esm.sh: Path Traversal via package.json browser field allows reading arbitrary server files

Summary

A Local File Inclusion (LFI) vulnerability exists in the esbuild plugin's handling of the browser field in package.json. An attacker can publish an npm package that causes the server to read and return arbitrary files from the host filesystem during the build process.

Details

The vulnerable code is in the OnResolve callback of the esbuild plugin:

https://github.com/esm-dev/esm.sh/blob/main/server/build.go

The plugin validates that resolved file paths stay within the package working directory. However, after this check, the browser field from package.json remaps the module path to an attacker-controlled value containing ../ sequences. No validation is performed after the remapping.

// Sandbox check passes for the original "./d1.txt" path
if !strings.HasPrefix(filename, ctx.wd+string(os.PathSeparator)) {
    return esbuild.OnResolveResult{}, fmt.Errorf("could not resolve module %s", specifier)
}

// ... later, browser field remaps to attacker-controlled path:
if len(pkgJson.Browser) > 0 && ctx.isBrowserTarget() {
	if path, ok := pkgJson.Browser[modulePath]; ok {
		if path == "" {
			return esbuild.OnResolveResult{
				Path:      args.Path,
				Namespace: "browser-exclude",
			}, nil
		}
		if !isRelPathSpecifier(path) {
			externalPath, sideEffects, err := ctx.resolveExternalModule(path, args.Kind, withTypeJSON, analyzeMode)
			if err != nil {
				return esbuild.OnResolveResult{}, err
			}
			return esbuild.OnResolveResult{
				Path:        externalPath,
				SideEffects: sideEffects,
				External:    true,
			}, nil
		}
		modulePath = path
	}
}


// path.Join collapses "../" sequences - escapes the package directory
filename = path.Join(ctx.wd, "node_modules", ctx.esmPath.PkgName, modulePath)
// No second sandbox check

File contents appear in both the bundled JS output and the source map sourcesContent array.

Readable files are constrained by esbuild's loader selection based on file extension: .json files must be valid JSON, .txt/.html/.md are read as raw text, files without a recognized extension must be syntactically valid JavaScript. The config.json of esm.sh is fully readable (valid JSON with .json extension).

Non-existent target paths do not cause build errors - the import simply remains unresolved. This allows probing many paths in a single package, acting as a file existence oracle.

PoC

The test package is published at

<img width="1720" height="796" alt="image" src="https://github.com/user-attachments/assets/ee1c9781-2c5c-4718-b436-f6cf453f0952" />