Esbuild version 0.4.1 emerges as a minor iterative update following closely on the heels of version 0.4.0, both versions maintaining the core promise of an exceptionally fast JavaScript bundler and minifier. Both are released on the same day, May 26th 2020. Developers familiar with 0.4.0 will find the upgrade to 0.4.1 seamless, as the fundamental functionality and API remain consistent. The description,license and repository fields are identical signifying stability in the project's overall structure and licensing. The primary distinction between the two lies in the dist metadata. Version 0.4.1 exhibits a slightly larger unpackedSize (19831 bytes) compared to version 0.4.0 (19674 bytes). This suggests that 0.4.1 likely incorporates bug fixes, performance enhancements, or minor feature additions that contribute to the increased size. The updated releaseDate, just a few hours later, confirms its nature as a quick follow-up to the initial 0.4.0 release. For developers already leveraging the speed and efficiency of esbuild, migrating to 0.4.1 is advisable to benefit from the incremental improvements it offers, ensuring they're working with the most refined version of the tool available on that day. The small difference in size is an indication that the changes are probably minimal.
All the vulnerabilities related to the version 0.4.1 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.