Esbuild version 0.8.40 represents a subtle but potentially significant update to the popular JavaScript bundler and minifier, following closely on the heels of version 0.8.39. Both versions maintain the core promise of esbuild: extremely fast bundling and minification, underpinned by its MIT license and open-source nature on GitHub. The primary difference lies in the details; specifically, the unpacked size of version 0.8.40 is slightly larger, at 68,676 bytes, compared to 0.8.39's 68,320 bytes. This increase, though small, suggests internal changes that could encompass bug fixes, performance tweaks, or minor feature additions.
Developers adopting esbuild into their workflow will be interested in these incremental improvements. While the file count remains constant at 6, hinting at no major structural modifications, the differing unpacked size released just days apart signifies active development and refinement. Users should expect continued stability and optimized performance. Although the core description remains the same, those seeking cutting-edge performance in their build processes will value the commitment to regular updates and fine-tuning reflected in this release. Version 0.8.40 cements esbuild's position as a compelling tool for modern JavaScript development, delivering fast, efficient builds in a constantly evolving landscape for web applications.
All the vulnerabilities related to the version 0.8.40 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.