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You have been subscribed to a public bug by Mauricio Faria de Oliveira (mfo):

[Impact]

 * Users of libc-client2007e (e.g., php7.x-imap) can no longer
   connect to GMail on Bionic and later, after introduction of
   TLSv1.3 with OpenSSL 1.1.1 (normal upgrade path in Bionic).

 * GMail requires Server Name Indication (SNI) to be set when
   TLSv1.3 is used, otherwise the server provided certificate
   fails verification in the client and connection is aborted.

 * The fix is to set SNI to the hostname that the client will
   perform verification on. The change is only enabled if the
   client is built with OpenSSL 1.1.1 or later (i.e., TLSv1.3
   support) so not to affect pre- TLSv1.3 support's behavior.

 * However it is functional nonetheless if the client is built
   with OpenSSL 1.1.1 or later but an earlier TLS version ends
   up used due to the handshake/negotiation/server TLS support
   (e.g., TLSv1.2); this shouldn't be a problem per test below.

 * Regression testing happened with a crawled list of IMAP/POP
   SSL servers (167 servers), and no regressions were observed.
   Actually, one more email provider/server has been fixed too.

 * OpenSSL-only demonstration with -(no)servername:

   $ echo QUIT \
     | openssl s_client \
       -connect imap.gmail.com:993 \
       -verify_hostname imap.gmail.com \
       -noservername `# or -servername imap.gmail.com` \
       -tls1_3 -brief 2>&1 \
     | grep -i ^verif

  Output with '-noservername':

  verify error:num=18:self signed certificate
  verify error:num=62:Hostname mismatch
  Verification error: Hostname mismatch

  Output with '-servername imap.gmail.com'

  Verification: OK
  Verified peername: imap.gmail.com

[Test Case]

 * Commands:

   $ sudo apt install uw-mailutils
   $ mailutil check "{imap.googlemail.com:993/imap/ssl}INBOX"

   $ sudo apt install php7.2-cli php7.2-imap
   $ php -r 'imap_open("{imap.gmail.com:993/imap/ssl}INBOX", "username", "password");'

 * Before:

   $ mailutil check "{imap.googlemail.com:993/imap/ssl}INBOX"
   Certificate failure for imap.googlemail.com: self signed certificate: /OU=No SNI provided; please fix your client./CN=invalid2.invalid
   Certificate failure for imap.googlemail.com: self signed certificate: /OU=No SNI provided; please fix your client./CN=invalid2.invalid

   $ php -r 'imap_open("{imap.gmail.com:993/imap/ssl}INBOX", "username", "password");'
   PHP Warning:  imap_open(): Couldn't open stream {imap.gmail.com:993/imap/ssl}INBOX in Command line code on line 1
   PHP Notice:  Unknown: Certificate failure for imap.gmail.com: self signed certificate: /OU=No SNI provided; please fix your client./CN=invalid2.invalid (errflg=2) in Unknown on line 0

 * After:

   $ mailutil check "{imap.googlemail.com:993/imap/ssl}INBOX"
   {ce-in-f16.1e100.net/imap} username:
   ^C

   $ php -r 'imap_open("{imap.gmail.com:993/imap/ssl}INBOX", "username", "password");'
   PHP Warning:  imap_open(): Couldn't open stream {imap.gmail.com:993/imap/ssl}INBOX in Command line code on line 1
   PHP Notice:  Unknown: Retrying PLAIN authentication after [ALERT] Invalid credentials (Failure) (errflg=1) in Unknown on line 0
   PHP Notice:  Unknown: Retrying PLAIN authentication after [ALERT] Invalid credentials (Failure) (errflg=1) in Unknown on line 0
   PHP Notice:  Unknown: Can not authenticate to IMAP server: [ALERT] Invalid credentials (Failure) (errflg=2) in Unknown on line 0

 * Regression testing scripts/results are provided in
attachments/comments.

[Regression Potential]

 * Theoretically possible, but not observed in hundred+ of (167)
   IMAP/POP SSL servers.

 * The change sends additional data (SNI) from client to server
   when connecting, if built with OpenSSL 1.1.1 or later, which
   is in the specification, so should be handled by the server.

 * The risk is servers that misbehave when provided such info
   (not observed in the 167 server test).

 * Less likely are servers that do not recognize the server name
   identified (this also not observed in test and unlikely since
   the client usually reaches the server by public/known address).

 * Even less likely are servers whose provided certificate doesn't
   contain the server name identified (again not observed and it's
   in the server, not client, to provide a certificate for address
   it doesn't recognize).

 * This both shows the SRU team that the risks have been considered,
   and provides guidance to testers in regression-testing the SRU.

[Other Info]

 * Reverse build dependencies have been patched for No Change Rebuilds
   and (re)built correctly on all architectures in Launchpad PPA for
   all affected releases (eoan, disco, bionic).

[Original Description]

After upgrading from PHP5 to PHP7.2 (from Bionic), calling imap_open() against Google's Gmail servers stopped working.
After researching, I've found that new OpenSSL version introduced TLSv13-related breaking changes.

Here are the relevant issues:
PHP: https://bugs.php.net/bug.php?id=77108
PHP (Debian): https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=916041
In fetchmail (solved): https://bugs.launchpad.net/ubuntu/+source/fetchmail/+bug/1798786

OpenSSL upstream devs have issues related to this:
https://github.com/openssl/openssl/issues/5944
https://github.com/openssl/openssl/pull/5947

Looks like to me that either adding the SNI server name to the openssl
open call would be needed, as done on fetchmail.

** Affects: php-imap (Ubuntu)
     Importance: Undecided
         Status: Invalid

** Affects: uw-imap (Ubuntu)
     Importance: Undecided
     Assignee: Mauricio Faria de Oliveira (mfo)
         Status: In Progress

** Affects: php-imap (Ubuntu Bionic)
     Importance: Undecided
         Status: Invalid

** Affects: uw-imap (Ubuntu Bionic)
     Importance: Undecided
     Assignee: Mauricio Faria de Oliveira (mfo)
         Status: In Progress

** Affects: php-imap (Ubuntu Disco)
     Importance: Undecided
         Status: Invalid

** Affects: uw-imap (Ubuntu Disco)
     Importance: Undecided
     Assignee: Mauricio Faria de Oliveira (mfo)
         Status: In Progress

** Affects: php-imap (Ubuntu Eoan)
     Importance: Undecided
         Status: Invalid

** Affects: uw-imap (Ubuntu Eoan)
     Importance: Undecided
     Assignee: Mauricio Faria de Oliveira (mfo)
         Status: In Progress


** Tags: patch
-- 
Regression for GMail after libssl upgrade with TLSv1.3
https://bugs.launchpad.net/bugs/1834340
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