The instructions for installing LinkMachine require setting the chmod permissions on my site's

Setting the directory's permissions to 777 allows the LinkMachine script to write to files inside that directory. This doesn't allow anyone to upload files to your site, or change files on your site, without their having your FTP login information.

Permissions set to 777 is a security risk when there is a script on the site that can be used to upload a file to the site. Hackers could take advantage of this to upload a malicious script to the site. However, LinkMachine does not allow visitors to upload any files to the site, so it doesn't provide any opportunity for a hacker to place a malicious script on your site.