The rule is triggering on successful RDP connection events because the Next operator between the two subpatterns is set to OR . The FortiSIEM Study Guide explains that multiple subpattern rules are used when patterns must occur within a specific time period or when one of several patterns proves that an incident condition exists. It lists the OR operator as: “Subpattern X OR Subpattern Y occurred within the Time Window.” The same Study Guide further explains that if multiple patterns are used, FortiSIEM requires a next operator, and in the OR example, “an event that matches either” subpattern will trigger. It also states that because the next operator is OR, the constraint between the two subpatterns is not enforced.

In the exhibit, Subpattern 1 matches RDP traffic on TCP/UDP port 3389 from FortiGate traffic-forward events, while Subpattern 2 matches logon failure events with COUNT(Matched Events) > = 3. Because the rule uses OR, FortiSIEM can trigger when only the RDP connection subpattern matches, even if the failed-logon subpattern does not match. The correct logic should require both subpatterns to match with the intended relationship constraints, not either subpattern independently.