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Test+fix handling "wrong" downgrade signals 

This accounts for cases that can only occur when een non-compliant server sends
the wrong downgrade signal. (TLS1.1 signal when negotiating TLS1.2 or TLS1.2
signal when negotiating TLS1.0/TLS1.1). According to the TLS1.3 RFC these
cases should be rejected:

RFC8446, section 4.1.3: TLS 1.3 clients receiving a ServerHello indicating
TLS 1.2 or below MUST check that the last 8 bytes are not equal to either of
these values. TLS 1.2 clients SHOULD also check that the last 8 bytes are
not equal to the second value if the ServerHello indicates TLS 1.1 or below.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/27518)
This commit is contained in:
Anton Tieleman 2025-04-28 13:49:25 +02:00 committed by Tomas Mraz
parent 7535f26210
commit 290fd4a0c8
2 changed files with 79 additions and 38 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

23
ssl/statem/statem_lib.c
View File

@ -2365,23 +2365,24 @@ int ssl_choose_client_version(SSL_CONNECTION *s, int version,
real_max = ver_max;
/* Check for downgrades */
if (s->version == TLS1_2_VERSION && real_max > s->version) {
if (memcmp(tls12downgrade,
/* TODO(DTLSv1.3): Update this code for DTLSv1.3 */
if (!SSL_CONNECTION_IS_DTLS(s) && real_max > s->version) {
/* Signal applies to all versions */
if (memcmp(tls11downgrade,
s->s3.server_random + SSL3_RANDOM_SIZE
- sizeof(tls12downgrade),
sizeof(tls12downgrade)) == 0) {
- sizeof(tls11downgrade),
sizeof(tls11downgrade)) == 0) {
s->version = origv;
SSLfatal(s, SSL_AD_ILLEGAL_PARAMETER,
SSL_R_INAPPROPRIATE_FALLBACK);
return 0;
}
} else if (!SSL_CONNECTION_IS_DTLS(s)
&& s->version < TLS1_2_VERSION
&& real_max > s->version) {
if (memcmp(tls11downgrade,
s->s3.server_random + SSL3_RANDOM_SIZE
- sizeof(tls11downgrade),
sizeof(tls11downgrade)) == 0) {
/* Only when accepting TLS1.3 */
if (real_max == TLS1_3_VERSION
&& memcmp(tls12downgrade,
s->s3.server_random + SSL3_RANDOM_SIZE
- sizeof(tls12downgrade),
sizeof(tls12downgrade)) == 0) {
s->version = origv;
SSLfatal(s, SSL_AD_ILLEGAL_PARAMETER,
SSL_R_INAPPROPRIATE_FALLBACK);

