  The ongoing integration of renewable generation and distributed energy
resources introduces new challenges to distribution network operation. Due to
the increasing volatility and uncertainty, distribution system operators (DSOs)
are seeking concepts to enable more active management and control. Flexibility
markets (FMs) offer a platform for economically efficient trading of
electricity flexibility between DSOs and other participants. The integration of
cyber, physical and market domains of multiple participants makes FMs a system
of cyber-physical systems (CPSs). While cross-domain integration sets the
foundation for efficient deployment of flexibility, it introduces new physical
and cyber vulnerabilities to participants. This work systematically formulates
threat scenarios for the CPSs of FMs, revealing several remaining security
challenges across all domains. Based on the threat scenarios, unresolved
monitoring requirements for secure participation of DSOs in FMs are identified,
providing the basis for future works that address these gaps with new technical
concepts.
