CVE-2026-26960: Tar Arbitrary File Read/Write Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-83g3-92jg-28cx",
  "modified" : "2026-02-20T16:47:48Z",
  "published" : "2026-02-18T00:57:13Z",
  "aliases" : [ "CVE-2026-26960" ],
  "summary" : "Arbitrary File Read/Write via Hardlink Target Escape Through Symlink Chain in node-tar Extraction",
  "details" : "### Summary\n`tar.extract()` in Node `tar` allows an attacker-controlled archive to create a hardlink inside the extraction directory that points to a file outside the extraction root, using default options.\n\nThis enables **arbitrary file read and write** as the extracting user (no root, no chmod, no `preservePaths`).\n\nSeverity is high because the primitive bypasses path protections and turns archive extraction into a direct filesystem access primitive.\n\n### Details\nThe bypass chain uses two symlinks plus one hardlink:\n\n1. `a/b/c/up -> ../..`\n2. `a/b/escape -> c/up/../..`\n3. `exfil` (hardlink) -> `a/b/escape/<target-relative-to-parent-of-extract>`\n\nWhy this works:\n\n- Linkpath checks are string-based and do not resolve symlinks on disk for hardlink target safety.\n  - See `STRIPABSOLUTEPATH` logic in:\n    - `../tar-audit-setuid - CVE/node_modules/tar/dist/commonjs/unpack.js:255`\n    - `../tar-audit-setuid - CVE/node_modules/tar/dist/commonjs/unpack.js:268`\n    - `../tar-audit-setuid - CVE/node_modules/tar/dist/commonjs/unpack.js:281`\n\n- Hardlink extraction resolves target as `path.resolve(cwd, entry.linkpath)` and then calls `fs.link(target, destination)`.\n  - `../tar-audit-setuid - CVE/node_modules/tar/dist/commonjs/unpack.js:566`\n  - `../tar-audit-setuid - CVE/node_modules/tar/dist/commonjs/unpack.js:567`\n  - `../tar-audit-setuid - CVE/node_modules/tar/dist/commonjs/unpack.js:703`\n\n- Parent directory safety checks (`mkdir` + symlink detection) are applied to the destination path of the extracted entry, not to the resolved hardlink target path.\n  - `../tar-audit-setuid - CVE/node_modules/tar/dist/commonjs/unpack.js:617`\n  - `../tar-audit-setuid - CVE/node_modules/tar/dist/commonjs/unpack.js:619`\n  - `../tar-audit-setuid - CVE/node_modules/tar/dist/commonjs/mkdir.js:27`\n  - `../tar-audit-setuid - CVE/node_modules/tar/dist/commonjs/mkdir.js:101`\n\nAs a result, `exfil` is created inside extraction root but linked to an external file. The PoC confirms shared inode and successful read+write via `exfil`.\n\n### PoC\n[hardlink.js](https://github.com/user-attachments/files/25240082/hardlink.js)\nEnvironment used for validation:\n\n- Node: `v25.4.0`\n- tar: `7.5.7`\n- OS: macOS Darwin 25.2.0\n- Extract options: defaults (`tar.extract({ file, cwd })`)\n\nSteps:\n\n1. Prepare/locate a `tar` module. If `require('tar')` is not available locally, set `TAR_MODULE` to an absolute path to a tar package directory.\n\n2. Run:\n\n```bash\nTAR_MODULE=\"$(cd '../tar-audit-setuid - CVE/node_modules/tar' && pwd)\" node hardlink.js\n```\n\n3. Expected vulnerable output (key lines):\n\n```text\nsame_inode=true\nread_ok=true\nwrite_ok=true\nresult=VULNERABLE\n```\n\nInterpretation:\n\n- `same_inode=true`: extracted `exfil` and external secret are the same file object.\n- `read_ok=true`: reading `exfil` leaks external content.\n- `write_ok=true`: writing `exfil` modifies external file.\n\n### Impact\nVulnerability type:\n\n- Arbitrary file read/write via archive extraction path confusion and link resolution.\n\nWho is impacted:\n\n- Any application/service that extracts attacker-controlled tar archives with Node `tar` defaults.\n- Impact scope is the privileges of the extracting process user.\n\nPotential outcomes:\n\n- Read sensitive files reachable by the process user.\n- Overwrite writable files outside extraction root.\n- Escalate impact depending on deployment context (keys, configs, scripts, app data).",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "tar"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "7.5.8"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/isaacs/node-tar/security/advisories/GHSA-83g3-92jg-28cx"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-26960"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/isaacs/node-tar/commit/2cb1120bcefe28d7ecc719b41441ade59c52e384"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/isaacs/node-tar/commit/d18e4e1f846f4ddddc153b0f536a19c050e7499f"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/isaacs/node-tar"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-22" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-02-18T00:57:13Z",
    "nvd_published_at" : "2026-02-20T02:16:53Z"
  }
}