Established Fact
Election Systems & Software (ESS) admitted in a 2018 letter to Sen. Ron Wyden that it had installed pcAnywhere remote access software on election management systems sold to customers between 2000 and 2006, directly contradicting its prior public representations that it had never done so. ESS refused to disclose which states and counties received affected systems.
Citations
ES&S Letter to Sen. Wyden: https://www.documentcloud.org/documents/4608550-elections-systems-amp-software-letter-received-4 | DocumentCloud
Voting machine vendor says it installed remote software connections on some systems: https://cyberscoop.com/es-s-voting-machine-remote-access-ron-wyden/ | CyberScoop
A Voting-Machine Manufacturer Installed Remote-Access Software: https://psmag.com/news/a-voting-machine-manufacturer-installed-remote-access-software/ | Pacific Standard