The war crimes court, which is based in the Hague in the Netherlands, said it could not confirm who behind the attack and did not disclose whether any sensitive information had been stolen.
"The evidence available thus far indicates a targeted and sophisticated attack with the objective of espionage," the court said in a statement reviewing the incident.
"The attack can therefore be interpreted as a serious attempt to undermine the court's mandate."
The ICC holds sensitive records related to alleged war crimes investigations including documents, images and testimony from witnesses who could be endangered if their identity were revealed.
" Should evidence be found that specific data entrusted to the court has been compromised, those affected would be contacted immediately," it said.
Prosecutors at the court are currently conducting 17 investigations into alleged atrocities in Ukraine, Uganda, Venezuela, Afghanistan and the Palestinian Territories, among others.
The court said it has recently been subjected to "daily" attempts to disrupt its systems and it anticipates that the court or its officials may be targeted by disinformation campaigns attempting to tarnish its image.
In March, the ICC issued an arrest warrant for Russian President Vladimir Putin on suspicion of illegally deporting children from Ukraine. The Kremlin rejects the accusations and the court's jurisdiction.
In 2022 the Dutch secret service said it had caught a Russian spy posing as a Brazilian in an attempt to infiltrate the court as an intern. Moscow denied the allegation and accused Dutch authorities of "planting information" to whip up "Russiaphobia".
The Dutch government is conducting a criminal probe into the September incident, which disrupted the court's activities for days. The court said it is strengthening its defences.
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