The Duchess of Cambridge's sister has taken High Court action against a "person or persons unknown".
Pippa Middleton is listed as the claimant in a case listed to be heard at the High Court in London on Wednesday.
The case is listed as "Middleton & anr v The Person or Persons Unknown".
Court officials said the claimant was Pippa Middleton.
The listing gives no indication of what the case is about.
A 35-year-old man has been released on police bail after being arrested by officers investigating claims that Miss Middleton's iCloud account had been hacked.
Police said the man had been arrested at an address in Northamptonshire late on Saturday, on suspicion of a Computer Misuse Act offence.
Officers said he had been released on bail pending further inquiries.
Pictures of the Duchess of Cambridge and her children, Prince George and Princess Charlotte, are reportedly among 3,000 images taken from Miss Middleton.
Private photographs were said to have been offered to The Sun and Daily Mail newspapers, via encrypted messaging service WhatsApp.
The Sun said it had been approached by someone using a pseudonym, and asking for £50,000 within 48 hours.
Miss Middleton has taken legal action in the Queen's Bench Division of the High Court - where judges analyse claims relating to issues including libel, personal injury, negligence and privacy.
Her case is listed for 10.30am in the Interim Applications Court - Court 37 at the Royal Courts of Justice - as an Application Notice before judge Mrs Justice Whipple.