CertPathValidatorException connecting to a Let's Encrypt host on Android M or earlier

[edit: If you are here for Let's Encrypt expiry event, this was extended to 2024 https://letsencrypt.org/2020/12/21/extending-android-compatibility.html], but is now ending this year https://letsencrypt.org/2023/07/10/cross-sign-expiration.html. This can be extended slightly by configuration until at least June 2024

Connecting to https://valid-isrgrootx1.letsencrypt.org/ via OkHttp on Android M or earlier fails, while the connection works on N or later.

    OkHttpClient client = new OkHttpClient();

    try {
      Request request = new Request.Builder()
              .url("https://valid-isrgrootx1.letsencrypt.org/robots.txt")
              .build();
      try (Response response = client.newCall(request).execute()) {
        assertTrue(response.code() == 200 || response.code() == 404);
        assertEquals(Protocol.HTTP_2, response.protocol());
      }
    } catch (SSLHandshakeException sslhe) {
      sslhe.printStackTrace();
    }

javax.net.ssl.SSLHandshakeException: java.security.cert.CertPathValidatorException: Trust anchor for certification path not found.
at com.android.org.conscrypt.OpenSSLSocketImpl.startHandshake(OpenSSLSocketImpl.java:322)
at okhttp3.internal.connection.RealConnection.connectTls(RealConnection.java:320)
at okhttp3.internal.connection.RealConnection.establishProtocol(RealConnection.java:284)
at okhttp3.internal.connection.RealConnection.connect(RealConnection.java:169)
at okhttp3.internal.connection.StreamAllocation.findConnection(StreamAllocation.java:258)
at okhttp3.internal.connection.StreamAllocation.findHealthyConnection(StreamAllocation.java:135)
at okhttp3.internal.connection.StreamAllocation.newStream(StreamAllocation.java:114)
at okhttp3.internal.connection.ConnectInterceptor.intercept(ConnectInterceptor.java:42)
at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:147)
at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:121)
at okhttp3.internal.cache.CacheInterceptor.intercept(CacheInterceptor.java:93)
at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:147)
at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:121)
at okhttp3.internal.http.BridgeInterceptor.intercept(BridgeInterceptor.java:93)
at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:147)
at okhttp3.internal.http.RetryAndFollowUpInterceptor.intercept(RetryAndFollowUpInterceptor.java:127)
at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:147)
at okhttp3.internal.http.RealInterceptorChain.proceed(RealInterceptorChain.java:121)
at okhttp3.RealCall.getResponseWithInterceptorChain(RealCall.java:257)
at okhttp3.RealCall.execute(RealCall.java:93)
at okhttp.regression.LetsEncryptTest.sendRequest(LetsEncryptTest.java:133)
at okhttp.regression.LetsEncryptTest.getFailsWithoutAdditionalCert(LetsEncryptTest.java:52)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at androidx.test.ext.junit.runners.AndroidJUnit4.run(AndroidJUnit4.java:154)
at org.junit.runners.Suite.runChild(Suite.java:128)
at org.junit.runners.Suite.runChild(Suite.java:27)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at org.junit.runner.JUnitCore.run(JUnitCore.java:115)
at androidx.test.internal.runner.TestExecutor.execute(TestExecutor.java:56)
at androidx.test.runner.AndroidJUnitRunner.onStart(AndroidJUnitRunner.java:395)
at android.app.Instrumentation$InstrumentationThread.run(Instrumentation.java:1853)
Caused by: java.security.cert.CertificateException: java.security.cert.CertPathValidatorException: Trust anchor for certification path not found.
at com.android.org.conscrypt.TrustManagerImpl.checkTrusted(TrustManagerImpl.java:318)
at com.android.org.conscrypt.TrustManagerImpl.checkServerTrusted(TrustManagerImpl.java:219)
at com.android.org.conscrypt.Platform.checkServerTrusted(Platform.java:114)
at com.android.org.conscrypt.OpenSSLSocketImpl.verifyCertificateChain(OpenSSLSocketImpl.java:550)
at com.android.org.conscrypt.NativeCrypto.SSL_do_handshake(Native Method)
at com.android.org.conscrypt.OpenSSLSocketImpl.startHandshake(OpenSSLSocketImpl.java:318)
... 50 more
Caused by: java.security.cert.CertPathValidatorException: Trust anchor for certification path not found.
... 56 more

[edit: If you are here for Let's Encrypt expiry event, this was extended to 2024 https://letsencrypt.org/2020/12/21/extending-android-compatibility.html], but is now ending this year https://letsencrypt.org/2023/07/10/cross-sign-expiration.html. This can be extended slightly by configuration until at least June 2024

As generic advice, if you need to add an root CA to older Android devices, this example shows how this can work.

The issue relates to the known expiry of the ISRG root certificate for Let's encrypt in 2021, delayed to 2024.

