diff --git a/README.md b/README.md
index d22bd8515..89ee42aa4 100644
--- a/README.md
+++ b/README.md
@@ -99,6 +99,7 @@ For both API client and WebSocket client, the following error codes apply:
   | Code | Reason                                                   |
   | ---- | -------------------------------------------------------- |
   | -200 | SSL handshake failed due to a missing client certificate |
+  | -299 | Server SSL certificate is not trusted or invalid         |
 
 ## Method Swizzling
 
diff --git a/android/src/main/java/com/mattermost/networkclient/NetworkClientBase.kt b/android/src/main/java/com/mattermost/networkclient/NetworkClientBase.kt
index 150a15084..7103af75e 100644
--- a/android/src/main/java/com/mattermost/networkclient/NetworkClientBase.kt
+++ b/android/src/main/java/com/mattermost/networkclient/NetworkClientBase.kt
@@ -18,9 +18,14 @@ import org.json.JSONArray
 import org.json.JSONObject
 import java.io.IOException
 import java.net.URI
+import java.security.SecureRandom
+import java.security.cert.CertificateException
+import java.security.cert.X509Certificate
 import java.util.*
-import kotlin.collections.ArrayList
-import kotlin.collections.HashMap
+import javax.net.ssl.HttpsURLConnection
+import javax.net.ssl.SSLContext
+import javax.net.ssl.SSLSocketFactory
+import javax.net.ssl.X509TrustManager
 import kotlin.reflect.KProperty
 
 internal open class NetworkClientBase(private val baseUrl: HttpUrl? = null) {
@@ -75,9 +80,14 @@ internal open class NetworkClientBase(private val baseUrl: HttpUrl? = null) {
 
         val handshakeCertificates = buildHandshakeCertificates(options)
         if (handshakeCertificates != null) {
+            val sslContext = SSLContext.getInstance("TLS")
+            val trustManager = getTrustManager(handshakeCertificates.trustManager)
+            val trustManagers = arrayOf(trustManager)
+            sslContext.init(null, trustManagers, SecureRandom())
+
             builder.sslSocketFactory(
-                    handshakeCertificates.sslSocketFactory(),
-                    handshakeCertificates.trustManager
+                    sslContext.socketFactory,
+                    getTrustManager(handshakeCertificates.trustManager)
             )
         }
 
@@ -115,11 +125,24 @@ internal open class NetworkClientBase(private val baseUrl: HttpUrl? = null) {
 
         val handshakeCertificates = buildHandshakeCertificates()
         if (handshakeCertificates != null) {
+            val sslContext = SSLContext.getInstance("TLS")
+            val trustManager = getTrustManager(handshakeCertificates.trustManager)
+            val trustManagers = arrayOf(trustManager)
+            sslContext.init(null, trustManagers, SecureRandom())
+
             okHttpClient = okHttpClient.newBuilder()
                     .sslSocketFactory(
-                            handshakeCertificates.sslSocketFactory(),
-                            handshakeCertificates.trustManager
+                            sslContext.socketFactory,
+                            getTrustManager(handshakeCertificates.trustManager)
                     )
+                    .hostnameVerifier {hostname, session ->
+                        val hv = HttpsURLConnection.getDefaultHostnameVerifier()
+                        val result = hv.verify(hostname, session)
+                        if (!result) {
+                            emitInvalidCertificateError()
+                        }
+                        result
+                    }
                     .build()
         }
     }
@@ -380,6 +403,37 @@ internal open class NetworkClientBase(private val baseUrl: HttpUrl? = null) {
         return null
     }
 
