untrusted comment: verify with openbsd-73-base.pub
RWQS90bYzZ4XFg7BHftxpQQigHQHY94HRDEZu8BzwfYfut2vq2bRC/0f+BXb5I/MsDifnGeJ46kc0jtjH7VCAh5uD+s3yTs63wc=
OpenBSD 7.3 errata 024, December 18, 2023:
An SSH protocol weakness (the Terrapin Attack) exists that allows an
on-path adversary to disable keystroke timing obfuscation.
Apply by doing:
signify -Vep /etc/signify/openbsd-73-base.pub -x 024_ssh.patch.sig \
-m - | (cd /usr/src && patch -p0)
And then rebuild and install OpenSSH:
cd /usr/src/usr.bin/ssh
make obj
make
make install
Index: usr.bin/ssh/PROTOCOL
===================================================================
RCS file: /cvs/src/usr.bin/ssh/PROTOCOL,v
diff -u -p -r1.48 PROTOCOL
--- usr.bin/ssh/PROTOCOL 7 Nov 2022 01:53:01 -0000 1.48
+++ usr.bin/ssh/PROTOCOL 13 Dec 2023 23:13:16 -0000
@@ -104,6 +104,32 @@ http://git.libssh.org/users/aris/libssh.
This is identical to curve25519-sha256 as later published in RFC8731.
+1.9 transport: strict key exchange extension
+
+OpenSSH supports a number of transport-layer hardening measures under
+a "strict KEX" feature. This feature is signalled similarly to the
+RFC8308 ext-info feature: by including a additional algorithm in the
+initiial SSH2_MSG_KEXINIT kex_algorithms field. The client may append
+"kex-strict-c-v00@openssh.com" to its kex_algorithms and the server
+may append "kex-strict-s-v00@openssh.com". These pseudo-algorithms
+are only valid in the initial SSH2_MSG_KEXINIT and MUST be ignored
+if they are present in subsequent SSH2_MSG_KEXINIT packets.
+
+When an endpoint that supports this extension observes this algorithm
+name in a peer's KEXINIT packet, it MUST make the following changes to
+the the protocol:
+
+a) During initial KEX, terminate the connection if any unexpected or
+ out-of-sequence packet is received. This includes terminating the
+ connection if the first packet received is not SSH2_MSG_KEXINIT.
+ Unexpected packets for the purpose of strict KEX include messages
+ that are otherwise valid at any time during the connection such as
+ SSH2_MSG_DEBUG and SSH2_MSG_IGNORE.
+b) After sending or receiving a SSH2_MSG_NEWKEYS message, reset the
+ packet sequence number to zero. This behaviour persists for the
+ duration of the connection (i.e. not just the first
+ SSH2_MSG_NEWKEYS).
+
2. Connection protocol changes
2.1. connection: Channel write close extension "eow@openssh.com"
Index: usr.bin/ssh/kex.c
===================================================================
RCS file: /cvs/src/usr.bin/ssh/kex.c,v
diff -u -p -r1.178 kex.c
--- usr.bin/ssh/kex.c 12 Mar 2023 10:40:39 -0000 1.178
+++ usr.bin/ssh/kex.c 13 Dec 2023 23:13:16 -0000
@@ -60,7 +60,7 @@
#include "xmalloc.h"
/* prototype */
-static int kex_choose_conf(struct ssh *);
+static int kex_choose_conf(struct ssh *, uint32_t seq);
static int kex_input_newkeys(int, u_int32_t, struct ssh *);
static const char * const proposal_names[PROPOSAL_MAX] = {
@@ -162,6 +162,18 @@ kex_names_valid(const char *names)
return 1;
}
+/* returns non-zero if proposal contains any algorithm from algs */
+static int
+has_any_alg(const char *proposal, const char *algs)
+{
+ char *cp;
+
+ if ((cp = match_list(proposal, algs, NULL)) == NULL)
+ return 0;
+ free(cp);
+ return 1;
+}
+
/*
* Concatenate algorithm names, avoiding duplicates in the process.
