Esbuild is a lightning-fast JavaScript bundler and minifier, and two closely released versions, 0.3.4 and 0.3.5, arrived on the same day, May 18th, 2020. While both versions share the same fundamental characteristics – MIT licensing, hosting on GitHub, identical file counts (6) and unpacked size (19025 bytes) in their distributed tarballs reflecting similar content – there's a slight difference in their release times. Version 0.3.4 was released at 20:45:48 UTC, while version 0.3.5 followed a few hours later at 23:45:16 UTC, indicating a quick fix or minor update.
For developers considering esbuild, this suggests active maintenance and responsiveness from the maintainers. The minimal difference between these adjacent releases highlights the stability of the tool. Whether you choose 0.3.4 or 0.3.5, you're essentially getting the same core functionality: a highly performant bundler designed to drastically improve build times compared to other solutions. Considering the unchanged file count, unpacked size and description, the upgrade from 0.3.4 to 0.3.5 probably includes bug fixes or very minor non-breaking changes. The release date proximity emphasizes the development team's dedication to swiftly resolving issues and providing a dependable tool for developers optimizing JavaScript code. Developers looking for efficient bundling and minification should find esbuild, in either version, very valuable.
All the vulnerabilities related to the version 0.3.5 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.