {
+ if(!session->kex_strict) {
+ if(datalen < 17) {
+ LIBSSH2_FREE(session, data);
+ session->packAdd_state = libssh2_NB_state_idle;
+ return _libssh2_error(session,
+ LIBSSH2_ERROR_BUFFER_TOO_SMALL,
+ "Data too short extracting kex");
+ }
+ else {
+ const unsigned char *strict =
+ (unsigned char *)"kex-strict-s-v00@openssh.com";
+ struct string_buf buf;
+ unsigned char *algs = NULL;
+ size_t algs_len = 0;
+
+ buf.data = (unsigned char *)data;
+ buf.dataptr = buf.data;
+ buf.len = datalen;
+ buf.dataptr += 17; /* advance past type and cookie */
+
+ if(_libssh2_get_string(&buf, &algs, &algs_len)) {
+ LIBSSH2_FREE(session, data);
+ session->packAdd_state = libssh2_NB_state_idle;
+ return _libssh2_error(session,
+ LIBSSH2_ERROR_BUFFER_TOO_SMALL,
+ "Algs too short");
+ }
+
+ if(algs_len == 0 ||
+ _libssh2_kex_agree_instr(algs, algs_len, strict, 28)) {
+ session->kex_strict = 1;
+ }
+ }
+ }
+
+ if(session->kex_strict && seq) {
+ LIBSSH2_FREE(session, data);
+ session->socket_state = LIBSSH2_SOCKET_DISCONNECTED;
+ session->packAdd_state = libssh2_NB_state_idle;
+ libssh2_session_disconnect(session, "strict KEX violation: "
+ "KEXINIT was not the first packet");
+
+ return _libssh2_error(session, LIBSSH2_ERROR_SOCKET_DISCONNECT,
+ "strict KEX violation: "
+ "KEXINIT was not the first packet");
+ }
+ }
+
+ if(session->kex_strict && session->fullpacket_required_type &&
+ session->fullpacket_required_type != msg) {
+ LIBSSH2_FREE(session, data);
+ session->socket_state = LIBSSH2_SOCKET_DISCONNECTED;
+ session->packAdd_state = libssh2_NB_state_idle;
+ libssh2_session_disconnect(session, "strict KEX violation: "
+ "unexpected packet type");
+
+ return _libssh2_error(session, LIBSSH2_ERROR_SOCKET_DISCONNECT,
+ "strict KEX violation: "
+ "unexpected packet type");
+ }
+ }
+
if(session->packAdd_state == libssh2_NB_state_allocated) {
/* A couple exceptions to the packet adding rule: */
switch(msg) {
@@ -1341,6 +1404,15 @@ _libssh2_packet_ask(LIBSSH2_SESSION * session, unsigned char packet_type,
return 0;
}
+ else if(session->kex_strict &&
+ (session->state & LIBSSH2_STATE_INITIAL_KEX)) {
+ libssh2_session_disconnect(session, "strict KEX violation: "
+ "unexpected packet type");
+
+ return _libssh2_error(session, LIBSSH2_ERROR_SOCKET_DISCONNECT,
+ "strict KEX violation: "
+ "unexpected packet type");
+ }
packet = _libssh2_list_next(&packet->node);
}
return -1;
@@ -1402,7 +1474,10 @@ _libssh2_packet_require(LIBSSH2_SESSION * session, unsigned char packet_type,
}
while(session->socket_state == LIBSSH2_SOCKET_CONNECTED) {
- int ret = _libssh2_transport_read(session);
+ int ret;
+ session->fullpacket_required_type = packet_type;
+ ret = _libssh2_transport_read(session);
+ session->fullpacket_required_type = 0;
if(ret == LIBSSH2_ERROR_EAGAIN)
return ret;
else if(ret < 0) {
diff --git a/src/packet.h b/src/packet.h
index 79018bc..6ea100a 100644
--- a/src/packet.h
+++ b/src/packet.h
@@ -71,6 +71,6 @@ int _libssh2_packet_burn(LIBSSH2_SESSION * session,
int _libssh2_packet_write(LIBSSH2_SESSION * session, unsigned char *data,
unsigned long data_len);
int _libssh2_packet_add(LIBSSH2_SESSION * session, unsigned char *data,
- size_t datalen, int macstate);
+ size_t datalen, int macstate, uint32_t seq);
#endif /* __LIBSSH2_PACKET_H */
diff --git a/src/session.c b/src/session.c
index a4d602b..f4bafb5 100644
--- a/src/session.c
+++ b/src/session.c
@@ -464,6 +464,8 @@ libssh2_session_init_ex(LIBSSH2_ALLOC_FUNC((*my_alloc)),
session->abstract = abstract;
session->api_timeout = 0; /* timeout-free API by default */
session->api_block_mode = 1; /* blocking API by default */
+ session->state = LIBSSH2_STATE_INITIAL_KEX;
+ session->fullpacket_required_type = 0;
session->packet_read_timeout = LIBSSH2_DEFAULT_READ_TIMEOUT;
session->flag.quote_paths = 1; /* default behavior is to quote paths
for the scp subsystem */
@@ -1186,6 +1188,7 @@ libssh2_session_disconnect_ex(LIBSSH2_SESSION *session, int reason,
const char *desc, const char *lang)
{
int rc;
+ session->state &= ~LIBSSH2_STATE_INITIAL_KEX;
session->state &= ~LIBSSH2_STATE_EXCHANGING_KEYS;
BLOCK_ADJUST(rc, session,
session_disconnect(session, reason, desc, lang));
diff --git a/src/transport.c b/src/transport.c
index 6d902d3..3b30ff8 100644
--- a/src/transport.c
+++ b/src/transport.c
@@ -187,6 +187,7 @@ fullpacket(LIBSSH2_SESSION * session, int encrypted /* 1 or 0 */ )
struct transportpacket *p = &session->packet;
int rc;
int compressed;
+ uint32_t seq = session->remote.seqno;
if(session->fullpacket_state == libssh2_NB_state_idle) {
session->fullpacket_macstate = LIBSSH2_MAC_CONFIRMED;
@@ -318,7 +319,7 @@ fullpacket(LIBSSH2_SESSION * session, int encrypted /* 1 or 0 */ )
if(session->fullpacket_state == libssh2_NB_state_created) {
rc = _libssh2_packet_add(session, p->payload,
session->fullpacket_payload_len,
- session->fullpacket_macstate);
+ session->fullpacket_macstate, seq);
if(rc == LIBSSH2_ERROR_EAGAIN)
return rc;
if(rc) {
@@ -329,6 +330,11 @@ fullpacket(LIBSSH2_SESSION * session, int encrypted /* 1 or 0 */ )
session->fullpacket_state = libssh2_NB_state_idle;
+ if(session->kex_strict &&
+ session->fullpacket_packet_type == SSH_MSG_NEWKEYS) {
+ session->remote.seqno = 0;
+ }
+
return session->fullpacket_packet_type;
}
@@ -1091,6 +1097,10 @@ int _libssh2_transport_send(LIBSSH2_SESSION *session,
session->local.seqno++;
+ if(session->kex_strict && data[0] == SSH_MSG_NEWKEYS) {
+ session->local.seqno = 0;
+ }
+
ret = LIBSSH2_SEND(session, p->outbuf, total_length,
LIBSSH2_SOCKET_SEND_FLAGS(session));
if(ret < 0)
--
2.33.0
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