* Caller must free returned string.
@@ -169,7 +181,7 @@ kex_names_valid(const char *names)
char *
kex_names_cat(const char *a, const char *b)
{
- char *ret = NULL, *tmp = NULL, *cp, *p, *m;
+ char *ret = NULL, *tmp = NULL, *cp, *p;
size_t len;
if (a == NULL || *a == '\0')
@@ -186,10 +198,8 @@ kex_names_cat(const char *a, const char
}
strlcpy(ret, a, len);
for ((p = strsep(&cp, ",")); p && *p != '\0'; (p = strsep(&cp, ","))) {
- if ((m = match_list(ret, p, NULL)) != NULL) {
- free(m);
+ if (has_any_alg(ret, p))
continue; /* Algorithm already present */
- }
if (strlcat(ret, ",", len) >= len ||
strlcat(ret, p, len) >= len) {
free(tmp);
@@ -319,15 +329,23 @@ kex_proposal_populate_entries(struct ssh
const char *defpropclient[PROPOSAL_MAX] = { KEX_CLIENT };
const char **defprop = ssh->kex->server ? defpropserver : defpropclient;
u_int i;
+ char *cp;
if (prop == NULL)
fatal_f("proposal missing");
+ /* Append EXT_INFO signalling to KexAlgorithms */
+ if (kexalgos == NULL)
+ kexalgos = defprop[PROPOSAL_KEX_ALGS];
+ if ((cp = kex_names_cat(kexalgos, ssh->kex->server ?
+ "kex-strict-s-v00@openssh.com" :
+ "ext-info-c,kex-strict-c-v00@openssh.com")) == NULL)
+ fatal_f("kex_names_cat");
+
for (i = 0; i < PROPOSAL_MAX; i++) {
switch(i) {
case PROPOSAL_KEX_ALGS:
- prop[i] = compat_kex_proposal(ssh,
- kexalgos ? kexalgos : defprop[i]);
+ prop[i] = compat_kex_proposal(ssh, cp);
break;
case PROPOSAL_ENC_ALGS_CTOS:
case PROPOSAL_ENC_ALGS_STOC:
@@ -348,6 +366,7 @@ kex_proposal_populate_entries(struct ssh
prop[i] = xstrdup(defprop[i]);
}
}
+ free(cp);
}
void
@@ -451,7 +470,12 @@ kex_protocol_error(int type, u_int32_t s
{
int r;
- error("kex protocol error: type %d seq %u", type, seq);
+ /* If in strict mode, any unexpected message is an error */
+ if ((ssh->kex->flags & KEX_INITIAL) && ssh->kex->kex_strict) {
+ ssh_packet_disconnect(ssh, "strict KEX violation: "
+ "unexpected packet type %u (seqnr %u)", type, seq);
+ }
+ error_f("type %u seq %u", type, seq);
if ((r = sshpkt_start(ssh, SSH2_MSG_UNIMPLEMENTED)) != 0 ||
(r = sshpkt_put_u32(ssh, seq)) != 0 ||
(r = sshpkt_send(ssh)) != 0)
@@ -529,7 +553,7 @@ kex_input_ext_info(int type, u_int32_t s
if (ninfo >= 1024) {
error("SSH2_MSG_EXT_INFO with too many entries, expected "
"<=1024, received %u", ninfo);
- return SSH_ERR_INVALID_FORMAT;
+ return dispatch_protocol_error(type, seq, ssh);
}
for (i = 0; i < ninfo; i++) {
if ((r = sshpkt_get_cstring(ssh, &name, NULL)) != 0)
@@ -645,7 +669,7 @@ kex_input_kexinit(int type, u_int32_t se
error_f("no kex");
return SSH_ERR_INTERNAL_ERROR;
}
- ssh_dispatch_set(ssh, SSH2_MSG_KEXINIT, NULL);
+ ssh_dispatch_set(ssh, SSH2_MSG_KEXINIT, &kex_protocol_error);
ptr = sshpkt_ptr(ssh, &dlen);
if ((r = sshbuf_put(kex->peer, ptr, dlen)) != 0)
return r;
@@ -681,7 +705,7 @@ kex_input_kexinit(int type, u_int32_t se
if (!(kex->flags & KEX_INIT_SENT))
if ((r = kex_send_kexinit(ssh)) != 0)
return r;
- if ((r = kex_choose_conf(ssh)) != 0)
