Method for monitoring the security of an information system comprising a plurality of elements (3-1; 3-4) and at least one point (6, 7) of access to these elements, at least one element of the information system being liable to be affected by a least one vulnerability, each vulnerability liable to affect an element being associated with a probability of occurrence and with an impact on the element, said monitoring method comprising, for at least one state of the information system: - a step of modelling the information system in said state with a graph comprising a plurality of nodes, each node of the graph corresponding to a pair formed from an element and from a vulnerability liable to affect it or to an access point, said graph being such that two nodes of the graph, called first and second nodes, are connected by an arc that is: ∘ bidirectional, if said nodes correspond to the same element; ∘ oriented from the first node to the second node, if the exploitation during an attack from a point of access of said vulnerability of the first node leads to the exploitation of said vulnerability of the second node, said arc being associated with a so-called accessibility metric that measures a frequency of connection in the information system between the element of the first node and the element of the second node corresponding to said state of the information system; - a step of evaluating, for each node of a set of nodes of the graph that are identified as critical, a security metric of the node representing a level of risk run by the element of the node of being affected by the vulnerability of this node during attacks carried out against the information system from said at least one point of access, the security metric of the node being evaluated using the impact associated with the vulnerability of the node, and the probabilities of occurrence of the vulnerabilities of the nodes and the accessibility metrics of the arcs of at least one path of the graph existing between the node and said at least one point of access; - a step of determining an overall security metric of the information system in said state by cumulating the evaluated security metrics of the nodes of the set of critical nodes of the graph; said monitoring method furthermore comprising a step (E80, E90) of monitoring the security of the information system using the determined overall security metric of the information system corresponding to said at least one state. - a step of modelling the information system in said state with a graph comprising a plurality of nodes, each node of the graph corresponding to a pair formed from an element and from a vulnerability liable to affect it or to an access point, said graph being such that two nodes of the graph, called first and second nodes, are connected by an arc that is: ∘ bidirectional, if said nodes correspond to the same element; ∘ oriented from the first node to the second node, if the exploitation during an attack from a point of access of said vulnerability of the first node leads to the exploitation of said vulnerability of the second node, said arc being associated with a so-called accessibility metric that measures a frequency of connection in the information system between the element of the first node and the element of the second node corresponding to said state of the information system; ∘ bidirectional, if said nodes correspond to the same element; ∘ oriented from the first node to the second node, if the exploitation during an attack from a point of access of said vulnerability of the first node leads to the exploitation of said vulnerability of the second node, - a step of evaluating, for each node of a set of nodes of the graph that are identified as critical, a security metric of the node representing a level of risk run by the element of the node of being affected by the vulnerability of this node during attacks carried out against the information system from said at least one point of access, the security metric of the node being evaluated using the impact associated with the vulnerability of the node, and the probabilities of occurrence of the vulnerabilities of the nodes and the accessibility metrics of the arcs of at least one path of the graph existing between the node and said at least one point of access; - a step of determining an overall security metric of the information system in said state by cumulating the evaluated security metrics of the nodes of the set of critical nodes of the graph;