A ten-year-old legal settlement could be the undoing of the SCO Group's crusade to be recognised as a copyright holder overpart of Linux.

Documents detailing an agreement that granted developers the right to redistribute much of the Unix source code that SCO claims to own appeared earlier this week on the Groklaw.net website, which has been closely following the situation.

Groklaw had used the Freedom of Information Act to gain access to documents relating to a long-standing dispute over the ownership of the Unix source code, fought in the early 1990s between the University of California and an AT&T subsidiary called Unix Systems Laboratories (USL).

SCO later acquired a number of USL's assets among which, it is believed, was the Unix code that it now claims is illegally contained within Linux. The documents are just another piece in what has become an enormously complex legal process, but nevertheless they raise another question mark over SCO's claimed rights.

"The agreement actually gives people rights to redistribute [Unix] software that they were not previously aware of," said Bruce Perens an open source advocate and one of the founders of the Open Source Initiative. "It makes it very clear that some of the Unix software that SCO is currently claiming as their own is distributable by the public as open source."

Though software developers generally understood that source code from one of the major variants of Unix, called the Berkeley Software Distribution (BSD), could be freely redistributed in the Linux operating system, the release of the agreement makes those terms public for the first time.

In December 2003, SCO provided a list of some of the files covered in the USL settlement, claiming that they "were never intended or authorised for unrestricted use or distribution under the GPL in Linux".

The settlement agreement, however, appears to say otherwise. Under its terms, the files in question may be "freely reproduced and redistributed by others," provided that they include a copyright notice.