+ if ((r = kex_choose_conf(ssh, seq)) != 0)
return r;
if (kex->kex_type < KEX_MAX && kex->kex[kex->kex_type] != NULL)
@@ -943,20 +967,14 @@ proposals_match(char *my[PROPOSAL_MAX],
return (1);
}
-/* returns non-zero if proposal contains any algorithm from algs */
static int
-has_any_alg(const char *proposal, const char *algs)
+kexalgs_contains(char **peer, const char *ext)
{
- char *cp;
-
- if ((cp = match_list(proposal, algs, NULL)) == NULL)
- return 0;
- free(cp);
- return 1;
+ return has_any_alg(peer[PROPOSAL_KEX_ALGS], ext);
}
static int
-kex_choose_conf(struct ssh *ssh)
+kex_choose_conf(struct ssh *ssh, uint32_t seq)
{
struct kex *kex = ssh->kex;
struct newkeys *newkeys;
@@ -981,13 +999,23 @@ kex_choose_conf(struct ssh *ssh)
sprop=peer;
}
- /* Check whether client supports ext_info_c */
- if (kex->server && (kex->flags & KEX_INITIAL)) {
- char *ext;
-
- ext = match_list("ext-info-c", peer[PROPOSAL_KEX_ALGS], NULL);
- kex->ext_info_c = (ext != NULL);
- free(ext);
+ /* Check whether peer supports ext_info/kex_strict */
+ if ((kex->flags & KEX_INITIAL) != 0) {
+ if (kex->server) {
+ kex->ext_info_c = kexalgs_contains(peer, "ext-info-c");
+ kex->kex_strict = kexalgs_contains(peer,
+ "kex-strict-c-v00@openssh.com");
+ } else {
+ kex->kex_strict = kexalgs_contains(peer,
+ "kex-strict-s-v00@openssh.com");
+ }
+ if (kex->kex_strict) {
+ debug3_f("will use strict KEX ordering");
+ if (seq != 0)
+ ssh_packet_disconnect(ssh,
+ "strict KEX violation: "
+ "KEXINIT was not the first packet");
+ }
}
/* Check whether client supports rsa-sha2 algorithms */
Index: usr.bin/ssh/kex.h
===================================================================
RCS file: /cvs/src/usr.bin/ssh/kex.h,v
diff -u -p -r1.118 kex.h
--- usr.bin/ssh/kex.h 6 Mar 2023 12:14:48 -0000 1.118
+++ usr.bin/ssh/kex.h 13 Dec 2023 23:13:16 -0000
@@ -141,6 +141,7 @@ struct kex {
u_int kex_type;
char *server_sig_algs;
int ext_info_c;
+ int kex_strict;
struct sshbuf *my;
struct sshbuf *peer;
struct sshbuf *client_version;
Index: usr.bin/ssh/packet.c
===================================================================
RCS file: /cvs/src/usr.bin/ssh/packet.c,v
diff -u -p -r1.309 packet.c
--- usr.bin/ssh/packet.c 3 Mar 2023 10:23:42 -0000 1.309
+++ usr.bin/ssh/packet.c 13 Dec 2023 23:13:17 -0000
@@ -1185,8 +1185,13 @@ ssh_packet_send2_wrapped(struct ssh *ssh
sshbuf_dump(state->output, stderr);
#endif
/* increment sequence number for outgoing packets */
- if (++state->p_send.seqnr == 0)
+ if (++state->p_send.seqnr == 0) {
+ if ((ssh->kex->flags & KEX_INITIAL) != 0) {
+ ssh_packet_disconnect(ssh, "outgoing sequence number "
+ "wrapped during initial key exchange");
+ }
logit("outgoing seqnr wraps around");
+ }
if (++state->p_send.packets == 0)
if (!(ssh->compat & SSH_BUG_NOREKEY))
return SSH_ERR_NEED_REKEY;
@@ -1194,6 +1199,11 @@ ssh_packet_send2_wrapped(struct ssh *ssh
state->p_send.bytes += len;
sshbuf_reset(state->outgoing_packet);