+    internal fun getTrustManager(defaultTrustManager: X509TrustManager): X509TrustManager {
+         return object : X509TrustManager {
+            @Throws(CertificateException::class)
+            override fun checkClientTrusted(chain: Array<X509Certificate>, authType: String) {
+                defaultTrustManager.checkClientTrusted(chain, authType)
+            }
+
+            @Throws(CertificateException::class)
+            override fun checkServerTrusted(chain: Array<X509Certificate>, authType: String) {
+                try {
+                    defaultTrustManager.checkServerTrusted(chain, authType)
+                } catch (ce: CertificateException) {
+                    emitInvalidCertificateError()
+                    throw ce
+                }
+            }
+
+            override fun getAcceptedIssuers(): Array<X509Certificate> {
+                return defaultTrustManager.acceptedIssuers
+            }
+        }
+    }
+
+    internal fun emitInvalidCertificateError() {
+        val data = Arguments.createMap()
+        data.putString("serverUrl", BASE_URL_STRING)
+        data.putInt("errorCode", -299)
+        data.putString("errorDescription", "The certificate for this server is invalid.\nYou might be connecting to a server that is pretending to be “${URI(BASE_URL_STRING).host}” which could put your confidential information at risk.")
+        APIClientModule.sendJSEvent(APIClientEvents.CLIENT_ERROR.event, data)
+    }
+
     internal fun buildHandshakeCertificates(options: ReadableMap?): HandshakeCertificates? {
         if (options != null) {
             // `trustSelfSignedServerCertificate` can be in `options.sessionConfiguration` for
@@ -390,11 +444,15 @@ internal open class NetworkClientBase(private val baseUrl: HttpUrl? = null) {
                         sessionConfiguration.getBoolean("trustSelfSignedServerCertificate")) {
                     trustSelfSignedServerCertificate = true
                     builder.hostnameVerifier { _, _ -> true }
+                } else {
+                    buildHostnameVerifier()
                 }
             } else if (options.hasKey("trustSelfSignedServerCertificate") &&
                     options.getBoolean("trustSelfSignedServerCertificate")) {
                 trustSelfSignedServerCertificate = true
                 builder.hostnameVerifier { _, _ -> true }
+            } else {
+                buildHostnameVerifier()
             }
 
             if (options.hasKey("clientP12Configuration")) {
@@ -420,15 +478,23 @@ internal open class NetworkClientBase(private val baseUrl: HttpUrl? = null) {
         return buildHandshakeCertificates()
     }
 
+    private fun buildHostnameVerifier() {
+        builder.hostnameVerifier {hostname, session ->
+            val hv = HttpsURLConnection.getDefaultHostnameVerifier()
+            val result = hv.verify(hostname, session)
+            if (!result) {
+                emitInvalidCertificateError()
+            }
+            result
+        }
+    }
+
     private fun buildHandshakeCertificates(): HandshakeCertificates? {
         if (baseUrl == null)
             return null
 
         val (heldCertificate, intermediates) = KeyStoreHelper.getClientCertificates(P12_ALIAS)
 
-        if (!trustSelfSignedServerCertificate && heldCertificate == null)
-            return null
-
         val builder = HandshakeCertificates.Builder()
                 .addPlatformTrustedCertificates()
 
diff --git a/ios/APIClient.swift b/ios/APIClient.swift
index c78cdbd25..5234cfcec 100644
--- a/ios/APIClient.swift
+++ b/ios/APIClient.swift
@@ -13,12 +13,14 @@ import React
 
 enum APIClientError: Error {
     case ClientCertificateMissing
+    case ServerCertificateInvalid
 }
 
 extension APIClientError: LocalizedError {
     var errorCode: Int {
         switch self {
         case .ClientCertificateMissing: return -200
+        case .ServerCertificateInvalid: return -299
         }
     }
     
@@ -26,6 +28,8 @@ extension APIClientError: LocalizedError {
         switch self {
         case .ClientCertificateMissing:
             return "Failed to authenticate: missing client certificate"
+        case .ServerCertificateInvalid:
+            return "Invalid or not trusted server certificate"
         }
     }
 }
@@ -61,6 +65,17 @@ class APIClientSessionDelegate: SessionDelegate {
 
         completionHandler(disposition, credential)
     }
+    
+    override open func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?) {
+        if let err = error as? NSError,
+           let urlSession = SessionManager.default.getSession(for: session),
+           err.domain == NSURLErrorDomain && err.code == NSURLErrorServerCertificateUntrusted {
+            NotificationCenter.default.post(name: Notification.Name(API_CLIENT_EVENTS["CLIENT_ERROR"]!),
+                                            object: nil,
+                                            userInfo: ["serverUrl": urlSession.baseUrl.absoluteString, "errorCode": APIClientError.ServerCertificateInvalid.errorCode, "errorDescription": err.localizedDescription])
+            super.urlSession(session, task: task, didCompleteWithError: error)
+        }
+    }
 }
 
 @objc(APIClient)