Esbuild is a blazing-fast JavaScript bundler and minifier, and versions 0.4.2 and 0.4.3 showcase its continuing development. Both versions maintain the core description of being an "extremely fast" solution under the MIT license, hosted on GitHub. Examining the dist object reveals subtle yet potentially impactful changes. While the fileCount remains constant at 6, the unpackedSize sees an increase from 19831 bytes in version 0.4.2 to 20198 bytes in version 0.4.3. This ~3% increase suggests code or asset additions, potentially bug fixes, performance enhancements or new features incorporated into the newer version.
The release dates indicate that version 0.4.3 was released on May 28, 2020, two days after version 0.4.2, released on May 26, 2020. Such a quick turnaround between releases usually signals critical updates or bug fixes deemed important enough to warrant an immediate patch.
For developers, the key takeaway is that while both versions represent a fast and efficient bundler, the newer version (0.4.3) likely contains improvements and fixes. Upgrading is generally recommended to benefit from these enhancements. The slight increase in unpacked size should be considered if disk space is a constraint, however, It is probably not going to be a relevant problem. As always it's recommended to check the changelog to understand specific changes that occurred between these versions of the library.
All the vulnerabilities related to the version 0.4.3 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.