+ if (type == SSH2_MSG_NEWKEYS && ssh->kex->kex_strict) {
+ debug_f("resetting send seqnr %u", state->p_send.seqnr);
+ state->p_send.seqnr = 0;
+ }
+
if (type == SSH2_MSG_NEWKEYS)
r = ssh_set_newkeys(ssh, MODE_OUT);
else if (type == SSH2_MSG_USERAUTH_SUCCESS && state->server_side)
@@ -1322,8 +1332,7 @@ ssh_packet_read_seqnr(struct ssh *ssh, u
/* Stay in the loop until we have received a complete packet. */
for (;;) {
/* Try to read a packet from the buffer. */
- r = ssh_packet_read_poll_seqnr(ssh, typep, seqnr_p);
- if (r != 0)
+ if ((r = ssh_packet_read_poll_seqnr(ssh, typep, seqnr_p)) != 0)
break;
/* If we got a packet, return it. */
if (*typep != SSH_MSG_NONE)
@@ -1393,29 +1402,6 @@ ssh_packet_read(struct ssh *ssh)
return type;
}
-/*
- * Waits until a packet has been received, verifies that its type matches
- * that given, and gives a fatal error and exits if there is a mismatch.
- */
-
-int
-ssh_packet_read_expect(struct ssh *ssh, u_int expected_type)
-{
- int r;
- u_char type;
-
- if ((r = ssh_packet_read_seqnr(ssh, &type, NULL)) != 0)
- return r;
- if (type != expected_type) {
- if ((r = sshpkt_disconnect(ssh,
- "Protocol error: expected packet type %d, got %d",
- expected_type, type)) != 0)
- return r;
- return SSH_ERR_PROTOCOL_ERROR;
- }
- return 0;
-}
-
static int
ssh_packet_read_poll2_mux(struct ssh *ssh, u_char *typep, u_int32_t *seqnr_p)
{
@@ -1606,10 +1592,16 @@ ssh_packet_read_poll2(struct ssh *ssh, u
if ((r = sshbuf_consume(state->input, mac->mac_len)) != 0)
goto out;
}
+
if (seqnr_p != NULL)
*seqnr_p = state->p_read.seqnr;
- if (++state->p_read.seqnr == 0)
+ if (++state->p_read.seqnr == 0) {
+ if ((ssh->kex->flags & KEX_INITIAL) != 0) {
+ ssh_packet_disconnect(ssh, "incoming sequence number "
+ "wrapped during initial key exchange");
+ }
logit("incoming seqnr wraps around");
+ }
if (++state->p_read.packets == 0)
if (!(ssh->compat & SSH_BUG_NOREKEY))
return SSH_ERR_NEED_REKEY;
@@ -1675,6 +1667,10 @@ ssh_packet_read_poll2(struct ssh *ssh, u
#endif
/* reset for next packet */
state->packlen = 0;
+ if (*typep == SSH2_MSG_NEWKEYS && ssh->kex->kex_strict) {
+ debug_f("resetting read seqnr %u", state->p_read.seqnr);
+ state->p_read.seqnr = 0;
+ }
if ((r = ssh_packet_check_rekey(ssh)) != 0)
return r;
@@ -1695,10 +1691,39 @@ ssh_packet_read_poll_seqnr(struct ssh *s
r = ssh_packet_read_poll2(ssh, typep, seqnr_p);
if (r != 0)
return r;
- if (*typep) {
- state->keep_alive_timeouts = 0;
- DBG(debug("received packet type %d", *typep));
+ if (*typep == 0) {
+ /* no message ready */
+ return 0;
}
+ state->keep_alive_timeouts = 0;
+ DBG(debug("received packet type %d", *typep));
+
+ /* Always process disconnect messages */
+ if (*typep == SSH2_MSG_DISCONNECT) {
+ if ((r = sshpkt_get_u32(ssh, &reason)) != 0 ||
+ (r = sshpkt_get_string(ssh, &msg, NULL)) != 0)
+ return r;
+ /* Ignore normal client exit notifications */
+ do_log2(ssh->state->server_side &&
+ reason == SSH2_DISCONNECT_BY_APPLICATION ?
