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main/dropbear: add mitigations for CVE-2023-48795

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This commit is contained in:
omni 2023-12-19 13:00:07 +00:00
parent 20bc60951b
commit b4abdeecd9
2 changed files with 232 additions and 1 deletions
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6
main/dropbear/APKBUILD
View File

@ -3,7 +3,7 @@
# Maintainer: Natanael Copa <ncopa@alpinelinux.org>
pkgname=dropbear
pkgver=2022.83
pkgrel=3
pkgrel=4
pkgdesc="small SSH 2 client/server designed for small memory environments"
url="https://matt.ucc.asn.au/dropbear/dropbear.html"
arch="all"
@ -21,9 +21,12 @@ source="https://matt.ucc.asn.au/dropbear/releases/dropbear-$pkgver.tar.bz2
dropbear.confd
dropbear-options_sftp-server_path.patch
dropbear-fix-utmp.patch
Implement-Strict-KEX-mode.patch
"
# secfixes:
# 2022.83-r4:
# - CVE-2023-48795
# 2020.79-r0:
# - CVE-2018-20685
# 2018.76-r2:
@ -93,4 +96,5 @@ c63afa615d64b0c8c5e739c758eb8ae277ecc36a4223b766bf562702de69910904cbc3ea98d22989
83f2c1eaf7687917a4b2bae7d599d4378c4bd64f9126ba42fc5d235f2b3c9a474d1b3168d70ed64bb4101cc251d30bc9ae20604da9b5d819fcd635ee4d0ebb0f dropbear.confd
df3fa2c64665b669257ec511afa2e04107eb6b0a2a4238ad9b33db3d322b90854b00506e14a09b648fe2d716dc6c140212cc1739f7cd03088f9b7b9781aaa499 dropbear-options_sftp-server_path.patch
e143b8b2083c96fdf425fbe179aa43892e44b95734686a57ae5d5612778f1bd7e78a725d8413c1701d37dcf1b24986b1faa7365eb039789e2204320272fed7a3 dropbear-fix-utmp.patch
d747d4b083255634962ee0d42454093564930555b5f8bff1fff64e2e5e399c47d3b53ddb54356bb3ad2cc7d8b7f017193dcd889ff49021b3d43d3f38b5b0d616 Implement-Strict-KEX-mode.patch
"

