  We demonstrate the first downgrade attacks against RPKI. The key design
property in RPKI that allows our attacks is the tradeoff between connectivity
and security: when networks cannot retrieve RPKI information from publication
points, they make routing decisions in BGP without validating RPKI. We exploit
this tradeoff to develop attacks that prevent the retrieval of the RPKI objects
from the public repositories, thereby disabling RPKI validation and exposing
the RPKI-protected networks to prefix hijack attacks.
  We demonstrate experimentally that at least 47% of the public repositories
are vulnerable against a specific version of our attacks, a rate-limiting
off-path downgrade attack. We also show that all the current RPKI relying party
implementations are vulnerable to attacks by a malicious publication point.
This translates to 20.4% of the IPv4 address space.
  We provide recommendations for preventing our downgrade attacks. However,
resolving the fundamental problem is not straightforward: if the relying
parties prefer security over connectivity and insist on RPKI validation when
ROAs cannot be retrieved, the victim AS may become disconnected from many more
networks than just the one that the adversary wishes to hijack. Our work shows
that the publication points are a critical infrastructure for Internet
connectivity and security. Our main recommendation is therefore that the
publication points should be hosted on robust platforms guaranteeing a high
degree of connectivity.