+ SYSLOG_LEVEL_INFO : SYSLOG_LEVEL_ERROR,
+ "Received disconnect from %s port %d:"
+ "%u: %.400s", ssh_remote_ipaddr(ssh),
+ ssh_remote_port(ssh), reason, msg);
+ free(msg);
+ return SSH_ERR_DISCONNECTED;
+ }
+
+ /*
+ * Do not implicitly handle any messages here during initial
+ * KEX when in strict mode. They will be need to be allowed
+ * explicitly by the KEX dispatch table or they will generate
+ * protocol errors.
+ */
+ if (ssh->kex != NULL &&
+ (ssh->kex->flags & KEX_INITIAL) && ssh->kex->kex_strict)
+ return 0;
+ /* Implicitly handle transport-level messages */
switch (*typep) {
case SSH2_MSG_IGNORE:
debug3("Received SSH2_MSG_IGNORE");
@@ -1713,19 +1738,6 @@ ssh_packet_read_poll_seqnr(struct ssh *s
debug("Remote: %.900s", msg);
free(msg);
break;
- case SSH2_MSG_DISCONNECT:
- if ((r = sshpkt_get_u32(ssh, &reason)) != 0 ||
- (r = sshpkt_get_string(ssh, &msg, NULL)) != 0)
- return r;
- /* Ignore normal client exit notifications */
- do_log2(ssh->state->server_side &&
- reason == SSH2_DISCONNECT_BY_APPLICATION ?
- SYSLOG_LEVEL_INFO : SYSLOG_LEVEL_ERROR,
- "Received disconnect from %s port %d:"
- "%u: %.400s", ssh_remote_ipaddr(ssh),
- ssh_remote_port(ssh), reason, msg);
- free(msg);
- return SSH_ERR_DISCONNECTED;
case SSH2_MSG_UNIMPLEMENTED:
if ((r = sshpkt_get_u32(ssh, &seqnr)) != 0)
return r;
@@ -2188,6 +2200,7 @@ kex_to_blob(struct sshbuf *m, struct kex
(r = sshbuf_put_u32(m, kex->hostkey_type)) != 0 ||
(r = sshbuf_put_u32(m, kex->hostkey_nid)) != 0 ||
(r = sshbuf_put_u32(m, kex->kex_type)) != 0 ||
+ (r = sshbuf_put_u32(m, kex->kex_strict)) != 0 ||
(r = sshbuf_put_stringb(m, kex->my)) != 0 ||
(r = sshbuf_put_stringb(m, kex->peer)) != 0 ||
(r = sshbuf_put_stringb(m, kex->client_version)) != 0 ||
@@ -2350,6 +2363,7 @@ kex_from_blob(struct sshbuf *m, struct k
(r = sshbuf_get_u32(m, (u_int *)&kex->hostkey_type)) != 0 ||
(r = sshbuf_get_u32(m, (u_int *)&kex->hostkey_nid)) != 0 ||
(r = sshbuf_get_u32(m, &kex->kex_type)) != 0 ||
+ (r = sshbuf_get_u32(m, &kex->kex_strict)) != 0 ||
(r = sshbuf_get_stringb(m, kex->my)) != 0 ||
(r = sshbuf_get_stringb(m, kex->peer)) != 0 ||
(r = sshbuf_get_stringb(m, kex->client_version)) != 0 ||
@@ -2674,6 +2688,7 @@ sshpkt_disconnect(struct ssh *ssh, const
vsnprintf(buf, sizeof(buf), fmt, args);
va_end(args);
+ debug2_f("sending SSH2_MSG_DISCONNECT: %s", buf);
if ((r = sshpkt_start(ssh, SSH2_MSG_DISCONNECT)) != 0 ||
