Esbuild version 0.9.0 introduces subtle but potentially impactful changes compared to its predecessor, version 0.8.57. Both versions maintain the core functionality of being exceptionally fast JavaScript bundlers and minifiers, licensed under the MIT license, and residing in the same GitHub repository. However, a closer examination reveals some intriguing differences.
The most immediate discrepancy lies in the release dates. Version 0.9.0 was published on March 9th, 2021, a single day after version 0.8.57's release on March 8th, 2021. This suggests a quick iteration, possibly addressing bugs or incorporating minor enhancements discovered shortly after the previous release.
Another notable difference is the unpacked size of the packages. Version 0.9.0 boasts a slightly smaller unpacked size of 69,973 bytes compared to version 0.8.57's 70,386 bytes. While seemingly insignificant, this reduction could indicate optimizations in code size or asset management.
For developers relying on esbuild for their bundling and minification needs, these changes necessitate a careful upgrade. While the core promise of speed and efficiency remains constant, the release notes or changelog accompanying version 0.9.0 should be consulted to understand the specific fixes or improvements implemented. The reduced unpacked size might also translate to faster installation times and a smaller footprint in deployment environments, representing a valuable, albeit subtle, benefit. Therefore, developers are encouraged to evaluate the new version in their development pipelines to leverage its potential advantages.
All the vulnerabilities related to the version 0.9.0 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.