The case represents the biggest clash between open source and proprietary software groups since open source software began to be recognised as a credible business alternative to proprietary products.
SCO is suing IBM for $3 billion, accusing it of infringing its rights in the UNIX operating system by leaking UNIX code in breach of a contract. SCO says that Linux contains its UNIX code and consequently is unauthorised derivative of UNIX. SCO is also demanding that commercial Linux users buy its licences.
The counterclaim filed on Thursday expands IBM's original argument that SCO is in breach of the General Public Licence, or GPL, which underpins the distribution of most open source software.
The new filing says that SCO has infringed and is infringing IBM's copyrights "by copying, modifying, sublicensing and/or distributing Linux products except as expressly provided under the GPL."
Another claim seeks redress for "promissory estoppel" – or SCO's broken promise to keep to the terms of the GPL – while a further claim asks the court to declare that "SCO is not entitled to impose restrictions on the copying, modifying or distributing of programs distributed by it under the GPL except as set out in the GPL."
SCO responded to the filing on Monday saying that IBM brought the GPL into the argument, not SCO, describing the GPL as "a shaky foundation on which to build a legal case."
It added, "The GPL has never faced a full legal test, and SCO believes that it will not stand up in court. We are confident that SCO will win the legal battle that IBM has now started over the GPL."