A cyber apocalypse that would shutter global stock markets is more than an idle nightmare. Almost 90 percent of the world’s financial exchanges believe that cyber-crime poses a systemic risk to the securities industry.

A report from two global exchange organizations states that a worst-case scenario could be pulled off, an “advanced persistent threat,” that would be so gradual in its execution that it would go unnoticed until havoc struck.

The cost of cyber-crime to society is already estimated as high as $1 trillion, and over half of worldwide financial exchanges have been hit by such attacks, according to the report from International Organization of Securities Exchanges and the World Federation of Exchanges.

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Quartz reported the world’s financial exchanges are not worried so much about typical malicious code or denial of service attacks, which can be overcome.

“Instead, what keeps them up at night is a drawn-out attack that slowly corrupts their systems from the inside, and could be absolutely devastating if not caught in time,” Quartz said.

The advanced persistent threats envisioned by the exchanges “involve stealth to persistently infiltrate a system over a long period of time,” the report stated.

In such a slowly executed cyber-attack, criminals could shut down markets for extended periods, stop clearing houses from processing trades, and block investors’ access to markets, a likely confidence-buster for investors, Quartz reported.

Quartz said one exchange representative interviewed for the report explained:

“A systemic risk scenario might involve infiltration of several exchanges, probably most easily by e-mail phishing campaigns involving stealth malware, access built up and maintained over a length of time, potentially involving contractors or malicious inside employee assistance, and over time enough reconnaissance done to identify key internal systems attackable from the infiltration point, and a coordinated attack from that internal toe-hold against multiple institutions.”

The Financial Times reported the exchange study highlighted a “shift away from financial gains toward more destabilizing aims” by hackers.

The Times said the report came as the separation of electronic trading from the Internet is becoming blurry, and noted a broad market sell-off occurred in April after a hacked Twitter account at the Associated Press falsely reported an attack on the White House.

The study revealed widespread concern by exchanges that current legislation on cyber-attacks may not be effective, the Times said.

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