(r = sshpkt_put_u32(ssh, SSH2_DISCONNECT_PROTOCOL_ERROR)) != 0 ||
(r = sshpkt_put_cstring(ssh, buf)) != 0 ||
Index: usr.bin/ssh/packet.h
===================================================================
RCS file: /cvs/src/usr.bin/ssh/packet.h,v
diff -u -p -r1.94 packet.h
--- usr.bin/ssh/packet.h 22 Jan 2022 00:49:34 -0000 1.94
+++ usr.bin/ssh/packet.h 13 Dec 2023 23:13:17 -0000
@@ -118,7 +118,6 @@ int ssh_packet_send2_wrapped(struct ssh
int ssh_packet_send2(struct ssh *);
int ssh_packet_read(struct ssh *);
-int ssh_packet_read_expect(struct ssh *, u_int type);
int ssh_packet_read_poll(struct ssh *);
int ssh_packet_read_poll2(struct ssh *, u_char *, u_int32_t *seqnr_p);
int ssh_packet_process_incoming(struct ssh *, const char *buf, u_int len);
Index: usr.bin/ssh/sshconnect2.c
===================================================================
RCS file: /cvs/src/usr.bin/ssh/sshconnect2.c,v
diff -u -p -r1.366 sshconnect2.c
--- usr.bin/ssh/sshconnect2.c 9 Mar 2023 07:11:05 -0000 1.366
+++ usr.bin/ssh/sshconnect2.c 13 Dec 2023 23:13:17 -0000
@@ -351,7 +351,6 @@ struct cauthmethod {
};
static int input_userauth_service_accept(int, u_int32_t, struct ssh *);
-static int input_userauth_ext_info(int, u_int32_t, struct ssh *);
static int input_userauth_success(int, u_int32_t, struct ssh *);
static int input_userauth_failure(int, u_int32_t, struct ssh *);
static int input_userauth_banner(int, u_int32_t, struct ssh *);
@@ -465,7 +464,7 @@ ssh_userauth2(struct ssh *ssh, const cha
ssh->authctxt = &authctxt;
ssh_dispatch_init(ssh, &input_userauth_error);
- ssh_dispatch_set(ssh, SSH2_MSG_EXT_INFO, &input_userauth_ext_info);
+ ssh_dispatch_set(ssh, SSH2_MSG_EXT_INFO, kex_input_ext_info);
ssh_dispatch_set(ssh, SSH2_MSG_SERVICE_ACCEPT, &input_userauth_service_accept);
ssh_dispatch_run_fatal(ssh, DISPATCH_BLOCK, &authctxt.success); /* loop until success */
pubkey_cleanup(ssh);
@@ -516,12 +515,6 @@ input_userauth_service_accept(int type,
return r;
}
-static int
-input_userauth_ext_info(int type, u_int32_t seqnr, struct ssh *ssh)
-{
- return kex_input_ext_info(type, seqnr, ssh);
-}
-
void
userauth(struct ssh *ssh, char *authlist)
{
@@ -600,6 +593,7 @@ input_userauth_success(int type, u_int32
free(authctxt->methoddata);
authctxt->methoddata = NULL;
authctxt->success = 1; /* break out */
+ ssh_dispatch_set(ssh, SSH2_MSG_EXT_INFO, dispatch_protocol_error);
return 0;
}