Adds comments from Coupang's conference call, background in paragraphs 6-9, 11-13; bullets

By Heekyong Yang and Bipasha Dey

Feb 26 (Reuters) - E-commerce giant Coupang CPNG.N swung to a fourth-quarter loss on Thursday and reported revenue below analysts' estimates, hurt by the fallout from a data breach in South Korea.

Coupang Korea, which generates more than 90% of the group's total revenue, has faced a public backlash following disclosure of a data breach in November involving some 34 million customers.

The company's revenue for the October-December period came in at $8.8 billion, compared with an $8.9 billion forecast from LSEG SmartEstimate, which is weighted toward analysts who are more consistently accurate.

The company swung to a loss of $26 million in the fourth quarter from a year-earlier profit. Coupang's New York-listed shares closed up 1.9%.

On an earnings call, Chief Financial Officer Gaurav Anand said active customers in its product commerce segment rose 8% from a year earlier to 24.6 million in the fourth quarter, though they fell from 24.7 million in the previous quarter, which appeared to be related to the data breach.

"We are seeing stabilization since the end of the fourth quarter, with a large number of customers reactivating their accounts and improving trends in customer growth," Anand said.

He added that constant-currency growth in its product commerce segment likely reached its lowest level in January, at about 4%, before showing signs of improvement in February. For the first quarter, Coupang expects consolidated constant-currency revenue growth of 5% to 10%.

Anand said growth and profitability would remain muted over the next few months, reflecting the lingering fallout from the data breach, though the impact should gradually diminish over the course of the year.

DATA BREACH DOWNFALL

The company has said that the breach exposed users' names, phone numbers, and shipping addresses, although payment details or login credentials were not compromised. Coupang said it would "take all necessary steps to prevent further harm and continue strengthening safeguards to prevent a recurrence."

Harold Rogers, the interim head of Coupang's South Korean business, said the company has not detected a misuse of customer data attributable to the incident and has found no evidence to date of "any secondary harm."

Rogers added that the breach stemmed from a targeted attack carried out by a former employee who leveraged inside knowledge of Coupang's systems, citing the findings of a cybersecurity firm's investigation.

However, South Korea's Science Ministry said this month the breach stemmed from management failures at Coupang rather than a sophisticated cyberattack.

Since the incident, its rivals have jumped in to lure shoppers away from the platform.

It also faces headwinds from a proposed regulatory change that could increase competition in ultra-fast overnight deliveries, which have been a cornerstone of its market dominance.

Separately, earlier on Thursday, South Korea's antitrust regulator fined Coupang 2.2 billion won ($1.53 million) for pressuring suppliers to cut prices and shoulder additional costs to meet profit margins, as well as for delaying payments to vendors. The fine is unrelated to the data breach.

($1 = 1,433.3200 won)

(Reporting by Heekyong Yang and Hyunjoo Jin in Seoul, Bipasha Dey in Bengaluru; Editing by Miyoung Kim, Maju Samuel, Rashmi Aich and Thomas Derpinghaus)

((Bipasha.Dey@thomsonreuters.com;))