Note: none of this advice applies in combination with CertificatePinner, if you choose to also pin certificates, then please discuss your strategy with your in-house security team.

    boolean androidNorEarlier = Build.VERSION.SDK_INT <= 25;

    OkHttpClient.Builder builder = new OkHttpClient.Builder();

    if (androidNorEarlier) {
      // TODO: download fresh from https://letsencrypt.org/certs/isrgrootx1.pem
      String isgCert =
              "-----BEGIN CERTIFICATE-----\n" +
              "MIIFazCCA1OgAwIBAgIRAIIQz7DSQONZRGPgu2OCiwAwDQYJKoZIhvcNAQELBQAw\n" +
              "TzELMAkGA1UEBhMCVVMxKTAnBgNVBAoTIEludGVybmV0IFNlY3VyaXR5IFJlc2Vh\n" +
              "cmNoIEdyb3VwMRUwEwYDVQQDEwxJU1JHIFJvb3QgWDEwHhcNMTUwNjA0MTEwNDM4\n" +
              "WhcNMzUwNjA0MTEwNDM4WjBPMQswCQYDVQQGEwJVUzEpMCcGA1UEChMgSW50ZXJu\n" +
              "ZXQgU2VjdXJpdHkgUmVzZWFyY2ggR3JvdXAxFTATBgNVBAMTDElTUkcgUm9vdCBY\n" +
              "MTCCAiIwDQYJKoZIhvcNAQEBBQADggIPADCCAgoCggIBAK3oJHP0FDfzm54rVygc\n" +
              "h77ct984kIxuPOZXoHj3dcKi/vVqbvYATyjb3miGbESTtrFj/RQSa78f0uoxmyF+\n" +
              "0TM8ukj13Xnfs7j/EvEhmkvBioZxaUpmZmyPfjxwv60pIgbz5MDmgK7iS4+3mX6U\n" +
              "A5/TR5d8mUgjU+g4rk8Kb4Mu0UlXjIB0ttov0DiNewNwIRt18jA8+o+u3dpjq+sW\n" +
              "T8KOEUt+zwvo/7V3LvSye0rgTBIlDHCNAymg4VMk7BPZ7hm/ELNKjD+Jo2FR3qyH\n" +
              "B5T0Y3HsLuJvW5iB4YlcNHlsdu87kGJ55tukmi8mxdAQ4Q7e2RCOFvu396j3x+UC\n" +
              "B5iPNgiV5+I3lg02dZ77DnKxHZu8A/lJBdiB3QW0KtZB6awBdpUKD9jf1b0SHzUv\n" +
              "KBds0pjBqAlkd25HN7rOrFleaJ1/ctaJxQZBKT5ZPt0m9STJEadao0xAH0ahmbWn\n" +
              "OlFuhjuefXKnEgV4We0+UXgVCwOPjdAvBbI+e0ocS3MFEvzG6uBQE3xDk3SzynTn\n" +
              "jh8BCNAw1FtxNrQHusEwMFxIt4I7mKZ9YIqioymCzLq9gwQbooMDQaHWBfEbwrbw\n" +
              "qHyGO0aoSCqI3Haadr8faqU9GY/rOPNk3sgrDQoo//fb4hVC1CLQJ13hef4Y53CI\n" +
              "rU7m2Ys6xt0nUW7/vGT1M0NPAgMBAAGjQjBAMA4GA1UdDwEB/wQEAwIBBjAPBgNV\n" +
              "HRMBAf8EBTADAQH/MB0GA1UdDgQWBBR5tFnme7bl5AFzgAiIyBpY9umbbjANBgkq\n" +
              "hkiG9w0BAQsFAAOCAgEAVR9YqbyyqFDQDLHYGmkgJykIrGF1XIpu+ILlaS/V9lZL\n" +
              "ubhzEFnTIZd+50xx+7LSYK05qAvqFyFWhfFQDlnrzuBZ6brJFe+GnY+EgPbk6ZGQ\n" +
              "3BebYhtF8GaV0nxvwuo77x/Py9auJ/GpsMiu/X1+mvoiBOv/2X/qkSsisRcOj/KK\n" +
              "NFtY2PwByVS5uCbMiogziUwthDyC3+6WVwW6LLv3xLfHTjuCvjHIInNzktHCgKQ5\n" +
              "ORAzI4JMPJ+GslWYHb4phowim57iaztXOoJwTdwJx4nLCgdNbOhdjsnvzqvHu7Ur\n" +
              "TkXWStAmzOVyyghqpZXjFaH3pO3JLF+l+/+sKAIuvtd7u+Nxe5AW0wdeRlN8NwdC\n" +
              "jNPElpzVmbUq4JUagEiuTDkHzsxHpFKVK7q4+63SM1N95R1NbdWhscdCb+ZAJzVc\n" +
              "oyi3B43njTOQ5yOf+1CceWxG1bQVs5ZufpsMljq4Ui0/1lvh+wjChP4kqKOJ2qxq\n" +
              "4RgqsahDYVvTH9w7jXbyLeiNdd8XM2w9U/t7y0Ff/9yi0GE44Za4rF2LN9d11TPA\n" +
              "mRGunUHBcnWEvgJBQl9nJEiU0Zsnvgc/ubhPgXRR4Xq37Z0j4r7g1SgEEzwxA57d\n" +
              "emyPxgcYxn/eR44/KJ4EBs+lVDR3veyJm+kXQ99b21/+jh5Xos1AnX5iItreGCc=\n" +
              "-----END CERTIFICATE-----";

