Esbuild version 0.8.4 refines the blazing-fast JavaScript bundling and minification experience already established by its predecessor, version 0.8.3. While both versions build upon the promise of incredible speed and efficiency for developers, a closer look reveals subtle but potentially impactful differences.
The core functionality remains consistent: both versions provide an exceptionally quick alternative to traditional JavaScript bundlers, significantly reducing build times for web applications and libraries. They both operate under the MIT license, giving developers freedom in how they use and distribute software built with esbuild and are managed in the same GitHub repository. Both contain the same number of files (6) after unpacking them.
However, the most notable distinction lies in the micro-improvements reflected in the unpacked size and release date. Version 0.8.4 has a slightly larger unpacked size (50746 bytes) compared to 0.8.3 (50738 bytes), meaning some internal files were slightly modified. As for the release date, version 0.8.4 was released on November 5, 2020, while version 0.8.3 was released on November 3, 2020. These nuances are crucial for developers concerned with the absolute latest changes, bug fixes, or minor feature enhancements within the esbuild ecosystem. The newer version will contain the latest bug fixes available. Despite their similar descriptions, it's recommended that users always opt for the newest stable release, as it will generally incorporate the newest improvements and bug fixes.
All the vulnerabilities related to the version 0.8.4 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.