Black: Arbitrary file writes from unsanitized user input in cache file name

Affected versions

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

• python blackbefore 26.3.1

Versions reported by the vendor

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

• psf black

  • >= 24.3.0, < 26.3.1affected

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-2026-31900Black's vulnerable version parsing leads to RCE in GitHub Action
  34Monitor

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.