Zero-Day Link

Zero-Day Link is an online risk management system comprised of four AI servers in Academy City, which records, sorts and processes information on the massive volume of online risks, viruses and other electronic vulnerabilities discovered daily.

Principles
Zero-Day Link is an AI server system which gathers information on the many viruses and electronic vulnerabilities discovered daily, tags them based on the risk they pose and sends prioritized data to the Virus Isolation Center for the development of countermeasures.

The system is in place because the increasing volume of online and electronic risks far exceeds the amount which humans can handle on their own. An average of 8000 malicious scripts are discovered daily, with their quality gradually rising. With programming being taught at the elementary school level, practically anyone can make a virus. As such, the engineers of the various online security groups have given up on manually identifying and developing countermeasures for every single virus and hacking method, turning to a largely automated process. Although the antivirus software in Academy City is supposedly all manually created in the storage center, the facility by itself would be insufficient and in truth, over 80% of the vulnerability patches are done entirely by machines, with humans handling the irregular parts left over.

The four different AI servers which make up Zero-Day Link are based in a university, company, organization and the government respectively. Given the nature of the system, it is closed to normal users with access only possible through the management of the individual servers.

Toaru Kagaku no Railgun SS3
During Misaka Mikoto's pursuit of Riot, Uiharu Kazari mentioned Zero-Day Link and reminded her that she couldn't use the same method of breaking through electronic doors twice because of it, though Mikoto used different approaches with her power each time and didn't even let them send an error report.

When Inoue Kasha attempted to kill Riot and Maeda, he planned for the ambulance they were in to collide with a driverless vehicle in a tunnel, planning the crash such that it would slip past Zero-Day Link's attention.

Zero-Day Link was also the focus of Blue Blood's plot to spread an Academy City-level computer virus across the rest of the world, by manually slipping an untraceable digital poison into the system which would enable them to do so, with the corporate server being the likeliest target as it would be easiest for the wealthy group to infiltrate.