esbuild known bugs

### Summary esbuild allows any websites to send any request to the development server and read the response due to default CORS settings. ### Details 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**: 1. The attacker serves a malicious web page (`http://malicious.example.com`). 1. The user accesses the malicious web page. 1. The attacker sends a `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. 1. The attacker gets the content of `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 - Fetching `/index.html`: normally you have a script tag here - Fetching `/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 - Connecting `/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))`) - Fetching URLs in the known file: once the attacker knows one file, the attacker can know the URLs imported from that file 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. ### PoC 1. Download [reproduction.zip](https://github.com/user-attachments/files/18561484/reproduction.zip) 2. Extract it and move to that directory 1. Run `npm i` 1. Run `npm run watch` 1. Run `fetch('http://127.0.0.1:8000/app.js').then(r => r.text()).then(content => console.log(content))` in a different website's dev tools.  ### Impact Users using the serve feature may get the source code stolen by malicious websites.

### Summary The development server contains a path traversal vulnerability on Windows when serving files from `servedir`. Due to the use of `path.Clean()` (which only normalizes forward-slash `/` separators) instead of a Windows-aware path normalization function, it is possible to craft requests using backslashes (`\`) that bypass the intended directory containment logic. An attacker can escape the configured `servedir` root and access arbitrary files on the filesystem. This issue affects Windows environments only. ### Details The request path is sanitized using: ```go // https://github.com/evanw/esbuild/blob/v0.27.3/pkg/api/serve_other.go#L165 queryPath := path.Clean(req.URL.Path)[1:] ``` However: - `path.Clean()` is POSIX-style and only understands `/` (docs: `https://pkg.go.dev/path#Clean`) - On Windows, `\` is a valid path separator - `path.Clean()` does not treat `\` as a separator Later, the server constructs the absolute path: ```go // https://github.com/evanw/esbuild/blob/v0.27.3/pkg/api/serve_other.go#L221 absPath := h.fs.Join(h.servedir, queryPath) ``` If `queryPath` contains sequences such as: ``` ..\..\..\..\..\..\..\Windows\system.ini ``` `path.Clean()` will not normalize them, but the Windows filesystem will interpret `\` as directory separators when resolving `absPath`. Because the implementation does not verify that the final resolved path remains within `servedir`, it allows directory traversal outside the intended root directory. ### Vulnerable Code ```go // https://github.com/evanw/esbuild/blob/v0.27.3/pkg/api/serve_other.go#L165 queryPath := path.Clean(req.URL.Path)[1:] .... // Check for a file in the "servedir" directory if h.servedir != "" && kind != fs.FileEntry { absPath := h.fs.Join(h.servedir, queryPath) if absDir := h.fs.Dir(absPath); absDir != absPath { if entries, err, _ := h.fs.ReadDirectory(absDir); err == nil { if entry, _ := entries.Get(h.fs.Base(absPath)); entry != nil && entry.Kind(h.fs) == fs.FileEntry { .... ``` ### Steps to reproduce ``` npm install --save-exact --save-dev esbuild echo "console.log(1)" > app.js .\node_modules\.bin\esbuild --version 0.27.3 .\node_modules\.bin\esbuild app.js --bundle --outdir=www --servedir=www --watch curl -i --path-as-is "http://localhost:8000/..\..\..\..\..\..\..\Windows\system.ini" <content of Windows\system.ini> ``` ### Impact - Arbitrary file read on Windows - Exposure of sensitive files