AT&T has filed a lawsuit against three former employees and a company that sells phone unlocking codes, claiming that they installed malware on AT&T's computer systems in order to illegally unlock hundreds of thousands of mobile phones.

Unlocking a phone allows it to be used with any carrier that has a compatible network. It is legal for consumers to unlock their own phones or to pay someone to unlock phones for them. But carriers are not obligated to unlock them except in certain circumstances, such as when customers have paid off their contracts or device financing plans.

AT&T, whose strict unlocking policies have drawn the ire of consumers, filed its complaint last week in US District Court in Seattle, Washington. AT&T claimed that phone locking software "is vital to AT&T's business because it allows AT&T to subsidize the cost of the phone to consumers while protecting AT&T’s investment in the phones through term contracts."

The carrier says a company called Swift Unlocks collaborated with AT&T employees Marc Sapatin, Nguyen Lam, and Kyra Evans while they worked in an AT&T call center in Washington in 2013. Prashant Vira, who operates Swift Unlocks, "paid Evans at least $20,000 for her placement and/or execution of the malware programs on AT&T’s protected computer systems for the purpose of securing the fraudulent unlocks," AT&T alleges. Swift Unlocks paid Sapatin at least $10,500, AT&T further alleged.

No payments to Lam were alleged, but AT&T says he also installed malware on AT&T computers. Lam was fired by AT&T, while Sapatin and Evans both left the company.

The "defendants perpetuated the Unlock Scheme by creating, distributing, and placing on AT&T’s computer systems a 'malware' program designed to fraudulently, and without authorization, transmit unlock requests that unlocked hundreds of thousands of phones from exclusive use on AT&T’s network," AT&T claimed.

The malware allowed commands to be issued from a remote, unauthorized server and used "valid customer service personnel identification numbers" to process automated unlock requests without proper authorization, AT&T wrote.

AT&T says it was able to trace the unlocking requests to specific AT&T employees, triggering its investigation into Sapatin, Lam, and Evans. AT&T also says there are 50 "John Doe Defendants" who developed the software used to perpetrate the alleged unlocking operation. Their names are unknown.

Early versions of the malware "gathered confidential and proprietary information regarding AT&T’s internal applications and computer systems and transmitted that information to John Doe Defendants 1-50 through the remote server," AT&T said. "The John Doe Defendants used that information to adjust the malware to specifically facilitate the hacking of [AT&T's customer service] application and then sent revised malware files to Evans for installation."

These John Doe defendants also re-sold fraudulently unlocked phones, AT&T alleged. The alleged scheme is similar to others "in which illegal operators buy or steal large quantities of phones (prepaid or with term contracts), unlock them, and resell them in foreign markets that do not subsidize the devices," AT&T wrote.

AT&T's complaint quotes Sapatin as saying "that there were many people across the country participating in the Unlock Scheme and others like it against different wireless carriers." The John Doe defendants include some of those people, AT&T wrote.

Sapatin allegedly tried to recruit other AT&T employees, telling one "that she would make $2,000 every two weeks through her participation in the Unlock Scheme," AT&T wrote.

Summons were issued to the defendants on Monday, giving them 21 days to respond to the complaint. We have sent a message to Swift Unlocks about the case but haven't heard back yet.

AT&T is seeking financial damages in an amount to be determined at trial, and injunctions preventing the defendants from continuing the alleged activity.

UPDATE: After this published, AT&T sent Ars a statement, saying, "We’re seeking damages and injunctive relief from several people who engaged in a scheme a couple of years ago to illegally unlock wireless telephones used on our network. It’s important to note that this did not involve any improper access of customer information, or any adverse effect on our customers."