fix: surface readable error when server app exception cannot be deserialized

lib/src/main/java/com/nextcloud/android/sso/api/AidlNetworkRequest.java

@@ -25,6 +25,7 @@
 
 import androidx.annotation.NonNull;
 import androidx.annotation.Nullable;
+import androidx.annotation.VisibleForTesting;
 
 import com.google.gson.Gson;
 import com.google.gson.internal.LinkedTreeMap;
@@ -229,10 +230,23 @@ private static <T> T deserializeObject(InputStream is) throws IOException, Class
         return (T) new ObjectInputStream(is).readObject();
     }
 
-    private ExceptionResponse deserializeObjectV2(InputStream is) throws IOException, ClassNotFoundException {
+    @VisibleForTesting
+    static ExceptionResponse deserializeObjectV2(InputStream is) throws IOException, ClassNotFoundException {
         final ObjectInputStream ois = new ObjectInputStream(is);
         final ArrayList<PlainHeader> headerList = new ArrayList<>();
-        final Exception exception = (Exception) ois.readObject();
+        Exception exception;
+        try {
+            exception = (Exception) ois.readObject();
+        } catch (ClassNotFoundException e) {
+            // The server app serializes the original exception object. If its class - or any
+            // class in its cause chain - only exists in the server app (e.g.
+            // com.owncloud.android.lib.common.network.CertificateCombinedException on SSL errors),
+            // deserialization fails here. Surface a plain exception carrying the original type
+            // name instead of a ClassNotFoundException that hides the actual error.
+            exception = new Exception("The Nextcloud Files app responded with an error that could not"
+                    + " be read by this app: " + e.getMessage()
+                    + ". Check the server connection (e.g. SSL certificate) or the Files app log for the actual error.");
+        }
 
         if (exception == null) {
             final String headers = (String) ois.readObject();
@@ -275,7 +289,7 @@ private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundE
         }
     }
 
-    private record ExceptionResponse(
+    record ExceptionResponse(
         @NonNull ArrayList<PlainHeader> headers,
         @Nullable Exception exception
     ) {

lib/src/test/java/com/nextcloud/android/sso/api/AidlNetworkRequestTest.kt

@@ -0,0 +1,103 @@
+/*
+ * Nextcloud Android SingleSignOn Library
+ *
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+package com.nextcloud.android.sso.api
+
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertNotNull
+import org.junit.Assert.assertNull
+import org.junit.Assert.assertTrue
+import org.junit.Test
+import java.io.ByteArrayInputStream
+import java.io.ByteArrayOutputStream
+import java.io.ObjectOutputStream
+import java.nio.charset.StandardCharsets
+
+class AidlNetworkRequestTest {
+
+    /**
+     * Exception class only used to be renamed in the serialized stream, simulating an exception
+     * type that exists in the Files app but not in the client app (e.g.
+     * `com.owncloud.android.lib.common.network.CertificateCombinedException`).
+     */
+    class OnlyInFilesAppException(message: String) : Exception(message)
+
+    @Test
+    fun deserializeObjectV2ExceptionClassMissingInClientAppReturnsReadableException() {
+        val data = serialize(OnlyInFilesAppException("certificate expired"), "[]")
+
+        // Rename the class inside the serialized stream to one that does not exist on this
+        // side of the IPC channel (same length, so the stream structure stays intact)
+        val tampered = replaceBytes(data, "OnlyInFilesAppException", "OnlyInF1lesAppException")
+
+        val response = AidlNetworkRequest.deserializeObjectV2(ByteArrayInputStream(tampered))
+
+        val exception = response.exception()
+        assertNotNull(exception)
+        assertFalse(exception is ClassNotFoundException)
+        assertNotNull(exception!!.message)
+        assertTrue(
+            "message should contain the original exception type",
+            exception.message!!.contains("OnlyInF1lesAppException")
+        )
+    }
+
+    @Test
+    fun deserializeObjectV2NoExceptionParsesHeaders() {
+        val data = serialize(null, "[{\"name\":\"Content-Type\",\"value\":\"application/json\"}]")
+
+        val response = AidlNetworkRequest.deserializeObjectV2(ByteArrayInputStream(data))
+
+        assertNull(response.exception())
+        assertEquals(1, response.headers().size)
+        assertEquals("Content-Type", response.headers()[0].name)
+        assertEquals("application/json", response.headers()[0].value)
+    }
+
+    @Test
+    fun deserializeObjectV2KnownExceptionReturnsItUnchanged() {
+        val data = serialize(IllegalStateException("CE_1"), "[]")
+
+        val response = AidlNetworkRequest.deserializeObjectV2(ByteArrayInputStream(data))
+
+        assertTrue(response.exception() is IllegalStateException)
+        assertEquals("CE_1", response.exception()?.message)
+    }
+
+    /** Mirrors `InputStreamBinder#serializeObjectToInputStreamV2` in the Files app. */
+    private fun serialize(exception: Exception?, headers: String): ByteArray {
+        val baos = ByteArrayOutputStream()
+        ObjectOutputStream(baos).use { oos ->
+            oos.writeObject(exception)
+            oos.writeObject(headers)
+        }
+        return baos.toByteArray()
+    }
+
+    private fun replaceBytes(data: ByteArray, search: String, replace: String): ByteArray {
+        val searchBytes = search.toByteArray(StandardCharsets.US_ASCII)
+        val replaceBytes = replace.toByteArray(StandardCharsets.US_ASCII)
+        assertEquals(searchBytes.size, replaceBytes.size)
+
+        val result = data.clone()
+        var i = 0
+        while (i <= result.size - searchBytes.size) {
+            var match = true
+            for (j in searchBytes.indices) {
+                if (result[i + j] != searchBytes[j]) {
+                    match = false
+                    break
+                }
+            }
+            if (match) {
+                System.arraycopy(replaceBytes, 0, result, i, replaceBytes.size)
+            }
+            i++
+        }
+        return result
+    }
+}