[Bug 1926254] Re: x509 Certificate verification fails when basicConstraints=CA:FALSE, pathlen:0 on self-signed leaf certs

Thu Apr 29 23:56:25 UTC 2021

Attached is a debdiff for openssl on Groovy which fixes this bug.

** Patch added: "Debdiff for openssl on Groovy"
   https://bugs.launchpad.net/ubuntu/+source/openssl/+bug/1926254/+attachment/5493443/+files/lp1926254_groovy.debdiff

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Title:
  x509 Certificate verification fails when
  basicConstraints=CA:FALSE,pathlen:0 on self-signed leaf certs

Status in openssl package in Ubuntu:
  Fix Released
Status in openssl source package in Focal:
  In Progress
Status in openssl source package in Groovy:
  In Progress
Status in openssl source package in Hirsute:
  Fix Released

Bug description:
  [Impact]

  In openssl 1.1.1f, the below commit was merged:

  commit ba4356ae4002a04e28642da60c551877eea804f7
  Author: Bernd Edlinger <bernd.edlinger at hotmail.de>
  Date:   Sat Jan 4 15:54:53 2020 +0100
  Subject: Fix error handling in x509v3_cache_extensions and related functions
  Link: https://github.com/openssl/openssl/commit/ba4356ae4002a04e28642da60c551877eea804f7

  This introduced a regression which caused certificate validation to
  fail when certificates violate RFC 5280 [1], namely, when a
  certificate has "basicConstraints=CA:FALSE,pathlen:0". This
  combination is commonly seen by self-signed leaf certificates with an
  intermediate CA before the root CA.

  Because of this, openssl 1.1.1f rejects these certificates and they
  cannot be used in the system certificate store, and ssl connections
  fail when you try to use them to connect to a ssl endpoint.

  The error you see when you try verify is:

  $ openssl verify -CAfile CA/rootCA_cert.pem -untrusted CA/subCA_cert.pem user1_cert.pem
  error 20 at 0 depth lookup: unable to get local issuer certificate
  error user1_cert.pem: verification failed

  The exact same certificates work fine on Xenial, Bionic and Hirsute.

  [1] https://tools.ietf.org/html/rfc5280.html

  [Testcase]

  We will create our own root CA, intermediate CA and leaf server
  certificate.

  Create necessary directories:

  $ mkdir reproducer
  $ cd reproducer
  $ mkdir CA

  Write openssl configuration files to disk for each CA and cert:

  $ cat << EOF >> rootCA.cnf
  [ req ]
  prompt              = no
  distinguished_name  = req_distinguished_name
  x509_extensions     = usr_cert

  [ req_distinguished_name ]
  C  = DE
  O  = Test Org
  CN = Test RSA PSS Root-CA

  [ usr_cert ]
  basicConstraints        = critical,CA:TRUE
  keyUsage                = critical,keyCertSign,cRLSign
  subjectKeyIdentifier    = hash
  authorityKeyIdentifier  = keyid:always
  EOF

  $ cat << EOF >> subCA.cnf
  [ req ]
  prompt              = no
  distinguished_name  = req_distinguished_name
  x509_extensions     = usr_cert

  [ req_distinguished_name ]
  C  = DE
  O  = Test Org
  CN = Test RSA PSS Sub-CA

  [ usr_cert ]
  basicConstraints        = critical,CA:TRUE,pathlen:0
  keyUsage                = critical,keyCertSign,cRLSign
  subjectKeyIdentifier    = hash
  authorityKeyIdentifier  = keyid:always
  EOF

  $ cat << EOF >> user.cnf
  [ req ]
  prompt              = no
  distinguished_name  = req_distinguished_name
  x509_extensions     = usr_cert

  [ req_distinguished_name ]
  C  = DE
  O  = Test Org
  CN = Test User

  [ usr_cert ]
  basicConstraints        = critical,CA:FALSE,pathlen:0
  keyUsage                = critical,digitalSignature,keyAgreement
  extendedKeyUsage        = clientAuth,serverAuth
  subjectKeyIdentifier    = hash
  authorityKeyIdentifier  = keyid:always
  EOF

  Then generate the necessary RSA keys and form certificates:

  $ openssl genpkey -algorithm RSA-PSS -out rootCA_key.pem -pkeyopt rsa_keygen_bits:2048
  $ openssl req -config rootCA.cnf -set_serial 01 -new -batch -sha256 -nodes -x509 -days 9125 -out CA/rootCA_cert.pem -key rootCA_key.pem -sigopt rsa_padding_mode:pss -sigopt rsa_pss_saltlen:-1

