Select Most Specific RDN in X500 Principal Extractor

SubjectX500PrincipalExtractor now returns the left-most, most-specific
matching RDN value, matching both convention and
SubjectDnX509PrincipalExtractor. Since LdapName#getRdns lists RDNs
most-significant first, the extractor reads them in reverse so that a
subject with more than one matching attribute (e.g. two CNs) resolves to
the most specific value.

Closes gh-19254

Signed-off-by: Josh Cummings <3627351+jzheaux@users.noreply.github.com>

web/src/main/java/org/springframework/security/web/authentication/preauth/x509/SubjectX500PrincipalExtractor.java

@@ -17,6 +17,8 @@
 package org.springframework.security.web.authentication.preauth.x509;
 
 import java.security.cert.X509Certificate;
+import java.util.ArrayList;
+import java.util.Collections;
 import java.util.List;
 
 import javax.naming.InvalidNameException;
@@ -72,7 +74,10 @@ public final class SubjectX500PrincipalExtractor implements X509PrincipalExtract
 
 	private List<Rdn> getDns(String subjectDn) {
 		try {
-			return new LdapName(subjectDn).getRdns();
+			// read most-specific first, see gh-19254
+			List<Rdn> rdns = new ArrayList<>(new LdapName(subjectDn).getRdns());
+			Collections.reverse(rdns);
+			return rdns;
 		}
 		catch (InvalidNameException ex) {
 			throw new BadCredentialsException("Failed to parse client certificate", ex);

web/src/test/java/org/springframework/security/web/authentication/preauth/x509/SubjectDnX509PrincipalExtractorTests.java

@@ -74,6 +74,13 @@ public class SubjectDnX509PrincipalExtractorTests {
 		assertThat(principal).isEqualTo("Duke");
 	}
 
+	// gh-19254
+	@Test
+	public void defaultCNPatternReturnsMostSpecificPrincipalWhenMultipleCns() throws Exception {
+		Object principal = this.extractor.extractPrincipal(X509TestUtils.buildTestCertificateWithMultipleCns());
+		assertThat(principal).isEqualTo("alice");
+	}
+
 	@Test
 	public void setMessageSourceWhenNullThenThrowsException() {
 		assertThatIllegalArgumentException().isThrownBy(() -> this.extractor.setMessageSource(null));

web/src/test/java/org/springframework/security/web/authentication/preauth/x509/SubjectX500PrincipalExtractorTests.java

@@ -60,6 +60,14 @@ public class SubjectX500PrincipalExtractorTests {
 		assertThat(principal).isEqualTo("luke");
 	}
 
+	// gh-19254
+	@Test
+	void extractWhenMultipleCnsThenExtractsMostSpecificCn() throws Exception {
+		Object principal = this.extractor.extractPrincipal(X509TestUtils.buildTestCertificateWithMultipleCns());
+
+		assertThat(principal).isEqualTo("alice");
+	}
+
 	@Test
 	void extractWhenEmailDnEmbeddedInCnThenExtractsEmail() throws Exception {
 		this.extractor.setExtractPrincipalNameFromEmail(true);

web/src/test/java/org/springframework/security/web/authentication/preauth/x509/X509TestUtils.java

@@ -185,4 +185,38 @@ public final class X509TestUtils {
 		return (X509Certificate) cf.generateCertificate(in);
 	}
 
+	/**
+	 * Builds an X.509 certificate whose subject contains more than one CN. The subject DN
+	 * is:
+	 *
+	 * <pre>
+	 *  CN=alice, CN=bob, O=Example Corp, C=US
+	 * </pre>
+	 *
+	 * where {@code CN=alice} is the most specific (left-most) RDN.
+	 */
+	public static X509Certificate buildTestCertificateWithMultipleCns() throws Exception {
+		String cert = "-----BEGIN CERTIFICATE-----\n"
+				+ "MIIDJzCCAg+gAwIBAgIIclOz1VulWC8wDQYJKoZIhvcNAQEMBQAwQjELMAkGA1UE\n"
+				+ "BhMCVVMxFTATBgNVBAoTDEV4YW1wbGUgQ29ycDEMMAoGA1UEAxMDYm9iMQ4wDAYD\n"
+				+ "VQQDEwVhbGljZTAeFw0yNjA2MDExOTQzMTVaFw0zNjA1MjkxOTQzMTVaMEIxCzAJ\n"
+				+ "BgNVBAYTAlVTMRUwEwYDVQQKEwxFeGFtcGxlIENvcnAxDDAKBgNVBAMTA2JvYjEO\n"
+				+ "MAwGA1UEAxMFYWxpY2UwggEiMA0GCSqGSIb3DQEBAQUAA4IBDwAwggEKAoIBAQCX\n"
+				+ "q8hZrTRHJEN7+D6yK65OKeCTVU+WccI6awz6g4T6O3aoC+1IiUljsEYn+xuDfx5L\n"
+				+ "L/O1kejjmbYt+vzRmiILoJ9xKfW3ERcB4+gEar959Dkj6wmpsgOKRjmOvcOFOkEe\n"
+				+ "gU1F7t04JHOou3DaAkHMNMQV+3jsWSh9Rry7XZBDkcT8XbbagoCgSIIef03Qtw31\n"
+				+ "zOBwqmJmd6CQhFJva5cl8cCE2xZinOIiz/2j7VprZTjhkud2M4bRnK3T0WExyOCg\n"
+				+ "jvjSf5ZoOKwC2Z9Q/Oyf8WKpren1+GfZhAKKmn7QZ2foHhdPPRtNjRGE6SqQyW2u\n"
+				+ "8+1tOXl+aRCF19rR7gIpAgMBAAGjITAfMB0GA1UdDgQWBBRfLtnK5WKU0q3zxIrr\n"
+				+ "y3GD+lYplzANBgkqhkiG9w0BAQwFAAOCAQEAe+/FHqErVPsF/sHrVHny8mIsn3ux\n"
+				+ "qE9P24KNF0oIfmBrAqqge6hoVQ8PS+JialyqFf//osuDjiuYaBEKBw7GCoA6I8mr\n"
+				+ "FA7wyFaGosfq7An5vxkJl7lap2u5oSVv3dCy13Bs0ziYmNlTkfHDLy9yh7jpH1wg\n"
+				+ "TvylH9O4Vc7y9rzzpIjMCuQJ/wJ4MuJ2mSarYZsx3UQHIRfpKtR/9jbFMX1Rbv/A\n"
+				+ "N0XD+NrtFjiikp71y3aCO1EHnGG7qPKCWh3PzaNoWFpyZKDBvud8ymW7RMiLPgv9\n"
+				+ "eTa82KGgnCwtKCNzGkszIn7fza/6xnCuzvh4y9tYE5BWP2mcMRtRmDD07w==\n" + "-----END CERTIFICATE-----";
+		ByteArrayInputStream in = new ByteArrayInputStream(cert.getBytes());
+		CertificateFactory cf = CertificateFactory.getInstance("X.509");
+		return (X509Certificate) cf.generateCertificate(in);
+	}
+
 }