One of the most successful U.S. National Security Agency spying programs involved intercepting IT equipment en route to customers and modifying it.

At secret workshops, backdoor surveillance tools were inserted into routers, servers and networking equipment before the equipment was repackaged and sent to customers outside the U.S.

The program, run by the NSA's Tailored Access Operations (TAO) group, was revealed by documents leaked by former NSA contractor Edward Snowden and reported by Der Spiegel and Glenn Greenwald.

It was one of many revelations about the NSA that caused widespread suspicion that U.S. technology products shouldn't be trusted, even if companies strenuously denied helping the agency.

And it appears some Cisco Systems customers have since taken steps to prevent NSA tampering.

The company has shipped equipment to addresses that are unrelated to a customer, said John Stewart, Cisco's chief security and trust officer, on Wednesday during a panel session at the Cisco Live conference in Melbourne.

In theory, that makes it harder for the NSA to target an individual company and scoop up their package. But supply chains are tough to secure, Stewart said, and once a piece of equipment is handed from Cisco to DHL or FedEx, it's gone.

Still, the risk of such tampering is pretty low for most customers. Cisco has been working on better ways for customers to verify the integrity of the systems it ships, but there will always be certain amount of risk that can't be mitigated, Stewart said.

"If a truly dedicated team is coming after you, and they're coming after you for a very long period of time, then the probability of them succeeding at least once does go up," Stewart said. "And its because they've got patience, they've got capacity and more often than not, they've got capability."

One of the leaked Snowden documents, dated June 2010, has two photos of an NSA interdiction operation, with a box that said Cisco on the side.

The document, labeled top secret, goes on to say that supply-chain interdiction operations "are some of the most productive operations in TAO, because they pre-position access points into hard target networks around the world."

In May 2014, Cisco CEO John Chambers sent a letter to President Barack Obama, arguing that the NSA's alleged actions undermine trust with its customers and more broadly hurt the U.S. technology industry. Cisco also asserted that it does not work with any government to intentionally weaken its products.

During the roundtable on Wednesday, Stewart was asked if Cisco ever identified any strange hardware put inside any of its products.

"No, we couldn't, because the only people who would know that for sure is the NSA," Stewart responded.

(Adam Bender of Computerworld Australia contributed to this report.)

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