  To protect against misuse of passwords compromised in a breach, consumers
should promptly change affected passwords and any similar passwords on other
accounts. Ideally, affected companies should strongly encourage this behavior
and have mechanisms in place to mitigate harm. In order to make recommendations
to companies about how to help their users perform these and other
security-enhancing actions after breaches, we must first have some
understanding of the current effectiveness of companies' post-breach practices.
To study the effectiveness of password-related breach notifications and
practices enforced after a breach, we examine---based on real-world password
data from 249 participants---whether and how constructively participants
changed their passwords after a breach announcement.
  Of the 249 participants, 63 had accounts on breached domains; only 33% of the
63 changed their passwords and only 13% (of 63) did so within three months of
the announcement. New passwords were on average 1.3x stronger than old
passwords (when comparing log10-transformed strength), though most were weaker
or of equal strength. Concerningly, new passwords were overall more similar to
participants' other passwords, and participants rarely changed passwords on
other sites even when these were the same or similar to their password on the
breached domain. Our results highlight the need for more rigorous
password-changing requirements following a breach and more effective breach
notifications that deliver comprehensive advice.
