It looks like someone was trying to brute force a remote file inclusion trick on your pagination. /etc/passwd is your unix password file. It’s encrypted, but the passwords can be cracked if they are common enough.
https://en.wikipedia.org/wiki/Remote_File_Inclusion
However, it wouldn’t make sense for WordPress to directly feed this variable into the command line, so I don’t think the culprits were successful. If anything, the thing you should watch out for there is SQL injection.
https://en.wikipedia.org/wiki/SQL_injection