Busca en el código fuente si hay alguna webshell, aunque suelen estar bastante camufladas.
Tipo:
https://github.com/pentestmonkey/php-reverse-shell/blob/master/php-reverse-shell.phpo
Msfvenom PHP Shell
O busca por funciones PHP potencialmente peligrosas tipo:
system
passthru
shell_exec
exec
proc_open
popen
fsockopen
socket_create
imap_open
mail
putenv
Aunque hay muchas más:
allow_url_fopen
allow_url_include
apache_child_terminate
apache_get_modules
apache_note
apache_setenv
curl_exec
curl_multi_exec
define_syslog_variables
diskfreespace
disk_free_space
disk_total_space
dl
escapeshellarg
escapeshellcmd
exec
passthru
shell_exec
system
popen
curl_exec
curl_multi_exec
pcntl_exec
pcntl_exec
putenv
proc_close
proc_get_status
proc_nice
proc_terminate
popen
pclose
ini_alter
virtual
openlog
escapeshellcmd
escapeshellarg
parse_ini_file
show_source
imap_open
ftp_connect
posix_uname
posix_getuid
posix_getgid
apache_setenv
define_syslog_variables
eval
ftp_connect
ftp_exec
ftp_get
ftp_login
ftp_nb_fput
ftp_put
ftp_raw
ftp_rawlist
highlight_file
ini_alter
ini_restore
inject_code
openlog
phpAds_remoteInfo
phpAds_XmlRpc
phpAds_xmlrpcDecode
phpAds_xmlrpcEncode
popen
posix_getpwuid
posix_kill
posix_mkfifo
posix_setpgid
posix_setsid
posix_setuid
posix_setuid
posix_uname
proc_close
proc_get_status
proc_nice
proc_open
proc_terminate
syslog
xmlrpc_entity_decode
pipe
parse_ini_file
show_source
dl
ini_alter
virtual
openlog
apc_add
apc_bin_dump
apc_bin_dumpfile
apc_bin_loadfile
apc_cache_info
apc_cas
apc_clear_cache
apc_compile_file
apc_dec
apc_define_constants
apc_delete_file
apc_delete
apc_exists
apc_fetch
apc_inc
apc_load_constants
apc_store
symlink
eval
extract
fsockopen
getcwd
getenv
getlastmo
getmygid
getmyinode
getmypid
getmyuid
get_cfg_var
get_current_user
ini_alter
ini_restore
dl
exec
shell
proc_close
ini_restore
ini_set
mail
parse_ini_file
pcntl_alarm
pcntl_exec
pcntl_fork
pcntl_getpriority
pcntl_get_last_error
pcntl_setpriority
pcntl_signal
pcntl_signal_dispatch
pcntl_sigprocmask
pcntl_sigtimedwait
pcntl_sigwaitinfo
pcntl_strerrorp
pcntl_wait
pcntl_waitpid
pcntl_wexitstatus
pcntl_wifexited
pcntl_wifsignaled
pcntl_wifstopped
pcntl_wstopsig
pcntl_wtermsig
phpinfo
php_uname
popen
posixc
posix_getlogin
posix_getpwuid
posix_kill
posix_mkfifo
posix_setpgid
posix_setsid
posix_setuid
posix_ttyname
posix_uname
proc_close
proc_get_status
proc_nice
proc_terminate
ps_aux
readlink
runkit_function_rename
show_source
socket_create
stream_select
symlink
syslog