Indonesian hackers have claimed to have attacked several Malaysian websites, following a blunder which saw the Indonesian flag printed upside down in a regional sports event's guidebook.
The mistake prompted fury in Indonesia over the weekend and President Joko Widodo demanded an official apology.
The book was printed for the Southeast Asian Games, which are being held in Kuala Lumpur.
Malaysian organisers and officials have apologised profusely.
The affected websites display a message in red and white - Indonesia's flag colours - which says: "My national flag is not a toy!"
At least 27 websites were affected, and were mostly for private businesses, according to Indonesian and Malaysian news outlets.
A BBC check on Monday found that several of the sites have since been taken offline.
Pictures of the upside down Indonesian flag began circulating online shortly after the guidebook was handed out at the opening of the games in Kuala Lumpur on Saturday.
Indonesia's Youth and Sports Minister Imam Nahrawi tweeted several images, saying it was a "fatal negligence that was very painful".
Mr Widodo said Indonesia wanted an official apology "as it concerns the pride and nationalistic spirit of our Indonesian people", according to reports.
Indonesian netizens also complained online, using the hashtag #ShameOnYouMalaysia, and several noted that the upside down flag resembled Poland's.
Malaysian minister Khairy Jamaluddin later apologised over Twitter and also met Mr Nahrawi in person.
"Just met @imam_nahrawi to officially apologise to the people of Indonesia for the mistake of printing the flag of RI," he said, using the abbreviation for the Republic of Indonesia.
But it was not the only mistake to plague the event.
A Malaysian newspaper also printed the Indonesian flag upside down in a graphic, while the wrong flag was used for Brunei at a swimming event on Friday.