CVE-2025-4138: Bypassing extraction filter to create symlinks to arbitrary targets outside extraction directory

Versions reported by the vendor

Affected version ranges reported by the assigning authority (PSF). Independent of NVD's CPE analysis and usually ahead of it.

• Python Software Foundation CPython

  • before 3.9.23affected · python
  • 3.10.0 and later · before 3.10.18affected · python
  • 3.11.0 and later · before 3.11.13affected · python
  • 3.12.0 and later · before 3.12.11affected · python
  • 3.13.0 and later · before 3.13.4affected · python
  • 3.14.0a1 and later · before 3.14.0b3affected · python

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

Other highest-scoring records for the same primary product.

• CVE-2026-115557Distrust compromised intermediate certificates in ssl module
  37Monitor
• CVE-2025-4517Arbitrary writes via tarfile realpath overflow
  37Monitor
• CVE-2026-19445Use-after-free of a server-side SSLContext when sni_callback switches contexts
  36Monitor

Published before and after this one in the same product

• ← EarlierCVE-2025-4516May 15, 2025 · Use-after-free in "unicode_escape" decoder with error handler
  23Monitor
• ← EarlierCVE-2024-12718Jun 3, 2025 · Bypass extraction filter to modify file metadata outside extraction directory
  21Monitor
• Later →CVE-2025-4330Jun 3, 2025 · Extraction filter bypass for linking outside extraction directory
  30Monitor

FIRST EPSS daily score; only changes of 0.01 or more are recorded (step chart).

Root cause

A user-supplied file name is joined with the allowed directory, but the result is never checked to confirm it is still inside that directory.

A representative example of this vulnerability class. Not the vendor's source code; it shows the faulty pattern and its fix.

1const file = path.join(ROOT, req.query.name);2res.sendFile(file);

1const file = path.resolve(ROOT, req.query.name);2if (!file.startsWith(ROOT + path.sep)) {3  return res.sendStatus(400);4}5res.sendFile(file);

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.