Php.Trojan.Spambot-1

We have recieved by our hosting company a warning about a threat found in our piwik installation.

FILE HIT LIST:
{CAV}Php.Trojan.Spambot-1 : /var/www/vhosts/xxx/httpdocs/piwik/tmp/templates_c/f1/defines.php

{CAV}Php.Trojan.Spambot-1 : /var/www/vhosts/xxx/httpdocs/piwik/libs/tcpdf/include/test.php

Hm. Maybe your server has been hacked?
Which version of Piwik are you using?
Both files should not exists is current version!