nginx 1.18.0 implicitly enables TLS 1.3 (with only "ssl_protocols TLSv1.2; " in nginx.conf config)

[Andreas Bartelt]

On 11/30/20 11:39 PM, Maxim Dounin wrote:
> Hello!
> 
> On Mon, Nov 30, 2020 at 06:41:18PM +0100, Andreas Bartelt wrote:
> 
>> On 11/30/20 4:07 PM, Maxim Dounin wrote:
>>> Hello!
>>>
>>> On Sun, Nov 29, 2020 at 04:01:07PM +0100, nginx at bartelt.name wrote:
>>>
>>>> I've noticed that nginx 1.18.0 always enables TLS 1.3 even if not
>>>> configured to do so. I've observed this behavior on OpenBSD with (nginx
>>>> 1.18.0 linked against LibreSSL 3.3.0) and on Ubuntu 20.04 (nginx 1.18.0
>>>> linked against OpenSSL 1.1.1f). I don't know which release of nginx
>>>> introduced this bug.
>>>>
>>>>    From nginx.conf:
>>>> ssl_protocols TLSv1.2;
>>>> --> in my understanding, this config statement should only enable TLS
>>>> 1.2 but not TLS 1.3. However, the observed behavior is that TLS 1.3 is
>>>> implicitly enabled in addition to TLS 1.2.
>>>
>>> As long as "ssl_protocols TLSv1.2;" is the only ssl_protocols in
>>> nginx configuration, TLSv1.3 shouldn't be enabled.  Much like when
>>> there are no "ssl_protocols" at all, as TLSv1.3 isn't enabled by
>>> default (for now, at least up to and including nginx 1.19.5).
>>>
>>
>> I've just retested this with my Ubuntu 20.04 based nginx test instance
>> from yesterday (nginx 1.18.0 linked against OpenSSL 1.1.1f) and noticed
>> that it works there as intended (i.e., "ssl_protocols TLSv1.2;" only
>> enables TLS 1.2 but not TLS 1.3). I don't know what I did wrong there
>> yesterday -- sorry for this.
>>
>> However, the problem persists on OpenBSD current with nginx 1.18.0
>> (built from ports with default options which links against LibreSSL
>> 3.3.0 from base). Setting "ssl_protocols TLSv1.2;" enables TLS 1.2 as
>> well as TLS 1.3 there.
> 
> I don't see any problems when testing with LibreSSL 3.3.0 as
> available on libressl.org and the very same configuration.  So
> it's probably something specific to your system.
> 
> Some possible reasons for the behaviour you are seeing, in no
> particular order:
> 
> - Given that OpenBSD current and LibreSSL from base implies some
>    arbitrary version of LibreSSL, this might be something with the
>    changes present on your system but not in LibreSSL 3.3.0
>    release.
> 
> - There may be something with the port you are using to compile
>    nginx.  Consider testing nginx compiled manually.
> 
> - You are testing the wrong server (the name resolves to a
>    different IP address, or the IP address is routed to a different
>    server).  Make sure you are seeing connection on nginx side,
>    something like "return 200 $ssl_protocol;" in the appropriate
>    server block and making a "GET / HTTP/1.0" request in s_client
>    would be a good test.
> 
> - The nginx version running differs from the one on disk, and you
>    are running an nginx version older than 1.15.6 built with an old
>    LibreSSL without TLSv1.3 but running with LibreSSL 3.3.0 with
>    TLSv1.3 enabled.  Check the "Server" header in the above test.
> 
> - There might be something wrong with headers on your system.  The
>    behaviour observed might happen if SSL_OP_NO_TLSv1_3, TLS1_3_VERSION,
>    and SSL_CTX_set_min_proto_version/SSL_CTX_set_max_proto_version are
>    not defined, yet TLSv1.3 is present in the library.
> 

I've just tested the same nginx.conf on two freshly installed OpenBSD 
based test systems:
1) release 6.8 (with nginx 1.18.0 / LibreSSL 3.2.2)
2) snapshot from today (with nginx 1.18.0 / LibreSSL 3.3.0 + more recent 
commits since it's a snapshot)

Both instances were installed from scratch with the official OpenBSD 
binary tarballs and the nginx binary package from ports, respectively.

Release 6.8 interprets "ssl_protocols TLSv1.2;" correctly. However, the 
snapshot instance enables TLS 1.2 and 1.3, i.e., this looks like a bug 
which has been recently introduced into OpenBSD current.

Although OpenBSD 6.8 and the snapshot both use nginx 1.18.0, it's built 
differently on current:
# cvs diff -r 1.145 -r 1.146 ports/www/nginx/Makefile
Index: ports/www/nginx/Makefile
===================================================================
RCS file: /cvs/ports/www/nginx/Makefile,v
retrieving revision 1.145
retrieving revision 1.146
diff -u -p -r1.145 -r1.146
--- ports/www/nginx/Makefile	27 Jul 2020 14:33:15 -0000	1.145
+++ ports/www/nginx/Makefile	23 Oct 2020 15:20:30 -0000	1.146
@@ -1,4 +1,4 @@
-# $OpenBSD: Makefile,v 1.145 2020/07/27 14:33:15 sthen Exp $
+# $OpenBSD: Makefile,v 1.146 2020/10/23 15:20:30 robert Exp $

  BROKEN-hppa=	src/core/ngx_rwlock.c:116:2: error: \#error 
ngx_atomic_cmp_set() is not defined!

@@ -21,7 +21,7 @@ VERSION=	1.18.0
  DISTNAME=	nginx-${VERSION}
  CATEGORIES=	www

-REVISION-main=	0
+REVISION-main=	1
  REVISION-xslt=	0

  VERSION-rtmp=	1.2.1
@@ -122,6 +122,8 @@ SUBST_VARS=	NGINX_DIR
  .for i in ${MODULE_PACKAGES}
  PREFIX$i=	${NGINX_DIR}/modules
  .endfor
+
+CFLAGS+=	-DTLS1_3_VERSION=0x0304

  CFLAGS+=	-Wall -Wpointer-arith \
  		-I "${LOCALBASE}/include/libxml2" \

Best regards
Andreas