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[nginx] Mail: connections with wrong ALPN protocols are now rejected.

Vladimir Homutov
October 20, 2021 01:28PM
details: https://hg.nginx.org/nginx/rev/dc955d274130
branches:
changeset: 7938:dc955d274130
user: Vladimir Homutov <vl@nginx.com>
date: Wed Oct 20 09:45:34 2021 +0300
description:
Mail: connections with wrong ALPN protocols are now rejected.

This is a recommended behavior by RFC 7301 and is useful
for mitigation of protocol confusion attacks [1].

For POP3 and IMAP protocols IANA-assigned ALPN IDs are used [2].
For the SMTP protocol "smtp" is used.

[1] https://alpaca-attack.com/
[2] https://www.iana.org/assignments/tls-extensiontype-values/

diffstat:

src/mail/ngx_mail.h | 1 +
src/mail/ngx_mail_imap_module.c | 1 +
src/mail/ngx_mail_pop3_module.c | 1 +
src/mail/ngx_mail_smtp_module.c | 1 +
src/mail/ngx_mail_ssl_module.c | 58 +++++++++++++++++++++++++++++++++++++++++
5 files changed, 62 insertions(+), 0 deletions(-)

diffs (126 lines):

diff -r db6b630e6086 -r dc955d274130 src/mail/ngx_mail.h
--- a/src/mail/ngx_mail.h Wed Oct 20 09:50:02 2021 +0300
+++ b/src/mail/ngx_mail.h Wed Oct 20 09:45:34 2021 +0300
@@ -324,6 +324,7 @@ typedef ngx_int_t (*ngx_mail_parse_comma

struct ngx_mail_protocol_s {
ngx_str_t name;
+ ngx_str_t alpn;
in_port_t port[4];
ngx_uint_t type;

diff -r db6b630e6086 -r dc955d274130 src/mail/ngx_mail_imap_module.c
--- a/src/mail/ngx_mail_imap_module.c Wed Oct 20 09:50:02 2021 +0300
+++ b/src/mail/ngx_mail_imap_module.c Wed Oct 20 09:45:34 2021 +0300
@@ -46,6 +46,7 @@ static ngx_str_t ngx_mail_imap_auth_met

static ngx_mail_protocol_t ngx_mail_imap_protocol = {
ngx_string("imap"),
+ ngx_string("\x04imap"),
{ 143, 993, 0, 0 },
NGX_MAIL_IMAP_PROTOCOL,

diff -r db6b630e6086 -r dc955d274130 src/mail/ngx_mail_pop3_module.c
--- a/src/mail/ngx_mail_pop3_module.c Wed Oct 20 09:50:02 2021 +0300
+++ b/src/mail/ngx_mail_pop3_module.c Wed Oct 20 09:45:34 2021 +0300
@@ -46,6 +46,7 @@ static ngx_str_t ngx_mail_pop3_auth_met

static ngx_mail_protocol_t ngx_mail_pop3_protocol = {
ngx_string("pop3"),
+ ngx_string("\x04pop3"),
{ 110, 995, 0, 0 },
NGX_MAIL_POP3_PROTOCOL,

diff -r db6b630e6086 -r dc955d274130 src/mail/ngx_mail_smtp_module.c
--- a/src/mail/ngx_mail_smtp_module.c Wed Oct 20 09:50:02 2021 +0300
+++ b/src/mail/ngx_mail_smtp_module.c Wed Oct 20 09:45:34 2021 +0300
@@ -39,6 +39,7 @@ static ngx_str_t ngx_mail_smtp_auth_met

static ngx_mail_protocol_t ngx_mail_smtp_protocol = {
ngx_string("smtp"),
+ ngx_string("\x04smtp"),
{ 25, 465, 587, 0 },
NGX_MAIL_SMTP_PROTOCOL,

diff -r db6b630e6086 -r dc955d274130 src/mail/ngx_mail_ssl_module.c
--- a/src/mail/ngx_mail_ssl_module.c Wed Oct 20 09:50:02 2021 +0300
+++ b/src/mail/ngx_mail_ssl_module.c Wed Oct 20 09:45:34 2021 +0300
@@ -14,6 +14,12 @@
#define NGX_DEFAULT_ECDH_CURVE "auto"


+#ifdef TLSEXT_TYPE_application_layer_protocol_negotiation
+static int ngx_mail_ssl_alpn_select(ngx_ssl_conn_t *ssl_conn,
+ const unsigned char **out, unsigned char *outlen,
+ const unsigned char *in, unsigned int inlen, void *arg);
+#endif
+
static void *ngx_mail_ssl_create_conf(ngx_conf_t *cf);
static char *ngx_mail_ssl_merge_conf(ngx_conf_t *cf, void *parent, void *child);

@@ -244,6 +250,54 @@ ngx_module_t ngx_mail_ssl_module = {
static ngx_str_t ngx_mail_ssl_sess_id_ctx = ngx_string("MAIL");


+#ifdef TLSEXT_TYPE_application_layer_protocol_negotiation
+
+static int
+ngx_mail_ssl_alpn_select(ngx_ssl_conn_t *ssl_conn, const unsigned char **out,
+ unsigned char *outlen, const unsigned char *in, unsigned int inlen,
+ void *arg)
+{
+ unsigned int srvlen;
+ unsigned char *srv;
+ ngx_connection_t *c;
+ ngx_mail_session_t *s;
+ ngx_mail_core_srv_conf_t *cscf;
+#if (NGX_DEBUG)
+ unsigned int i;
+#endif
+
+ c = ngx_ssl_get_connection(ssl_conn);
+ s = c->data;
+
+#if (NGX_DEBUG)
+ for (i = 0; i < inlen; i += in[i] + 1) {
+ ngx_log_debug2(NGX_LOG_DEBUG_MAIL, c->log, 0,
+ "SSL ALPN supported by client: %*s",
+ (size_t) in[i], &in[i + 1]);
+ }
+#endif
+
+ cscf = ngx_mail_get_module_srv_conf(s, ngx_mail_core_module);
+
+ srv = cscf->protocol->alpn.data;
+ srvlen = cscf->protocol->alpn.len;
+
+ if (SSL_select_next_proto((unsigned char **) out, outlen, srv, srvlen,
+ in, inlen)
+ != OPENSSL_NPN_NEGOTIATED)
+ {
+ return SSL_TLSEXT_ERR_ALERT_FATAL;
+ }
+
+ ngx_log_debug2(NGX_LOG_DEBUG_MAIL, c->log, 0,
+ "SSL ALPN selected: %*s", (size_t) *outlen, *out);
+
+ return SSL_TLSEXT_ERR_OK;
+}
+
+#endif
+
+
static void *
ngx_mail_ssl_create_conf(ngx_conf_t *cf)
{
@@ -394,6 +448,10 @@ ngx_mail_ssl_merge_conf(ngx_conf_t *cf,
cln->handler = ngx_ssl_cleanup_ctx;
cln->data = &conf->ssl;

+#ifdef TLSEXT_TYPE_application_layer_protocol_negotiation
+ SSL_CTX_set_alpn_select_cb(conf->ssl.ctx, ngx_mail_ssl_alpn_select, NULL);
+#endif
+
if (ngx_ssl_ciphers(cf, &conf->ssl, &conf->ciphers,
conf->prefer_server_ciphers)
!= NGX_OK)
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Subject Author Views Posted

[nginx] Mail: connections with wrong ALPN protocols are now rejected.

Vladimir Homutov 130 October 20, 2021 01:28PM



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