According to reporting from The Los Angeles Times, the company believes the production of other newspapers may also be affected but has yet to confirm details.
A source told the Times the source of the attack was a "foreign entity".
The Ryuk ransomware strain is the primary suspect in a cyberattack that caused printing and delivery disruptions for several major United States newspapers over the weekend. "We apologize to all of our readers for the inconvenience".
As a result, several Saturday newspapers will be distributed in their slimmed-down versions on Sunday.
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The Southern California editions of the Wall Street Journal and The New York Times were affected as well, according to Light. "Thank you for your patience and support as we address the infection".
A request for comment from Tribune Publishing was not immediately returned.
This type of ransomware was first described in a Check Point report published over the summer. When subscribers tried to call the paper, they were erroneously told that the numbers were out of service. The Los Angeles Times, like the Union-Tribune, used to be owned by Tribune Publishing and still uses some of its systems.
"Usually when someone tries to disrupt a significant digital resource like a newspaper, you're looking at an experienced and sophisticated hacker", said Pam Dixon, executive director of the World Privacy Forum, a nonprofit public interest research group.
"Modern malware is all about the long game", Dixon said. "When people think of malware, the impression may be, 'It's a little program that runs on my computer, '" Dixon said.