The Spectre vulnerability, which stems from vulnerabilities at the CPU design level, has been known for over 3 years now. What’s so interesting about this PoC is that its feasibility for leaking the end-user’s data has now been proven for web applications, meaning that it’s no longer just theoretical.

The vulnerability in affected CPUs has to do with speculative execution, which in certain situations can leave behind observable side-effects and result in data leakage to the attacker. All the attacker needs is a way to execute exploit code in the same executing context as other JavaScript handling sensitive data.

The attacker could use the web supply chain, for instance, presenting itself as a useful library so that victims voluntarily add it to their webpages, or deliberately compromise a third-party library as a way to attack websites that use it. Another vehicle would be to find an injection vulnerability on the website and combine that with the Spectre exploit.

Regardless of the method, the list of victims would be long, as Spectre exploits the JavaScript engines of browsers across several different operating systems, processor architectures, and hardware generations.

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