Virtual organizations are dynamic associations of business partners that join forces to achieve a common goal. Because partners live in very competitive environments and they do not fully trust each other, they establish contractual agreements to help reduce the risk of participating in the virtual organization. This naturally puts severe demands on the security of the process leading to the establishment of a contract, including negotiation of terms and conditions. The open context in which virtual organizations are created raises further security concerns. This papers discusses the security requirements over contract negotiation that arise in this context, examines current negotiation protocols and proposes enhancements to one of them.