Efectivamente, como ha mencionado @MinusFour, ese archivo ejecuta directamente el php, es vulnerable.
Lo que si he podido aislar es el ataque..
176.31.251.103 - - [06/Jul/2014:21:23:24 -0300] "GET /wp-login.php HTTP/1.1" 200 2827 "-" "Opera/9.80 (Windows NT 6.0); U) Presto/2.10.289 Version/12.02"
176.31.251.103 - - [06/Jul/2014:21:23:25 -0300] "POST /wp-login.php HTTP/1.1" 302 - "-" "Opera/9.80 (Windows NT 6.0); U) Presto/2.10.289 Version/12.02"
176.31.251.103 - - [06/Jul/2014:21:23:26 -0300] "GET /wp-admin/ HTTP/1.1" 200 69338 "-" "Opera/9.80 (Windows NT 6.0); U) Presto/2.10.289 Version/12.02"
176.31.251.103 - - [06/Jul/2014:21:23:29 -0300] "GET /wp-admin/ HTTP/1.1" 200 69338 "-" "Opera/9.80 (Windows NT 6.0); U) Presto/2.10.289 Version/12.02"
176.31.251.103 - - [06/Jul/2014:21:23:31 -0300] "GET /wp-admin/theme-editor.php HTTP/1.1" 200 33381 "-" "Opera/9.80 (Windows NT 6.0); U) Presto/2.10.289 Version/12.02"
176.31.251.103 - - [06/Jul/2014:21:23:32 -0300] "GET /wp-admin/theme-editor.php?file=404.php&theme=wilborada HTTP/1.1" 200 18746 "-" "Opera/9.80 (Windows NT 6.0); U) Presto/2.10.289 Version/12.02"
176.31.251.103 - - [06/Jul/2014:21:23:33 -0300] "POST /wp-admin/theme-editor.php HTTP/1.1" 302 - "-" "Opera/9.80 (Windows NT 6.0); U) Presto/2.10.289 Version/12.02"
176.31.251.103 - - [06/Jul/2014:21:23:34 -0300] "GET /theme-editor.php?file=404.php&theme=wilborada&scrollto=0&updated=true HTTP/1.1" 404 151 "-" "Opera/9.80 (Windows NT 6.0); U) Presto/2.10.289 Version/12.02"
176.31.251.103 - - [06/Jul/2014:21:23:35 -0300] "POST /wp-content/themes/wilborada/404.php HTTP/1.1" 200 47 "-" "Opera/9.80 (Windows NT 6.0); U) Presto/2.10.289 Version/12.02"
176.31.251.103 - - [06/Jul/2014:21:23:36 -0300] "GET /wp-content/themes/wilborada/systemcash.php HTTP/1.1" 200 120 "-" "Opera/9.80 (Windows NT 6.0); U) Presto/2.10.289 Version/12.02"
176.31.251.103 - - [06/Jul/2014:21:23:38 -0300] "POST /wp-admin/theme-editor.php HTTP/1.1" 302 - "-" "Opera/9.80 (Windows NT 6.0); U) Presto/2.10.289 Version/12.02"
176.31.251.103 - - [06/Jul/2014:21:23:39 -0300] "GET /theme-editor.php?file=404.php&theme=wilborada&scrollto=0&updated=true HTTP/1.1" 404 3 "-" "Opera/9.80 (Windows NT 6.0); U) Presto/2.10.289 Version/12.02"
El atacante obtuvo aceso logueandose, modifico el 404.php del tema, lo abrio y lo volvio a modificar. Tambien se ve el primer acceso a systemcash.php.. despues, hay entradas de bots con UA (Mozilla 3.0) que acceden a el muchisimas veces.
Todos los archivos sospechosos están borrados sin embargo (y no he sido yo) xD
Estoy mirando el archivo ese de joomla.
Saludos