Esbuild, a bundler and minifier, underwent a significant update from version 0.0.0 to 0.0.4, marking a notable evolution in its early development stages. The initial release, version 0.0.0, was a barebones iteration, lacking a detailed description and simply stating the author as Evan Wallace under the ISC license. This foundational version provided the groundwork for what was to come.
Version 0.0.4 introduced several immediate changes. The package gained a description, albeit a simple "This is a test. Please ignore," suggesting an experimental phase. More importantly, it includes details of a distributable package, including a .tgz archive, file count (3), and unpacked size (2375 bytes). This indicates a transition from a purely conceptual stage to a tangible, distributable library. This version also got pushed to npm registry, indicating it was intended to be consumed by developers. Developers should note the considerable time gap between versions; 2017 and 2020, showing sporadic development in its early phases. The tarball URL points to the specific location of the package on the npm registry, allowing for direct access and installation. While 0.0.4 is marked as a test, the inclusion of distribution details suggests a step towards a more practical and usable tool for developers interested in bundling and minifying JavaScript projects.
All the vulnerabilities related to the version 0.0.4 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.