A man has been jailed for 18 months for hacking into 900 phones belonging to the insurance company Aviva.
Richard Neale, 40, pleaded guilty to carrying out the attack as revenge after falling out with colleagues.
He was previously a director at Esselar, a company contracted by Aviva to run its security network. Prosecutors said data had been wiped on hundreds of devices.
Esselar missed out on £500,000 of future business deals as a result.
Neale admitted to hacking into the Aviva system in May 2014, on the night that Esselar was giving a security demonstration.
Prosecutors said that this had led Aviva to end its relationship with Esselar, meaning that the latter had missed out on an £80,000-per-year contract.
Neale had also created a fake identity within his former company's system and used it to reject expenses claims from his erstwhile colleagues, the court heard.
In a separate offence, he hacked into Esselar's Twitter account and replaced its logo with a bleeding heart - a calling card meant to make it clear that its security had been compromised.
At a sentencing hearing at Guildford Crown Court on Monday, judge Neil Stewart said that Neale's actions had "damaged confidence and reputations in a way that can be far-reaching and serious".
According to a report of the case in the Daily Mail, the prosecutor, Fiona Alexander, said: "The aim of the attack was to ridicule Esselar. There was a degree of sophisticated planning.
"The offending persisted over a period of five months. The defendant was motivated by revenge - a serious aggravating feature. There was a grave breach of trust.
"It wasn't intended to target just Esselar but also... Aviva. Over 900 devices were wiped by the defendant's actions."
She told the court Esselar's "tangible" losses amounted to £528,000. But the company said the full extent of its losses was "simply incalculable".
"Yes, we survived, but there were times we thought we may not. Our brand was damaged to the point we felt we needed to rebrand," it said, in a statement read to the court by Ms Alexander.
Neale, who set up Esselar with Shane Taylor and Simon Rogan in 2009, sold his shares in the company after a falling out over an insurance payment in 2013. He had subsequently harboured resentment, according to his lawyer, Kevin Barry.
In mitigation, Mr Barry had told the court that the relationship between Esselar and Aviva had been "on a knife edge" before Neale's actions, the Mail reported.
'Foolish and childish'
According to the newspaper, he had said that Neale's resentment "festered" and that his actions had been "foolish and childish". He added that "no data was actually lost or permanently compromised".
Judge Stewart told Neale: "You parted on terms and in circumstances that left you nursing resentment.
"The prosecution describe these offences as revenge; you use the expression causing mischief. What form of words you use is beside the point: it was plainly borne of your resentment."
Neale had pleaded guilty to four counts of unauthorised or reckless acts with intent to impair computer operation, under the Computer Misuse Act 1990, at an earlier hearing.