Zero-Day Link

Zero-Day Link (ゼロデイ・リンク) is an online risk management system, comprised of four 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 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.

Every day, the server system gathers information over 8000 new virus varieties, hacking methods, electronic vulnerabilities and other online risks,  including the large number of error reports which corporations and universities specializing in security receive from users. The four servers work in tandem to analyze the massive amount of information, exchanging data and methods, and tag the threats according to their risk level. The highest priority threats are sent to the storage center for human-made countermeasures to be constructed,   while the machines produce patches for the lower priority items. The system is complex enough that even the programmer who designed it wouldn't be able to determine how an answer provided by the system was reached.

The four different 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. The corporate server is located in School District 15.

Although Zero-Day Link means that most methods cannot be used a second time once registered, the mechanical approach has several downsides. According to Uiharu Kazari, the standard for tagging is not particularly great and many digital criminals can slip through the gaps which are left. If the methods are not tagged as high priority, they won't be sent to humans for countermeasures to be manually developed. Due to the complexity of the system's internal processes, which produce over 80% of the patches, although the system is inaccessible to normal users, if it were compromised on-site and a poison manually introduced, the trouble might go unnoticed for a considerable period of time.

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 notice.

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.

Although Sakibasu Ryouu reached the server room, the plot was ultimately thwarted due to the efforts of Misaka Mikoto, Shokuhou Misaki, Shirai Kuroko, Hokaze Junko and Sakibasu Yuri.