[Bug 1928989] Re: expiring trust anchor compatibility issue

[Dimitri John Ledkov]

** Information type changed from Public to Public Security

** Tags removed: letsencrypt
** Tags added: letsencryptexpiry

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Title:
  expiring trust anchor compatibility issue

Status in openssl package in Ubuntu:
  Fix Released
Status in openssl source package in Trusty:
  New
Status in openssl source package in Xenial:
  New

Bug description:
  [Impact]

   * openssl fails to talk to letsencrypt website past September 2021,
  despite trusting the letsencrypt root certificate.

  [Test Plan]

   * Import staging cert equivalent to ISRG Root X1
  https://letsencrypt.org/certs/staging/letsencrypt-stg-root-x1.pem

   * Import expired staging cert equivalen tto DST Root CA X3
  https://letsencrypt.org/certs/staging/letsencrypt-stg-root-dst.pem

   * Test connectivity to the expired-root-ca test website
  https://expired-root-ca-test.germancoding.com

  setup:

  apt install openssl
  wget https://letsencrypt.org/certs/staging/letsencrypt-stg-root-x1.pem
  wget https://letsencrypt.org/certs/staging/letsencrypt-stg-root-dst.pem
  cat letsencrypt-stg-root-x1.pem letsencrypt-stg-root-dst.pem >> ca.pem

  test case:
  openssl s_client -connect expired-root-ca-test.germancoding.com:443 -servername expired-root-ca-test.germancoding.com -verify 1 -verifyCAfile ca.pem

  bad result:
  connection failed

  good result:
  connection successful

  Connection should be successful and trusted with correctly working
  openssl s_client that can manage to ignore expired CA, and build a
  valid trust path using non-expired CA in the chain.

  [Where problems could occur]

   * Changes as to how the trust paths are built in TLS connection may
  result in introducing bugs (failure to connect to valid sites) and/or
  security vulnerabilities (connecting to invalid sites successfully).

  [Other Info]

   * Background info
   * The current chain from letsencrypt is expiring, they are adding a new chain, but also keeping the expiring one. This will result in connectivity issues when using old gnutls/openssl against websites using the default letsencrypt configuration after September 2021.

  https://community.letsencrypt.org/t/openssl-client-compatibility-changes-for-let-s-encrypt-certificates/143816
  https://community.letsencrypt.org/t/questions-re-openssl-client-compatibility-changes-for-let-s-encrypt-certificates/143817

  Currently openssl in xenial and earlier will not establish a
  connection, if any parts of the trust chain have expired, even though
  alternative non-expired chains are available.

  This has been fixed in OpenSSL 1.1.0+ by setting
  X509_V_FLAG_TRUSTED_FIRST by default see
  https://github.com/openssl/openssl/commit/0daccd4dc1f1ac62181738a91714f35472e50f3c

  gnutls bug for this is
  https://bugs.launchpad.net/ubuntu/bionic/+source/gnutls28/+bug/1928648

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