BGP Communities: Even more Worms in the Routing Can

BGP communities are a mechanism widely used by operators to manage policy, mitigate attacks, and engineer traffic; e.g., to drop unwanted traffic, filter announcements, adjust local preference, and prepend paths to influence peer selection. Unfortunately, we show that BGP communities can be exploited by remote parties to influence routing in unintended ways. The BGP community-based vulnerabilities we expose are enabled by a combination of complex policies, error-prone configurations, a lack of cryptographic integrity and authenticity over communities, and the wide extent of community propagation. Due in part to their ill-defined semantics, BGP communities are often propagated far further than a single routing hop, even though their intended scope is typically limited to nearby ASes. Indeed, we find 14% of transit ASes forward received BGP communities onward. Given the rich inter-connectivity of transit ASes, this means that communities effectively propagate globally. As a consequence, remote adversaries can use BGP communities to trigger remote blackholing, steer traffic, and manipulate routes even without prefix hijacking. We highlight examples of these attacks via scenarios that we tested and measured both in the lab as well as in the wild. While we suggest what can be done to mitigate such ill effects, it is up to the Internet operations community whether to take up the suggestions.