Ad injection

When you visit a site, you expect its own ads. With ad injection, software on your device or network adds extra ads, or swaps the site's ads for different ones. The publisher earns nothing from those ads, the user gets a cluttered or unsafe page, and the advertiser may end up next to content that never agreed to host it.

The usual culprits are malicious or shady browser extensions, bundled adware installed alongside free software, and occasionally compromised Wi-Fi or ISPs that tamper with pages. Injected ads are often low quality and can lead to malvertising.

From a measurement standpoint, injected impressions are SIVT: the MRC IVT guidelines list adware, malware and hijacked sessions or ad tags as sophisticated invalid traffic. Detection involves checking whether an ad's rendering context matches the declared page, spotting ad containers the publisher did not define, and identifying known injector extensions. Browser stores have tightened extension policies, but injection still appears, particularly through adware on desktops.

Think of it like this

Ad injection is like someone sneaking their own flyers inside every copy of a magazine at the newsstand, pocketing the advertising money while the magazine gets nothing.

An example

A publisher in Poland gets reader complaints about pop-up casino ads it never sold. Testing shows that a popular coupon extension rewrites its pages to add three extra ad slots.