Is your ISP inserting ads in your browser? Possibly. Some of them do.
What’s going on: some Internet Service Providers passively monitor the HTML flowing through their network to your PC. In a scheme to collect advertising revenue, they intercept the HTML and substitute their ad content. They do this via a transparent proxy server that watches for opportunities to strip out advertising content flowing from a web server and inserting advertising content that the local ISP has sold.
This means that the advertiser who originally paid for your impressions has had their advertising replaced by another.
Here is an analogy, in case you’re having difficulty following this (and I wouldn’t blame you).
A national billboard company erects a billboard along a busy street and sells the space to a national company, and proceeds to put up an ad for that national company. The national company pays the billboard company a fee for this service.
Then a local advertising company solicits advertising business, and illicitly replaces the ad on that billboard with one of its own. The national company that paid for the advertising, nor the national billboard company, are aware of this.
And why is this a security issue? Because the three principles of security are Confidentiality, Availability, and Integrity. This gimmickry of replacing ad content violates the integrity of the content that the user requested from the website.
Is your ISP doing this to you? Go to this website and find out. It is run by the University of Washington and the International Computer Science Institute, as a public service and an experiment to see how many ISPs are doing this. They are going to make their results public at the end of their experiment. If your ISP is inserting ads, please PDF your results and send them to me, so that I can show others what such a result looks like.