Site isolation is a feature in certain web browsers that allow cross-origin sites to be isolated from each other. The feature was originally proposed by Charles Reis and others, with subsequent iterations from Microsoft, in the form of their implementation of the feature in the Gazelle research browser. However, the feature failed to gain traction due to issues surrounding its implementation and performance concerns.
In 2018, following the disclosure of the Spectre and Meltdown vulnerabilities to the public, Google started work on adding site isolation in Chrome eventually culminating in a 2019 release of the feature. In 2021, Firefox also launched their own version of site isolation which they had been working on under the codename Project Fission.
Despite the security benefits of this feature, researchers have also found security issues surrounding various aspects of this feature. These include issues with the perceived protection against transient attacks such as Spectre and Meltdown, as well as new timing and resource exhaustion attacks enabled by this feature.