Fix post-issuance validation logic.

Rather than adding intermediates in the certificate bundle we now
validate the leaf certificate with intermediates as untrusted (used for
chain building only).  Only the root certificates are used as trust
anchor.

Not pining intermediate certificates anymore is in line with Let's
Encrypt's latest recommendations:

    Rotating the set of intermediates we issue from helps keep the
    Internet agile and more secure.  It encourages automation and
    efficiency, and discourages outdated practices like key pinning.
    “Key Pinning” is a practice in which clients — either ACME clients
    getting certificates for their site, or apps connecting to their own
    backend servers — decide to trust only a single issuing intermediate
    certificate rather than delegating trust to the system trust store.
    Updating pinned keys is a manual process, which leads to an
    increased risk of errors and potential business continuity failures.
    — https://letsencrypt.org/2024/03/19/new-intermediate-certificates:

3 files changed, 76 insertions, 40 deletions

diff --git a/Makefile b/Makefile
index 10e55c5..cb2f4ed 100644
--- a/Makefile
+++ b/Makefile
@@ -16,17 +16,13 @@ $(MANUAL_FILES): $(BUILDDIR)/%: $(BUILDDIR)/%.md
 # used for validation, see https://letsencrypt.org/certificates/
 $(BUILDDIR)/certs/ca-certificates.crt: \
         certs/isrgrootx1.pem \
-        certs/isrg-root-x2.pem \
-        certs/lets-encrypt-r[34].pem \
-        certs/lets-encrypt-e[12].pem
+        certs/isrg-root-x2.pem
 	mkdir -pv -- $(@D)
 	cat -- $^ >$@
 
 # Staging Environment for tests, see https://letsencrypt.org/docs/staging-environment/
 $(BUILDDIR)/certs-staging/ca-certificates.crt: \
-        certs-staging/letsencrypt-stg-root-x[12].pem \
-        certs-staging/letsencrypt-stg-int-r[34].pem \
-        certs-staging/letsencrypt-stg-int-e[12].pem
+        certs-staging/letsencrypt-stg-root-x[12].pem
 	mkdir -pv -- $(@D)
 	cat -- $^ >$@
 
diff --git a/lacme b/lacme
index 19d78a9..b167f9b 100755
--- a/lacme
+++ b/lacme
@@ -822,21 +822,31 @@ elsif ($COMMAND eq 'newOrder' or $COMMAND eq 'new-cert') {
             next;
         }
 
-        my $cert;
+        my @chain;
         eval {
             my $mem = Net::SSLeay::BIO_s_mem() or die;
             my $bio = Net::SSLeay::BIO_new($mem) or die;
             die "incomplete write" unless
                 Net::SSLeay::BIO_write($bio, $chain) == length($chain);
-            my $x509 = Net::SSLeay::PEM_read_bio_X509($bio);
-            $cert = Net::SSLeay::PEM_get_string_X509($x509);
+
+            my $sk_x509_info = Net::SSLeay::PEM_X509_INFO_read_bio($bio);
+
+            my $n = Net::SSLeay::sk_X509_INFO_num($sk_x509_info);
+            for (my $i = 0; $i < $n; $i++) {
+                my $x509_info = Net::SSLeay::sk_X509_INFO_value($sk_x509_info, $i);
+                my $x509 = Net::SSLeay::P_X509_INFO_get_x509($x509_info);
+                my $cert = Net::SSLeay::PEM_get_string_X509($x509);
+                push @chain, $cert;
+            }
+
             Net::SSLeay::BIO_free($bio) or die;
         };
-        if ($@) {
+        if ($@ or !@chain) {
             print STDERR "[$s] Error: Received bogus X.509 certificate from ACME server!\n";
             $rv = 1;
             next;
         }
+        my $cert = shift @chain; # leave only the intermediate in @chain
 
         # extract pubkeys from CSR and cert, and ensure they match
         # XXX would be nice to use X509_get_X509_PUBKEY and X509_REQ_get_X509_PUBKEY here,
@@ -852,9 +862,15 @@ elsif ($COMMAND eq 'newOrder' or $COMMAND eq 'new-cert') {
 
         # verify certificate validity against the CA bundle
         if ((my $CAfile = $conf->{CAfile} // '@@datadir@@/lacme/ca-certificates.crt') ne '') {
+            my $chain_tmp = File::Temp::->new(SUFFIX => '.crt', TMPDIR => 1) // die;
+            $chain_tmp->say($_) foreach @chain;
+            $chain_tmp->flush();
+
             my %args = (in => $cert);
             $args{out} = \*STDERR if $OPTS{debug};
-            my @options = ('-trusted', $CAfile, '-purpose', 'sslserver', '-x509_strict');
+            my @options = ('-trusted', $CAfile);
+            push @options, '-untrusted', $chain_tmp->filename() if @chain;
+            push @options, ('-purpose', 'sslserver', '-x509_strict');
             push @options, '-show_chain' if $OPTS{debug};
             if (spawn(\%args, 'openssl', 'verify', @options)) {
                 print STDERR "[$s] Error: Received invalid X.509 certificate from ACME server!\n";
diff --git a/tests/cert-install b/tests/cert-install
index e24fe34..279309f 100644
--- a/tests/cert-install
+++ b/tests/cert-install
@@ -28,6 +28,55 @@ EOF
 grepstderr -Fxq "[bad3] Warning: Couldn't generate CSR, skipping"
 
