The company said in a statement that the breach, discovered late last week, exploited an external service provider's account.
It has notified Poland's data protection authority and specialist law enforcement agencies.
Żabka added that the unauthorised access was detected and blocked in line with security procedures, and insisted that customer transaction data and its "Żappka" loyalty app remained unaffected.
Deputy Prime Minister and Digital Affairs Minister Krzysztof Gawkowski confirmed the incident did not involve Żappka app customer data, transaction records or store operations.
"That's important information for millions of customers," he wrote on X.
He said Żabka had moved quickly to block access and had begun working with Poland's cyber-response agency (CERT) and cybercrime bureau (CBZC).
However, cybersecurity news outlet Niebezpiecznik reported on Monday evening that personal data belonging to Żabka employees and partners with access to company systems had leaked, and that some customer information may also have been exposed.
According to the report, the attackers put the stolen data up for sale at a notably low price – EUR 5,000 – and emailed details of the breach to journalists and companies linked to Żabka.
A sample of the leaked records released by the hackers suggested they had accessed at least the firm's Jira system, used for project management and tracking technical issues.
Separately, Żabka stores suffered a technical fault on Sunday that caused incorrect discounts to be applied at checkouts; the company said the issue was fixed in around 90 percent of outlets by the afternoon.
The Polish chain has not said whether that malfunction was linked to the cyberattack.
The breach comes as the company is at the centre of a possible ownership change.
Canadian retailer Alimentation Couche-Tard announced on Friday plans to launch a tender offer for 100 percent of Żabka Group at PLN 32 (EUR 7.5) per share.
Couche-Tard chief executive Alex Miller told Bloomberg he expects pressure from Żabka shareholders to raise the , price, but said he was confident the deal would go through.
The chain's share price had fluctuated over the previous fortnight mid speculation about a possible sale, with Japanese retail group Seven & i Holdings also previously reported to have shown interest.
(ał)
Source: PAP, tvp.info