Link spam is the category in Google's spam policies that covers links made mainly to manipulate rankings. It includes link schemes like buying and trading links, and also spam placed on other people's sites, such as comment spam and hacked pages stuffed with links. Google fights it with automated systems and manual actions, and it has released several updates aimed at it.
Link spam you create versus link spam aimed at you
Link spam in your profile usually comes from one of three places: links you or your agency built through schemes, links spammers placed on your site (in comments, forums or through a hack), or junk links that scrapers and spam sites point at you without your involvement.
Only the first two are your responsibility. The third is normal background noise, and Google has said it generally ignores it.
How Google handles it
Google's SpamBrain system, which Google describes as its AI-based spam prevention system, is used to detect both sites buying links and sites used to pass outgoing links. Google has said that link spam updates aim to neutralize spammy links, which means any benefit from them is lost. Manual actions for unnatural links are a separate, human-reviewed route.
Penguin, released in 2012, was the first major algorithm focused on link spam; details belong in its own entry.
Protecting your own site
Moderate user-generated content, apply ugc or nofollow to user links, keep software patched to prevent link injection, and avoid selling links on your own properties. Watch the Security issues and Manual actions reports in Search Console.
Example
A hacked blog
An illustrative IoT platform, Sensorhaven, runs its blog on an old CMS install. An outdated plugin lets attackers inject hidden links to gambling sites into post templates. The team spots it when Search Console shows unexpected pages and queries. They patch the CMS, remove the injected links and request review if a manual action has been applied.
Common mistakes
Treating all low-quality links as spam you must fix
Junk links you did not create are usually ignored by Google.
Overlooking your own outbound links
Link spam includes links leaving your site. Hacks and sold footer links can cause manual actions too.
Questions
It is an algorithm update Google announces to improve how its systems detect and neutralize link spam. Its effect is usually to remove credit from those links.
