  Recently, bug-bounty programs have gained popularity and become a significant
part of the security culture of many organizations. Bug-bounty programs enable
organizations to enhance their security posture by harnessing the diverse
expertise of crowds of external security experts (i.e., bug hunters).
Nonetheless, quantifying the benefits of bug-bounty programs remains elusive,
which presents a significant challenge for managing them. Previous studies
focused on measuring their benefits in terms of the number of vulnerabilities
reported or based on the properties of the reported vulnerabilities, such as
severity or exploitability. However, beyond these inherent properties, the
value of a report also depends on the probability that the vulnerability would
be discovered by a threat actor before an internal expert could discover and
patch it. In this paper, we present a data-driven study of the Chromium and
Firefox vulnerability-reward programs. First, we estimate the difficulty of
discovering a vulnerability using the probability of rediscovery as a novel
metric. Our findings show that vulnerability discovery and patching provide
clear benefits by making it difficult for threat actors to find
vulnerabilities; however, we also identify opportunities for improvement, such
as incentivizing bug hunters to focus more on development releases. Second, we
compare the types of vulnerabilities that are discovered internally vs.
externally and those that are exploited by threat actors. We observe
significant differences between vulnerabilities found by external bug hunters,
internal security teams, and external threat actors, which indicates that
bug-bounty programs provide an important benefit by complementing the expertise
of internal teams, but also that external hunters should be incentivized more
to focus on the types of vulnerabilities that are likely to be exploited by
threat actors.
