Not all advertisers have the size and financial wherewithal to do the right thing in the era of fake news.

Advertising on the internet has never been easier. Data and automation increasingly allow companies large and small to reach millions of people every month, and to tailor ads to specific groups based on their browsing habits or demographics.

Now, however, the marketing industry is facing a moral quandary in the face of a national debate over the role that fake news played in the presidential election and the realization that many websites that promote false and misleading stories are motivated by the money they can make from online advertising.

In the zeal to follow consumers wherever they may roam on the internet, advertisers now risk bankrolling sites that are toxic to society, whether by amplifying manufactured political stories or by spreading conspiracy theories virulent enough to drive a man to walk into a Washington pizzeria with a gun. That has inserted a new ethical cost into the automated advertising equation, which promises companies large, desired audiences at low prices with little need for human intervention.

“I would much rather pay a little premium as a brand and go for verified sites,” Raja Rajamannar, the chief marketing officer of MasterCard, said, noting that the company mostly advertised on sites it had evaluated and approved. “But it’s a question again of how much and where. And I think all brands are doing this soul-searching at this point in time.”

The problem is, most advertisers do not have the size or the financial wherewithal of MasterCard to simply opt out of the automated system as it now exists. And even if they do, there are tantalizing financial incentives to stay put.

The online digital ad market is dysfunctional.

Teenagers overseas and entrepreneurs in the United States discovered this year that they could earn thousands of dollars a month by writing wholly fictionalized or wildly exaggerated partisan political news intended to be spread on Facebook. They then reaped money from Google Ads and other networks after credulous readers in the United States clicked through to their sites.

“A lot of ad buying systems are trying to show the right ad to the right person at the right time, and you see that mantra of those three variables across the industry,” said Michael Tiffany, the chief executive and a founder of White Ops, an ad fraud detection company. “Note how ‘on the right site’ doesn’t make the list.”

Workable, a software start-up based in Boston, recently joined companies like Kellogg and Allstate in pulling ads from the hard-right website Breitbart and blaming automated ad technology for their appearance.

It is also watching out for its ads on fake news sites, which can slip through the cracks. Workable has about 110 employees and 6,000 customers but advertises on more than 50,000 sites a month through Google.

“Typically, Google has done a good job of keeping us off of low-quality sites,” said John Short, vice president for demand generation at Workable.

He added, “With fake news sites, or whatever you would like to call them, we need to be more alert.”

Facebook and Google have both said they plan to target misinformation. Facebook announced this month that it was experimenting with allowing users to report false stories and forging partnerships with outside fact-checking organizations.

This month, Robert Thomson, the chief executive of News Corporation, which owns The Wall Street Journal, described the digital ad market as “dysfunctional.”