227
main/dropbear/Implement-Strict-KEX-mode.patch Normal file
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@ -0,0 +1,227 @@
From the CHANGES file, Fixes:
Add "Strict KEX" support. This mitigates a SSH protocol flaw which lets
a MITM attacker silently remove packets immediately after the
first key exchange. At present the flaw does not seem to reduce Dropbear's
security (the only packet affected would be a server-sig-algs extension,
which is used for compatibility not security).
For Dropbear, chacha20-poly1305 is the only affected cipher.
Both sides of the connection must support Strict KEX for it to be used.
The protocol flaw is tracked as CVE-2023-48795, details
at https://terrapin-attack.com . Thanks to the researchers Fabian Bäumer,
Marcus Brinkmann, and Jörg Schwenk. Thanks to OpenSSH for specifying
strict KEX mode.
diff --git a/cli-session.c b/cli-session.c
index 5981b247..d261c8f8 100644
--- a/cli-session.c
+++ b/cli-session.c
@@ -46,6 +46,7 @@ static void cli_finished(void) ATTRIB_NORETURN;
static void recv_msg_service_accept(void);
static void cli_session_cleanup(void);
static void recv_msg_global_request_cli(void);
+static void cli_algos_initialise(void);
struct clientsession cli_ses; /* GLOBAL */
@@ -117,6 +118,7 @@ void cli_session(int sock_in, int sock_out, struct dropbear_progress_connection
}
chaninitialise(cli_chantypes);
+ cli_algos_initialise();
/* Set up cli_ses vars */
cli_session_init(proxy_cmd_pid);
@@ -487,3 +489,12 @@ void cli_dropbear_log(int priority, const char* format, va_list param) {
fflush(stderr);
}
+static void cli_algos_initialise(void) {
+ algo_type *algo;
+ for (algo = sshkex; algo->name; algo++) {
+ if (strcmp(algo->name, SSH_STRICT_KEX_S) == 0) {
+ algo->usable = 0;
+ }
+ }
+}
+
diff --git a/common-algo.c b/common-algo.c
index 378f0ca8..f9d46ebb 100644
--- a/common-algo.c
+++ b/common-algo.c
@@ -307,6 +307,12 @@ algo_type sshkex[] = {
/* Set unusable by svr_algos_initialise() */
{SSH_EXT_INFO_C, 0, NULL, 1, NULL},
#endif
+#endif
+#if DROPBEAR_CLIENT
+ {SSH_STRICT_KEX_C, 0, NULL, 1, NULL},
+#endif
+#if DROPBEAR_SERVER
+ {SSH_STRICT_KEX_S, 0, NULL, 1, NULL},
#endif
{NULL, 0, NULL, 0, NULL}
};
diff --git a/common-kex.c b/common-kex.c
index ac884424..8e33b12a 100644
--- a/common-kex.c
+++ b/common-kex.c
@@ -183,6 +183,10 @@ void send_msg_newkeys() {
gen_new_keys();
switch_keys();
+ if (ses.kexstate.strict_kex) {
+ ses.transseq = 0;
+ }
+
TRACE(("leave send_msg_newkeys"))
}
@@ -193,7 +197,11 @@ void recv_msg_newkeys() {
ses.kexstate.recvnewkeys = 1;
switch_keys();
-
+
+ if (ses.kexstate.strict_kex) {
+ ses.recvseq = 0;
+ }
+
TRACE(("leave recv_msg_newkeys"))
}
@@ -550,6 +558,10 @@ void recv_msg_kexinit() {
ses.kexstate.recvkexinit = 1;
+ if (ses.kexstate.strict_kex && !ses.kexstate.donefirstkex && ses.recvseq != 1) {
+ dropbear_exit("First packet wasn't kexinit");
+ }
+
TRACE(("leave recv_msg_kexinit"))
}
@@ -859,6 +871,18 @@ static void read_kex_algos() {
}
#endif
+ if (!ses.kexstate.donefirstkex) {
+ const char* strict_name;
+ if (IS_DROPBEAR_CLIENT) {
+ strict_name = SSH_STRICT_KEX_S;
+ } else {
+ strict_name = SSH_STRICT_KEX_C;
+ }
+ if (buf_has_algo(ses.payload, strict_name) == DROPBEAR_SUCCESS) {
+ ses.kexstate.strict_kex = 1;
+ }
+ }
+
algo = buf_match_algo(ses.payload, sshkex, kexguess2, &goodguess);
allgood &= goodguess;
if (algo == NULL || algo->data == NULL) {
diff --git a/kex.h b/kex.h
index 77cf21a3..7fcc3c25 100644
--- a/kex.h
+++ b/kex.h
@@ -83,6 +83,9 @@ struct KEXState {
unsigned our_first_follows_matches : 1;
+ /* Boolean indicating that strict kex mode is in use */
+ unsigned int strict_kex;
+
time_t lastkextime; /* time of the last kex */
unsigned int datatrans; /* data transmitted since last kex */
unsigned int datarecv; /* data received since last kex */
diff --git a/process-packet.c b/process-packet.c
index 94541602..133a152d 100644
--- a/process-packet.c
+++ b/process-packet.c
@@ -44,6 +44,7 @@ void process_packet() {
unsigned char type;
unsigned int i;
+ unsigned int first_strict_kex = ses.kexstate.strict_kex && !ses.kexstate.donefirstkex;
time_t now;
TRACE2(("enter process_packet"))
@@ -54,22 +55,24 @@ void process_packet() {
now = monotonic_now();
ses.last_packet_time_keepalive_recv = now;
- /* These packets we can receive at any time */
- switch(type) {
- case SSH_MSG_IGNORE:
- goto out;
- case SSH_MSG_DEBUG:
- goto out;
+ if (type == SSH_MSG_DISCONNECT) {
+ /* Allowed at any time */
+ dropbear_close("Disconnect received");
+ }
- case SSH_MSG_UNIMPLEMENTED:
- /* debugging XXX */
- TRACE(("SSH_MSG_UNIMPLEMENTED"))
- goto out;
-
- case SSH_MSG_DISCONNECT:
- /* TODO cleanup? */
- dropbear_close("Disconnect received");
+ /* These packets may be received at any time,
+ except during first kex with strict kex */
+ if (!first_strict_kex) {
+ switch(type) {
+ case SSH_MSG_IGNORE:
+ goto out;
+ case SSH_MSG_DEBUG:
+ goto out;
+ case SSH_MSG_UNIMPLEMENTED:
+ TRACE(("SSH_MSG_UNIMPLEMENTED"))
+ goto out;
+ }
}
/* Ignore these packet types so that keepalives don't interfere with
@@ -98,7 +101,8 @@ void process_packet() {
if (type >= 1 && type <= 49
&& type != SSH_MSG_SERVICE_REQUEST
&& type != SSH_MSG_SERVICE_ACCEPT
- && type != SSH_MSG_KEXINIT)
+ && type != SSH_MSG_KEXINIT
+ && !first_strict_kex)
{
TRACE(("unknown allowed packet during kexinit"))
recv_unimplemented();
diff --git a/ssh.h b/ssh.h
index 1b4fec65..ef3efdca 100644
--- a/ssh.h
+++ b/ssh.h
@@ -100,6 +100,10 @@
#define SSH_EXT_INFO_C "ext-info-c"
#define SSH_SERVER_SIG_ALGS "server-sig-algs"
+/* OpenSSH strict KEX feature */
+#define SSH_STRICT_KEX_S "kex-strict-s-v00@openssh.com"
+#define SSH_STRICT_KEX_C "kex-strict-c-v00@openssh.com"
+
/* service types */
#define SSH_SERVICE_USERAUTH "ssh-userauth"
#define SSH_SERVICE_USERAUTH_LEN 12
diff --git a/svr-session.c b/svr-session.c
index 769f0731..a538e2c5 100644
--- a/svr-session.c
+++ b/svr-session.c
@@ -370,6 +370,9 @@ static void svr_algos_initialise(void) {
algo->usable = 0;
}
#endif
+ if (strcmp(algo->name, SSH_STRICT_KEX_C) == 0) {
+ algo->usable = 0;
+ }
}
}