Cyber Attack Ensnares $4.3 Trillion Muni Market’s Key Site (2)

A ransomware attack is disrupting state and local borrowers’ ability to post debt documents on the $4.3 trillion municipal-bond market’s main distribution platform.

MuniOS, a website operated by Ann Arbor, Michigan-based tech company ImageMaster LLC, has been out of service for several days due to the cyber attack, according to people familiar with the matter who asked not to be named discussing the private matter.

Borrowers use the website to showcase their bond offering documents, and it’s a popular service used by investors and analysts for information about transactions before they are sold. While market participants said they hadn’t seen ...