 
+check_spki() {
+    local p1="$1" p2="$2" s1 s2
+    s1="$(openssl x509 -in "$p1" -noout -pubkey \
+        | openssl pkey -pubin -outform DER \
+        | openssl dgst -sha256 \
+        | sed 's/.*=\s*//')"
+    s2="$(openssl pkey -in "$p2" -pubout -outform DER \
+        | openssl dgst -sha256 \
+        | sed 's/.*=\s*//')"
+    if [ -n "$s1" ] && [ "$s1" = "$s2" ]; then
+        return 0
+    else
+        printf "%s != %s\\n" "$s1" "$s2" >&2
+        return 1
+    fi
+}
+check_chain() {
+    local priv="$1" chain="$2" leaf="${3-}" pem0
+
+    csplit -f "${chain%.crt}.chain.pem" "$chain" \
+        "/-----BEGIN CERTIFICATE-----/" "{*}"
+
+    pem0="${chain%.crt}.chain.pem00"
+    if [ ! -s "$pem0" ]; then
+        # 00 is empty, leaf cert is at 01
+        rm -f -- "$pem0"
+        pem0="${chain%.crt}.chain.pem01"
+    fi
+    test -s "$pem0" || return 1
+    check_spki "$pem0" "$priv"
+
+    if [ -n "$leaf" ]; then
+        diff --ignore-blank-lines --unified "$pem0" "$leaf" || return 1
+    fi
+
+    leaf="${chain%.crt}.leaf.pem"
+    mv -T -- "$pem0" "$leaf"
+
+    intermediates="${chain%.crt}.intermediates.pem"
+    sed "/^$/d" "${chain%.crt}.chain.pem"[0-9]* >"$intermediates"
+    test -s "$intermediates" || return 1 # ensure there is at least one intermediate
+
+    openssl verify -trusted /usr/share/lacme/ca-certificates.crt \
+        -untrusted "$intermediates" \
+        -purpose sslserver -x509_strict \
+        -show_chain \
+        -- "$leaf" || return 1
+}
+
 # 'certificate' installs only the leaf certificate
 openssl genpkey -algorithm RSA -out /etc/lacme/test1.key
 subject="/CN=$(head -c10 /dev/urandom | base32 -w0 | tr "A-Z" "a-z").$DOMAINNAME"
@@ -42,23 +91,9 @@ lacme newOrder test1 2>"$STDERR" || fail newOrder test1
 test /etc/lacme/test1.crt -nt /etc/lacme/test1.key
 sed -n "0,/^-----END CERTIFICATE-----$/ p" /etc/lacme/test1.crt >/etc/lacme/test1.pem
 diff --unified /etc/lacme/test1.crt /etc/lacme/test1.pem
+check_spki /etc/lacme/test1.crt /etc/lacme/test1.key
 
 
-check_hash() {
-    local p1="$1" p2 s1 s2
-    s1="$(openssl x509 -in "$p1" -noout -hash)"
-    for p2 in /usr/share/lacme/ca-certificates.pem.*; do
-        s2="$(openssl x509 -in "$p2" -noout -hash)"
-        if [ "$s1" = "$s2" ]; then
-            return 0
-        fi
-    done
-    return 1
-}
-csplit -f /usr/share/lacme/ca-certificates.pem. /usr/share/lacme/ca-certificates.crt \
-    "/-----BEGIN CERTIFICATE-----/" "{*}"
-rm -f /usr/share/lacme/ca-certificates.pem.00
-
 # 'certificate-chain' appends the chain of trust
 openssl genpkey -algorithm RSA -out /etc/lacme/test2.key
 cat >"/etc/lacme/lacme-certs.conf.d/test2.conf" <<- EOF
@@ -70,16 +105,7 @@ EOF
 
 lacme newOrder test2 2>"$STDERR" || fail newOrder test2
 test /etc/lacme/test2.crt -nt /etc/lacme/test2.key
-csplit -f /etc/lacme/test2.chain.pem /etc/lacme/test2.crt \
-    "/-----BEGIN CERTIFICATE-----/" "{*}"
-test -s /etc/lacme/test2.chain.pem01 # leaf cert (00 is empty)
-rm -f /etc/lacme/test2.chain.pem0[01]
-test -s /etc/lacme/test2.chain.pem02 # depth 1
-
-# all certificates at depth >=1 must be in our CA bundle
-for p in /etc/lacme/test2.chain.pem*; do
-    check_hash "$p"
-done
+check_chain /etc/lacme/test2.key /etc/lacme/test2.crt
 
 # 'certificate' + 'certificate-chain'
 openssl genpkey -algorithm RSA -out /etc/lacme/test3.key
@@ -94,10 +120,8 @@ EOF
 lacme newOrder test3 2>"$STDERR" || fail newOrder test3
 test /etc/lacme/test3.pem -nt /etc/lacme/test3.key
 test /etc/lacme/test3.crt -nt /etc/lacme/test3.key
-csplit -f /etc/lacme/test3.chain.pem /etc/lacme/test3.crt \
-    "/-----BEGIN CERTIFICATE-----/" "{*}"
-sed -i "/^$/d" /etc/lacme/test3.chain.pem*
-diff -q /etc/lacme/test3.chain.pem01 /etc/lacme/test3.pem
+check_chain /etc/lacme/test3.key /etc/lacme/test3.crt /etc/lacme/test3.pem
+
 st="$(stat -c "%U:%G %#a" /etc/lacme/test3.pem)"
 [ "$st" = "root:root 0644" ]
 st="$(stat -c "%U:%G %#a" /etc/lacme/test3.crt)"