Hackers are again using war dialling to break into corporate networks, security experts have warned.

NTA Monitor said that companies are not taking sufficient precautions against war dialling, which involves scanning telephone lines to find unsecured modems and create backdoors into corporate networks.

The practice emerged as an issue in the early 1980s but, while most companies have forgotten about it, hackers are now reacting to increased security in corporate networks.

A survey conducted by NTA Monitor between August and September showed that the issue had been widely overlooked, with 22 per cent having no knowledge of the hacking method.

Almost a quarter of respondents admitted that there are unauthorised modems attached to their systems, with as many as 20 rogue modems present at one site.

Over a third of organisations had found unauthorised modems in the past, but 68 per cent had no controls in place to detect modem scanning attempts on their systems.

This means those companies have no way of knowing whether they have been attacked or whether they have unsecured modems attached to their systems.

Four out of five firms had never conducted a third-party war dialling test run against their systems.

On average, modems are found in 0.75 per cent of a corporate's telephone number range, so a mid-sized company with 10,000 numbers will typically contain 75 modems.

"This should cause major concern, as it only takes one unsecured modem to permit a hacker to gain access to an organisation's systems," said NTA Monitor's technical director Roy Hills.

"Imagine the situation for a company with 5,000 extensions over 20 sites. How can it ever be sure that no rogue modems are attached to any of those lines, without testing them?"