      CertificateFactory cf = CertificateFactory.getInstance("X.509");
      Certificate isgCertificate = cf.generateCertificate(new ByteArrayInputStream(isgCert.getBytes("UTF-8")));

      HandshakeCertificates certificates = new HandshakeCertificates.Builder()
              .addTrustedCertificate((X509Certificate) isgCertificate)
              // Uncomment to allow connection to any site generally, but could possibly cause
              // noticeable memory pressure in Android apps.
//              .addPlatformTrustedCertificates()
              .build();

      builder.sslSocketFactory(certificates.sslSocketFactory(), certificates.trustManager());
    }

    OkHttpClient client = builder.build();

    Request request = new Request.Builder()
            .url("https://valid-isrgrootx1.letsencrypt.org/robots.txt")
            .build();
    try (Response response = client.newCall(request).execute()) {
      assertTrue(response.code() == 200 || response.code() == 404);
      assertEquals(Protocol.HTTP_2, response.protocol());
    }

The new host certificates are root signed by the ISRG Root X1 CA.

 ./cft --host valid-isrgrootx1.letsencrypt.org           
CN:     valid-isrgrootx1.letsencrypt.org
Pin:    sha256/489aa1610850a89c720217b9d9dbdc7f80918119f32b88c2dd3bcfaf1de29079
SAN:    valid-isrgrootx1.letsencrypt.org
Key Usage: DigitalSignature, KeyEncipherment
Ext Key Usage: serverAuth, clientAuth
Authority Info Access:
    ocsp: http://ocsp.int-x3.letsencrypt.org
    caIssuers: http://cert.int-x3.letsencrypt.org/
Valid:  2020-10-14T15:00:50Z..2021-01-12T15:00:50Z (1 months)
CA: false

CN:     Let's Encrypt Authority X3
Pin:    sha256/60b87575447dcba2a36b7d11ac09fb24a9db406fee12d2cc90180517616e8a18
SAN:    <N/A>
Key Usage: DigitalSignature, KeyCertSign, CRLSign
Authority Info Access:
    ocsp: http://ocsp.root-x1.letsencrypt.org/
    caIssuers: http://cert.root-x1.letsencrypt.org/
Valid:  2016-10-06T15:43:55Z..2021-10-06T15:43:55Z (10 months)
CA: true Max Intermediate: 0

CN:     ISRG Root X1 (signed by locally-trusted root)
Pin:    sha256/0b9fa5a59eed715c26c1020c711b4f6ec42d58b0015e14337a39dad301c5afc3
SAN:    <N/A>
Key Usage: KeyCertSign, CRLSign
Valid:  2015-06-04T11:04:38Z..2035-06-04T11:04:38Z (14 years)
CA: true

Strict Transport Security: max-age=604800

OCSP status: GOOD

Existing certificates are signed by DST Root CA X3 which expires in September 2021.

$  ./cft --host letsencrypt.org 
CN:     lencr.org
Pin:    sha256/b93116ebda5e22efe089e7710b221557eb80a2e13c60a58687c0ce0369afd68a
SAN:    lencr.org, letsencrypt.org, www.lencr.org, www.letsencrypt.org
Key Usage: DigitalSignature, KeyEncipherment
Ext Key Usage: serverAuth, clientAuth
Authority Info Access:
    ocsp: http://ocsp.int-x3.letsencrypt.org
    caIssuers: http://cert.int-x3.letsencrypt.org/
Valid:  2020-11-03T21:00:55Z..2021-02-01T21:00:55Z (2 months)
CA: false

CN:     Let's Encrypt Authority X3
Pin:    sha256/60b87575447dcba2a36b7d11ac09fb24a9db406fee12d2cc90180517616e8a18
SAN:    <N/A>
Key Usage: DigitalSignature, KeyCertSign, CRLSign
Authority Info Access:
    ocsp: http://isrg.trustid.ocsp.identrust.com
    caIssuers: http://apps.identrust.com/roots/dstrootcax3.p7c
Valid:  2016-03-17T16:40:46Z..2021-03-17T16:40:46Z (4 months)
CA: true Max Intermediate: 0

CN:     DST Root CA X3 (signed by locally-trusted root)
Pin:    sha256/563b3caf8cfef34c2335caf560a7a95906e8488462eb75ac59784830df9e5b2b
SAN:    <N/A>
Key Usage: KeyCertSign, CRLSign
Valid:  2000-09-30T21:12:19Z..2021-09-30T14:01:15Z (10 months)
CA: true

Strict Transport Security: max-age=31536000

OCSP status: GOOD

Available Fixes