SCO Group has sent letters to 6,000 Unix licence holders requiring them to certify that they are in full compliance with their agreements and are not using Unix code in Linux.
The move is an escalation in SCO's fight to assert its ownership of key parts of the Unix code, which it alleges have been incorporated into the Linux kernel.
SCO's targets range from suppliers such as IBM and Hewlett-Packard to some of the world's largest financial services, manufacturing and pharmaceutical companies.
The letter demands that Unix licensees certify that their employees and contractors have not taken eight specific actions involving Linux. If no certification is provided, SCO reserves the right to take legal action, including the termination of licences, the letter said.
As SCO issued its challenge, Novell, which bought Linux distributor SuSE Linux in November, said it had registered copyright for some of the same Unix System V code that SCO registered last year.
"Novell believes it owns the copyrights in Unix, and has applied for and received copyright registrations pertaining to Unix consistent with that position," the company said in a statement. "Contrary to SCO's public statements SCO has been well aware that Novell continues to assert ownership of the Unix copyrights."
Novell purchased rights to the Unix System V code for £82m from AT&T in 1992. It later sold them and they were eventually acquired by SCO. However, Novell said the terms of the sale meant it had retained copyright over the Unix source code.
Darl McBride, SCO's chief executive, accused Novell of registering the Unix copyright to help IBM with the ongoing legal dispute between SCO and IBM over the use of Unix code in Linux.
How should you respond to SCO's demands?
Law firm Bird & Bird has outlined a five-step plan for Unix users to help them respond to SCO's demands within the 30-day deadline set out by the company in its letter:
Identify from your internal records what Unix agreements you have and gather together full copies of all your licence documentation
Review these urgently with your lawyers
Determine whether compliance with each licence agreement requires a response to SCO's letter, either in the terms alleged by SCO or otherwise
Determine whether the possible consequences of not responding, or of a non-compliant response, include (as alleged by SCO) termination of the Unix licence
Ensure that any response needed in order to comply with the terms of the Unix licence agreement is provided in time.