94
test/recipes/70-test_tls13downgrade.t
View File

@ -38,16 +38,25 @@ use constant {
DOWNGRADE_TO_TLS_1_2 => 0,
DOWNGRADE_TO_TLS_1_1 => 1,
FALLBACK_FROM_TLS_1_3 => 2,
DOWNGRADE_TO_TLS_1_2_WITH_TLS_1_1_SIGNAL => 3,
DOWNGRADE_TO_TLS_1_1_WITH_TLS_1_2_SIGNAL => 4,
};
#Test 1: Downgrade from TLSv1.3 to TLSv1.2
$proxy->filter(\&downgrade_filter);
my $testtype = DOWNGRADE_TO_TLS_1_2;
$proxy->start() or plan skip_all => "Unable to start up Proxy for tests";
plan tests => 6;
plan tests => 8;
ok(is_illegal_parameter_client_alert(), "Downgrade TLSv1.3 to TLSv1.2");
#Test 2: Client falls back from TLSv1.3 (server does not support the fallback
#Test 2: Downgrade from TLSv1.3 to TLSv1.2 (server sends TLSv1.1 signal)
$proxy->clear();
$testtype = DOWNGRADE_TO_TLS_1_2_WITH_TLS_1_1_SIGNAL;
$proxy->start();
ok(is_illegal_parameter_client_alert(),
"Downgrade from TLSv1.3 to TLSv1.2 (server sends TLSv1.1 signal)");
#Test 3: Client falls back from TLSv1.3 (server does not support the fallback
# SCSV)
$proxy->clear();
$testtype = FALLBACK_FROM_TLS_1_3;
@ -56,13 +65,13 @@ $proxy->start();
ok(is_illegal_parameter_client_alert(), "Fallback from TLSv1.3");
SKIP: {
skip "TLSv1.1 disabled", 4 if disabled("tls1_1");
skip "TLSv1.1 disabled", 5 if disabled("tls1_1");
my $client_flags = "-min_protocol TLSv1.1 -cipher DEFAULT:\@SECLEVEL=0";
my $server_flags = "-min_protocol TLSv1.1";
my $ciphers = "AES128-SHA:\@SECLEVEL=0";
#Test 3: Downgrade from TLSv1.3 to TLSv1.1
#Test 4: Downgrade from TLSv1.3 to TLSv1.1
$proxy->clear();
$testtype = DOWNGRADE_TO_TLS_1_1;
$proxy->clientflags($client_flags);
@ -71,7 +80,17 @@ SKIP: {
$proxy->start();
ok(is_illegal_parameter_client_alert(), "Downgrade TLSv1.3 to TLSv1.1");
#Test 4: Downgrade from TLSv1.2 to TLSv1.1
#Test 5: Downgrade from TLSv1.3 to TLSv1.1 (server sends TLSv1.2 signal)
$proxy->clear();
$testtype = DOWNGRADE_TO_TLS_1_1_WITH_TLS_1_2_SIGNAL;
$proxy->clientflags($client_flags);
$proxy->serverflags($server_flags);
$proxy->ciphers($ciphers);
$proxy->start();
ok(is_illegal_parameter_client_alert(),
"Downgrade TLSv1.3 to TLSv1.1 (server sends TLSv1.2 signal)");
#Test 6: Downgrade from TLSv1.2 to TLSv1.1
$proxy->clear();
$testtype = DOWNGRADE_TO_TLS_1_1;
$proxy->clientflags($client_flags." -max_protocol TLSv1.2");
@ -80,7 +99,7 @@ SKIP: {
$proxy->start();
ok(is_illegal_parameter_client_alert(), "Downgrade TLSv1.2 to TLSv1.1");
#Test 5: A client side protocol "hole" should not be detected as a downgrade
#Test 7: A client side protocol "hole" should not be detected as a downgrade
$proxy->clear();
$proxy->filter(undef);
$proxy->clientflags($client_flags." -no_tls1_2");
@ -89,7 +108,7 @@ SKIP: {
$proxy->start();
ok(TLSProxy::Message->success(), "TLSv1.2 client-side protocol hole");
#Test 6: A server side protocol "hole" should not be detected as a downgrade
#Test 8: A server side protocol "hole" should not be detected as a downgrade
$proxy->clear();
$proxy->filter(undef);
$proxy->clientflags($client_flags);
@ -115,33 +134,54 @@ sub downgrade_filter
{
my $proxy = shift;
# We're only interested in the initial ClientHello
if ($proxy->flight != 0) {
# We're only interested in the initial ClientHello and ServerHello
if ($proxy->flight > 1) {
return;
}
my $message = ${$proxy->message_list}[0];
my $message = ${$proxy->message_list}[$proxy->flight];
my $ext;
if ($testtype == FALLBACK_FROM_TLS_1_3) {
#The default ciphersuite we use for TLSv1.2 without any SCSV
my @ciphersuites = (TLSProxy::Message::CIPHER_RSA_WITH_AES_128_CBC_SHA);
$message->ciphersuite_len(2 * scalar @ciphersuites);
$message->ciphersuites(\@ciphersuites);
} else {
if ($testtype == DOWNGRADE_TO_TLS_1_2) {
$ext = pack "C3",
0x02, # Length
0x03, 0x03; #TLSv1.2
} else {
$ext = pack "C3",
0x02, # Length
0x03, 0x02; #TLSv1.1
# ServerHello
if ($proxy->flight == 1 && defined($message)) {
# Update the last byte of the downgrade signal
if ($testtype == DOWNGRADE_TO_TLS_1_2_WITH_TLS_1_1_SIGNAL) {
$message->random(substr($message->random, 0, 31) . "\0");
$message->repack();
} elsif ($testtype == DOWNGRADE_TO_TLS_1_1_WITH_TLS_1_2_SIGNAL) {
$message->random(substr($message->random, 0, 31) . "\1");
$message->repack();
}
$message->set_extension(TLSProxy::Message::EXT_SUPPORTED_VERSIONS, $ext);
return;
}
$message->repack();
# ClientHello
if ($proxy->flight == 0) {
my $ext;
if ($testtype == FALLBACK_FROM_TLS_1_3) {
#The default ciphersuite we use for TLSv1.2 without any SCSV
my @ciphersuites = (TLSProxy::Message::CIPHER_RSA_WITH_AES_128_CBC_SHA);
$message->ciphersuite_len(2 * scalar @ciphersuites);
$message->ciphersuites(\@ciphersuites);
}
else {
if ($testtype == DOWNGRADE_TO_TLS_1_2
|| $testtype == DOWNGRADE_TO_TLS_1_2_WITH_TLS_1_1_SIGNAL) {
$ext = pack "C3",
0x02, # Length
0x03, 0x03; #TLSv1.2
}
else {
$ext = pack "C3",
0x02, # Length
0x03, 0x02; #TLSv1.1
}
$message->set_extension(TLSProxy::Message::EXT_SUPPORTED_VERSIONS,
$ext);
}
$message->repack();
}
}