The hack, which occurred between mid-May and late July and involved the theft of data such as birth dates, Social Security and credit card numbers, has led to multiple state and federal investigations, a criminal probe by the U.S. Justice Department and scores of class action law suits.
The company said it was not possible to estimate the cost of litigation and government investigations.
They have dropped around 25 percent since the company's September 7 disclosure of the breach that exposed sensitive data on 145.5 million consumers.
The company warned there could be further attacks. "I think they can get there, but they aren't there now".
Equifax said it would hold a conference call to discuss the results on Friday morning, giving Wall Street a chance to publicly grill the company for the first time since the breach was disclosed on September 7.
Johnson estimated the costs from the breach will total in the hundreds of millions of dollars, but that lost revenue could be even larger.More news: Pixel 2 problems surface again, this time with Google Assistant
Investors are looking for clues to help assess whether the breach will have a long-term impact on the company's sales and profit, Stephens Inc analyst Brett Huff said.
The company is now facing more than 240 lawsuits related to the breach and investigations launched by state and federal authorities, including 50 state attorneys general, the Federal Trade Commission, the Consumer Financial Protection Bureau and the Securities and Exchange Commission.
Equifax's net income of $96.3 million was down 27% from $132.8 million a year ago and more than 71% from $165.4 million in the second quarter.
The company said in a third-quarter report released a day earlier that the breach was responsible for about $87.5 million in related expenses that period, including investigative and legal fees as well as expenses related to offering free protection to affected customers.
Shares of Equifax, which reported after the closing bell, were down a penny in aftermarket trading at $108.94. The company expects revenue of $825 million to $835 million, compared to the average analyst forecast of $833.65 million.
The company may also give an update on its efforts to find a new chief executive officer, as well as new chief security and chief information officers, to replace executives that left after the breach was made public.