TalkTalk customers have been hit by a cyber attack believed to be of the same type that took out up to 900,000 users of Germany's Deutsche Telekom last week.
It comes after Post Office Telecom admitted being the subject to a similar attack that, it claims, started on Sunday, which left some customers without broadband for days.
Both companies have been targeted due to security weaknesses identified in the routers they issue to their customers.
The attacks are being directed via a modified version of the Mirai Internet of Things botnet, which exploits unsecure connected devices, typically cheap, home-security camera systems, which are linked up via Linux-based digital video recorders.
The DVRs typically run outdated versions of Linux and use universal plug-and-play to punch a hole through users' home security so that they can be accessed via mobile apps. That, however, also opens up chronic insecurities on users' home networks, making the devices easy to exploit.
According to TalkTalk, users of its Dlink DSL-3780 have been targeted. Kingston-upon-Hull-based Kcom may also be affected as it uses the same Zyxel AMG1302 device that Post Office Telecom issues to its customers.
"We would like to reassure customers that no personal data or devices have been compromised," said a Post Office Telecom spokesman. They added: "We have identified the source of the problem and implemented a resolution which is currently being rolled out to all customers."
Kcom claimed that the vast majority of its customers were unaffected by the attacks and added that it had implemented additional network security to prevent similar future attacks.
Home Telecom, meanwhile, which issues Technicolour and Zyxel routers, has also come under attack. In an advisory on its website, it claimed that the problems it was experiencing "seems to be router related". Like Post Office Telecom, Home Telecom suggested that the problems could be overcome by resetting the router.
Home Telecom issues Technicolour TG582n fibre and broadband routers, as well as the Zyxel AMG1202 and AMG 1302-T10B routers.
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