We present the first analysis of the popular Tor anonymity network that indicates the security of typical users against reasonably realis- tic adversaries in the Tor network or in the underlying Internet. Our results show that Tor users are far more susceptible to compromise than indicated by prior work. Specific contributions of the paper include (1) a model of various typical kinds of users, (2) an adver- sary model that includes Tor network relays, autonomous systems (ASes), Internet exchange points (IXPs), and groups of IXPs drawn from empirical study, (3) metrics that indicate how secure users are over a period of time, (4) the most accurate topological model to date of ASes and IXPs as they relate to Tor usage and network con- figuration, (5) a novel realistic Tor path simulator (TorPS), and (6) analyses of security making use of all the above. To show that our approach is useful to explore alternatives and not just Tor as cur- rently deployed, we also analyze a published alternative path se- lection algorithm, Congestion-Aware Tor. We create an empirical model of Tor congestion, identify novel attack vectors, and show that it too is more vulnerable than previously indicated.