  Almost all popular Internet services are hosted in a select set of countries,
forcing other nations to rely on international connectivity to access them. We
infer instances where traffic towards a large portion of a country is serviced
by a small number of Autonomous Systems, and, therefore, may be exposed to
observation or selective tampering. We introduce the Country-level Transit
Influence (CTI) metric to quantify the significance of a given AS on the
international transit service of a particular country. By studying the CTI
values for the top ASes in each country, we find that 32 nations have transit
ecosystems that render them particularly exposed, with traffic destined to over
40% of their IP addresses privy to a single AS. In the nations where we are
able to validate our findings with in-country operators, we obtain 83% accuracy
on average. In the countries we examine, CTI reveals two classes of networks
that play a particularly prominent role: submarine cable operators and
state-owned ASes.
