Esbuild, a blazing-fast JavaScript bundler and minifier, saw a minor version bump from 0.4.1 to 0.4.2 on May 26, 2020. Both versions maintain the same core promise of speed, aiming to significantly reduce build times for web development projects. Developers familiar with esbuild will find the familiar MIT license and GitHub repository remain unchanged, providing continuity in terms of usage rights and access to the source code. Functionally, both versions have identical unpacked sizes (19831 bytes) and the same number of files in the packages (6), suggesting that the changes between the versions are minimal. The key difference between 0.4.1 and 0.4.2 is the release time. Version 0.4.1 came out at 10:14:44.544Z, while version 0.4.2 came out at 21:22:44.384Z, about 11 hours later. This could mean that version 0.4.2 fixes minor bugs or includes small improvements reported shortly after the release of version 0.4.1. For developers already using esbuild, upgrading from 0.4.1 to 0.4.2 is advisable to ensure they have the latest bug fixes and improvements. For developers new to esbuild, choosing the latest version, 0.4.2, ensures they start with the most up-to-date and stable release of this high-performance tool.
All the vulnerabilities related to the version 0.4.2 of the package
esbuild enables any website to send any requests to the development server and read the response
esbuild allows any websites to send any request to the development server and read the response due to default CORS settings.
esbuild sets Access-Control-Allow-Origin: * header to all requests, including the SSE connection, which allows any websites to send any request to the development server and read the response.
https://github.com/evanw/esbuild/blob/df815ac27b84f8b34374c9182a93c94718f8a630/pkg/api/serve_other.go#L121 https://github.com/evanw/esbuild/blob/df815ac27b84f8b34374c9182a93c94718f8a630/pkg/api/serve_other.go#L363
Attack scenario:
http://malicious.example.com).fetch('http://127.0.0.1:8000/main.js') request by JS in that malicious web page. This request is normally blocked by same-origin policy, but that's not the case for the reasons above.http://127.0.0.1:8000/main.js.In this scenario, I assumed that the attacker knows the URL of the bundle output file name. But the attacker can also get that information by
/index.html: normally you have a script tag here/assets: it's common to have a assets directory when you have JS files and CSS files in a different directory and the directory listing feature tells the attacker the list of files/esbuild SSE endpoint: the SSE endpoint sends the URL path of the changed files when the file is changed (new EventSource('/esbuild').addEventListener('change', e => console.log(e.type, e.data)))The scenario above fetches the compiled content, but if the victim has the source map option enabled, the attacker can also get the non-compiled content by fetching the source map file.
npm inpm run watchfetch('http://127.0.0.1:8000/app.js').then(r => r.text()).then(content => console.log(content)) in a different website's dev tools.Users using the serve feature may get the source code stolen by malicious websites.