  $ openssl genpkey -algorithm RSA-PSS -out subCA_key.pem -pkeyopt rsa_keygen_bits:2048
  $ openssl req -config subCA.cnf -new -out subCA_req.pem -key subCA_key.pem -sigopt rsa_padding_mode:pss -sigopt rsa_pss_saltlen:-1
  $ openssl x509 -req -sha256 -in subCA_req.pem -CA CA/rootCA_cert.pem -CAkey rootCA_key.pem -out CA/subCA_cert.pem -CAserial rootCA_serial.txt -CAcreateserial -extfile subCA.cnf -extensions usr_cert -days 4380 -sigopt rsa_padding_mode:pss -sigopt rsa_pss_saltlen:-1
  $ c_rehash CA

  $ openssl genpkey -algorithm RSA-PSS -out user1_key.pem -pkeyopt rsa_keygen_bits:2048
  $ openssl req -config user.cnf -new -out user1_req.pem -key user1_key.pem -sigopt rsa_padding_mode:pss -sigopt rsa_pss_saltlen:-1
  $ openssl x509 -req -sha256 -in user1_req.pem -CA CA/subCA_cert.pem -CAkey subCA_key.pem -out user1_cert.pem -CAserial subCA_serial.txt -CAcreateserial -extfile user.cnf -extensions usr_cert -days 1825 -sigopt rsa_padding_mode:pss -sigopt rsa_pss_saltlen:-1

  Now, let's try verify the generated certificates:

  $ openssl version
  OpenSSL 1.1.1f  31 Mar 2020
  $ openssl verify -CAfile CA/rootCA_cert.pem -untrusted CA/subCA_cert.pem user1_cert.pem
  error 20 at 0 depth lookup: unable to get local issuer certificate
  error user1_cert.pem: verification failed

  There are test packages available in the following ppa:

  https://launchpad.net/~mruffell/+archive/ubuntu/sf308725-test

  If you install these test packages, and attempt to verify, things work
  as planned.

  $ openssl verify -CAfile CA/rootCA_cert.pem -untrusted CA/subCA_cert.pem user1_cert.pem
  user1_cert.pem: OK

  [Where problems could occur]

  If a regression were to occur, it would occur around x509 certificate
  parsing and validation. It may cause certificates which had previously
  passed to fail, or certificates which failed to pass. It could
  potentially cause all certificates to fail or pass.

  Since this is the openssl package and the package is security-
  critical, this will need a lot of validation, testing, and likely a
  review by the security team.

  One of the commits which fixes the issue adds two testcases to the
  openssl testsuite, which tests the "CA:FALSE, pathlen:0" certificates
  with and without -x509_strict, and tests to see if it passes without,
  and fails with.

  [Other info]

  This was reported in the upstream issue #11456 [2]:

  [2] https://github.com/openssl/openssl/issues/11456

  I believe these three commits fix the issue:

  commit 00a0da2f021e6a0bc9519a6a9e5be66d45e6fc91
  Author: Tomas Mraz <tmraz at fedoraproject.org>
  Date:   Thu Apr 2 15:56:12 2020 +0200
  Subject: Allow certificates with Basic Constraints CA:false, pathlen:0
  Link: https://github.com/openssl/openssl/commit/00a0da2f021e6a0bc9519a6a9e5be66d45e6fc91

  commit 29e94f285f7f05b1aec6fa275e320bc5fa37ab1e
  Author: Tomas Mraz <tmraz at fedoraproject.org>
  Date:   Thu Apr 2 17:31:21 2020 +0200
  Subject: Set X509_V_ERR_INVALID_EXTENSION error for invalid basic constraints
  Link: https://github.com/openssl/openssl/commit/29e94f285f7f05b1aec6fa275e320bc5fa37ab1e

  commit e78f2a8f269a4dcf820ca994e2b89b77972d79e1
  Author: Tomas Mraz <tmraz at fedoraproject.org>
  Date:   Fri Apr 3 10:24:40 2020 +0200
  Subject: Add test cases for the non CA certificate with pathlen:0
  Link: https://github.com/openssl/openssl/commit/e78f2a8f269a4dcf820ca994e2b89b77972d79e1

  These landed in openssl 1.1.1g, and hirsute already has these fixes.

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