The State of the SameSite: Studying the Usage, Effectiveness, and Adequacy of SameSite Cookies

Chromium-based browsers now restrict cookies’ scope to a same-site context by changing the default policy for cookies, thus requiring developers to adapt their websites. The extent of the adoption and effectiveness of the SameSite policy has not been studied yet, and, in this paper, we undertake one of the first evaluations of the state of the SameSite cookie policy. We conducted a set of large-scale, longitudinal, both automated and manual measurements of the Alexa top 1K, 10K, 100K, and 500K sites across the main rollout dates of the SameSite policies, covering both SameSite usage and cross-site functionality breakage caused by the new default policy. Also, we performed an extensive evaluation of threats against the new Lax-by-default policy’s effectiveness, looking at the adequacy of the coverage provided by the Lax policy and bypass caused by website developers’ mistakes.Our study shows that the growth of sites using a SameSite policy has slowed down considerably after the enforcement dates. Then, the new Lax-by-default policy has affected about 19% of the functionalities implemented via cross-site requests without an explicit SameSite policy, most of which are for online ads. Third, our study observes a significant mismatch between the request contexts covered by Lax and the ones actually used by websites in the wild, making it possible to perform XS attacks also against popular websites such as Tumblr, Twitch, SoundCloud, Mailchimp, and Pixiv. Even when using Lax or Strict policies, much of their effectiveness depends on developers’ awareness of SameSite policies’ implications, who could introduce vulnerabilities or inconsistent policies, leading to SameSite policy bypasses. For example, we identified bypass in IMDB, Paypal, and Meetup. Also, we discovered a widespread SSO IdP abuse that attackers could use to attack target websites even when using stricter SameSite policies. Finally, in this paper, we also look at SameSite implementations in popular browsers and the default configuration in web frameworks.