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CVE-2026-26960

ADVISORY - github

Summary

Summary

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.

This enables arbitrary file read and write as the extracting user (no root, no chmod, no preservePaths).

Severity is high because the primitive bypasses path protections and turns archive extraction into a direct filesystem access primitive.

Details

The bypass chain uses two symlinks plus one hardlink:

  1. a/b/c/up -> ../..
  2. a/b/escape -> c/up/../..
  3. exfil (hardlink) -> a/b/escape/<target-relative-to-parent-of-extract>

Why this works:

  • Linkpath checks are string-based and do not resolve symlinks on disk for hardlink target safety.

    • See STRIPABSOLUTEPATH logic in:
      • ../tar-audit-setuid - CVE/node_modules/tar/dist/commonjs/unpack.js:255
      • ../tar-audit-setuid - CVE/node_modules/tar/dist/commonjs/unpack.js:268
      • ../tar-audit-setuid - CVE/node_modules/tar/dist/commonjs/unpack.js:281

  • Hardlink extraction resolves target as path.resolve(cwd, entry.linkpath) and then calls fs.link(target, destination).

    • ../tar-audit-setuid - CVE/node_modules/tar/dist/commonjs/unpack.js:566
    • ../tar-audit-setuid - CVE/node_modules/tar/dist/commonjs/unpack.js:567
    • ../tar-audit-setuid - CVE/node_modules/tar/dist/commonjs/unpack.js:703

  • Parent directory safety checks (mkdir + symlink detection) are applied to the destination path of the extracted entry, not to the resolved hardlink target path.

    • ../tar-audit-setuid - CVE/node_modules/tar/dist/commonjs/unpack.js:617
    • ../tar-audit-setuid - CVE/node_modules/tar/dist/commonjs/unpack.js:619
    • ../tar-audit-setuid - CVE/node_modules/tar/dist/commonjs/mkdir.js:27
    • ../tar-audit-setuid - CVE/node_modules/tar/dist/commonjs/mkdir.js:101

As 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.

PoC

hardlink.js Environment used for validation:

  • Node: v25.4.0
  • tar: 7.5.7
  • OS: macOS Darwin 25.2.0
  • Extract options: defaults (tar.extract({ file, cwd }))

Steps:

  1. Prepare/locate a tar module. If require('tar') is not available locally, set TAR_MODULE to an absolute path to a tar package directory.

  2. Run:

TAR_MODULE="$(cd '../tar-audit-setuid - CVE/node_modules/tar' && pwd)" node hardlink.js
  1. Expected vulnerable output (key lines):
same_inode=true
read_ok=true
write_ok=true
result=VULNERABLE

Interpretation:

  • same_inode=true: extracted exfil and external secret are the same file object.
  • read_ok=true: reading exfil leaks external content.
  • write_ok=true: writing exfil modifies external file.

Impact

Vulnerability type:

  • Arbitrary file read/write via archive extraction path confusion and link resolution.

Who is impacted:

  • Any application/service that extracts attacker-controlled tar archives with Node tar defaults.
  • Impact scope is the privileges of the extracting process user.

Potential outcomes:

  • Read sensitive files reachable by the process user.
  • Overwrite writable files outside extraction root.
  • Escalate impact depending on deployment context (keys, configs, scripts, app data).

Common Weakness Enumeration (CWE)

ADVISORY - github

CWE-22⁠

Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')

Impacted images

Advisories

GitHub

CREATED

UPDATED

ADVISORY IDGHSA-83g3-92jg-28cx⁠
EXPLOITABILITY SCORE

1.8

EXPLOITS FOUND
-
COMMON WEAKNESS ENUMERATION (CWE)
CWE-22⁠

CVSS SCORE

7.1high
PackageTypeOS NameOS VersionAffected RangesFix Versions
tarnpm--<7.5.87.5.8

CVSS:3 Severity and metrics

The CVSS metrics represent different qualitative aspects of a vulnerability that impact the overall score, as defined by the CVSS Specification⁠.

The vulnerable component is not bound to the network stack and the attacker's path is via read/write/execute capabilities. Either: The attacker exploits the vulnerability by accessing the target system locally (e.g., keyboard, console), or remotely (e.g., SSH); or the attacker relies on User Interaction by another person to perform actions required to exploit the vulnerability (e.g., using social engineering techniques to trick a legitimate user into opening a malicious document).

Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success when attacking the vulnerable component.

The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.

Successful exploitation of this vulnerability requires a user to take some action before the vulnerability can be exploited. For example, a successful exploit may only be possible during the installation of an application by a system administrator.

An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.

There is a total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact. For example, an attacker steals the administrator's password, or private encryption keys of a web server.

There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any or all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the impacted component.

There is no impact to availability within the impacted component.