Key Muni Market Website MuniOS Is Back Online After Outage (1)

A key website for the $4.3 trillion municipal-bond market is back online on Thursday morning after an outage that lasted several days.

MuniOS, a website operated by Ann Arbor, Michigan-based tech company ImageMaster LLC, was out of service due to a cyber attack.

One of the company’s firewalls had a vulnerability that was exploited by “bad actors,” Albert Rodriguez, manager at ImageMaster, said in an email.

“They successfully executed an attack on our server computer infrastructure, essentially encrypting all the drives,” Rodriguez said. “This caused all servers to ultimately fail. They did not get any data. We had to ...