The Consequences of Subdomain Spam on a Primary Website
Many websites today use subdomains as part of their website (and SEO strategy) but could a manual action from Google on a particular subdomain negatively impact the good standing of the entire website?
That was the question from danielfoley22 in this Google product forum help thread. The issue was that his company used a subdomain to host a development website (a very common practice) but it was eventually hit with spam due to a CMS exploit which went unnoticed for an undetermined amount of time. The company wisely removed the sub-domain altogether but the issue was that the Google warning that resulted from the spam/exploit came to the main domain. The worry was that the primary domain itself would suffer a ranking loss.
So, would an affected subdomain, one that received a manual action from Google, impact rankings on a top-level domain?
Google's John Mueller responded, indicating that "If the manual action was specific to the subdomain, then that wouldn't - from a manual actions point of view - have an effect on the rest of your site. If you've taken care to remove all of that, then after a reconsideration request things should be fine."
This does call into question Google's long-held assertion that subdomains and subfolders are ultimately treated the same. The lesson here might just be that if you're going to run development environments (or test some aggressive SEO tactics) the best location for that is likely on a subdomain, not a sub folder.
Many sites use subdomains today to give users their own customized URL.
Ultimately, this should signal that even if there are some bad actors, it should not affect the good standing of the primary or top-level domain.