An administrator node (130) adjusts a trustworthy-measure associated with nodes (110) that are suspected of unauthorized modifications of content material. The original provider of the content material to a network binds an identifying code to it. Upon receiving the material from a source node (110), a target node (120) computes an associated code for the received material. If the computed code and the identifying code differ, the material is determined to be modified, and a discrepancy report is submitted to the administrator node (130). The administrator node (130) effects a penalty against the root source if the modification is confirmed; or against the target node (120) if the discrepancy report is unfounded. The penalties include downgrading of the trustworthiness-measure associated with each node, and these trustworthiness-measures are available for use by potential target nodes in their selection of preferred source nodes.