Ad injection is the technique of surreptitiously inserting ads on a publisher’s website without their consent. This can be done by placing the ads over the originally shown ads, replacing the original ads altogether or by placing ads on websites that otherwise never show ads. Ad injection is typically perpetrated as a background task by a user-downloaded browser extension, plugin or app. Without the knowledge of the user, the software injects ads into the user’s browser as they visit websites.
The impacts of ad injection are multifold. They block the content on the publisher’s website impacting the user experience; they often contain malware that can compromise user security; lastly, they obscure the website’s own advertising from the user’s view.
Source: TrafficGuard Glossary