Host certificates: check both public keys for not being revoked

It could be that the certificate is perfectly valid, but the
certified host key is listed itself as being revoked.

Author: tomaswolf

sshd-core/src/main/java/org/apache/sshd/client/keyverifier/KnownHostsServerKeyVerifier.java

@@ -298,6 +298,18 @@ protected boolean acceptKnownHostEntries(
                 .filter(e -> isCert == "cert-authority".equals(e.getHostEntry().getMarker()))
                 .collect(Collectors.toList());
         if (!keyMatches.isEmpty()) {
+            if (serverKey instanceof OpenSshCertificate) {
+                // Also check whether the certified key has been revoked.
+                PublicKey certifiedKey = ((OpenSshCertificate) serverKey).getCertPubKey();
+                String certKeyType = KeyUtils.getKeyType(certifiedKey);
+                if (hostMatches.stream() //
+                        .filter(entry -> certKeyType.equals(entry.getHostEntry().getKeyEntry().getKeyType()))
+                        .filter(k -> KeyUtils.compareKeys(k.getServerKey(), certifiedKey))
+                        .anyMatch(entry -> "revoked".equals(entry.getHostEntry().getMarker()))) {
+                    handleRevokedKey(clientSession, remoteAddress, certifiedKey);
+                    return false;
+                }
+            }
             return true;
         }
 
@@ -306,7 +318,8 @@ protected boolean acceptKnownHostEntries(
         }
 
         Optional<HostEntryPair> anyNonRevokedMatch = hostMatches.stream()
-                .filter(k -> !"revoked".equals(k.getHostEntry().getMarker()))
+                .filter(k -> !"revoked".equals(k.getHostEntry().getMarker())
+                        && !"cert-authority".equals(k.getHostEntry().getMarker()))
                 .findAny();
 
         if (!anyNonRevokedMatch.isPresent()) {

sshd-core/src/test/java/org/apache/sshd/common/signature/KnownHostsCertificateTest.java

@@ -20,6 +20,7 @@
 
 import java.nio.file.Files;
 import java.nio.file.Path;
+import java.nio.file.StandardOpenOption;
 import java.security.KeyPair;
 import java.util.Arrays;
 import java.util.Collections;
@@ -199,4 +200,24 @@ void testHostCertificateWithWildcardSucceeds(String principals) throws Exception
             s.auth().verify(AUTH_TIMEOUT);
         }
     }
+
+    @Test
+    void testHostCertificateWithRejectedHostKeyFails() throws Exception {
+        initKeys(KeyUtils.EC_ALGORITHM, 256, KeyUtils.EC_ALGORITHM, 256, "ecdsa-sha2-nistp256", "cert-authority");
+        Path knownHosts = tmp.resolve("known_hosts");
+        StringBuilder line = new StringBuilder();
+        line.append("@revoked ");
+        line.append("[localhost]:").append(port).append(",[127.0.0.1]:").append(port).append(' ');
+        line.append(PublicKeyEntry.toString(hostKey.getPublic()));
+        line.append('\n');
+        Files.write(knownHosts, Collections.singletonList(line.toString()), StandardOpenOption.APPEND);
+        client.setServerKeyVerifier(new KnownHostsServerKeyVerifier(AcceptAllServerKeyVerifier.INSTANCE, knownHosts));
+        assertThrows(SshException.class, () -> {
+            try (ClientSession s = client.connect(getCurrentTestName(), TEST_LOCALHOST, port).verify(CONNECT_TIMEOUT)
+                    .getSession()) {
+                s.addPasswordIdentity(getCurrentTestName());
+                s.auth().verify(AUTH_TIMEOUT);
+            }
+        });
+    }
 }