Fujitsu - Japan's computing standard-bearer

Fujitsu is one of the oldest names in Japanese computing and, through its acquisition of ICL, the inheritor of much of Britain's. The reason it belongs in this section is a case with no attacker in it at all, which makes it a different kind of warning from every other entry here: nobody broke into Horizon. The people it harmed were harmed by trusting it.

Horizon was the accounting system Fujitsu operated for the British Post Office from 1999, at the time the largest non-military IT project in Europe. It calculated how much cash each branch should hold. Sub-postmasters were self-employed contractors, and their contract made them liable for any shortfall between the cash on the counter and the figure the system produced. Reports of unexplained shortfalls began almost immediately.

Between 2000 and 2015 about nine hundred of them were prosecuted for theft, fraud and false accounting on the strength of Horizon's figures. Some went to prison; some were bankrupted; more than two thousand people were affected, and some took their own lives. Fujitsu knew the system had errors that could produce phantom losses, and its witnesses told courts the system was robust.

The Court of Appeal, quashing thirty-nine convictions in April 2021, put the mechanism in one sentence: by presenting the system as reliable, the prosecution had effectively reversed the burden of proof. The computer's figure was presumed true, so the defendant had to prove a negative. And the record that could have settled it - a complete log of every keystroke, held by the operator - was routinely not disclosed. This catalogue argues elsewhere that the record of who did what must be kept where nobody with an interest can reach it; here the record existed, the party with the interest held it, and the people it would have cleared never saw it.

The remedy took twenty-five years and, in the end, a television drama. Computer Weekly broke the story in 2009; the public inquiry opened in 2021; and it was a four-part dramatisation in January 2024 that produced the public anger which produced emergency legislation to exonerate the convicted. At the inquiry Fujitsu's European head conceded the company had missed the opportunity to prevent the convictions, and Fujitsu apologised and accepted what it called a moral obligation toward redress. As of 2025 most of the victims had not been fully compensated.

For a networking and security catalogue the lesson is uncomfortable because it is not technical. Every safeguard described in these pages - provenance, logging, testing, review - assumes that when the system and a human disagree, somebody will look. Horizon shows what happens when an institution decides in advance that the system is right: the safeguards do not fail, they are never consulted, and a software defect becomes a criminal record.