---
id: CVE-2024-21503
aliases:
  - GHSA-fj7x-q9j7-g6q6
  - PYSEC-2024-48
title: Black vulnerable to Regular Expression Denial of Service (ReDoS)
summary: Black vulnerable to Regular Expression Denial of Service (ReDoS)
severity: medium
cvss: 5.3
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L'
vendor: black
product: black
ecosystem: pip
affected:
  - black < 24.3.0
patched:
  - black 24.3.0
published: '2024-03-19'
updated: '2026-09-10'
sourceUpdated: '2026-09-10T03:50:10.486429507Z'
source: OSV
sourceUrl: 'https://osv.dev/vulnerability/GHSA-fj7x-q9j7-g6q6'
references:
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2024-21503'
  - url: >-
      https://github.com/psf/black/commit/f00093672628d212b8965a8993cee8bedf5fe9b8
  - url: 'https://github.com/psf/black'
  - url: 'https://github.com/psf/black/releases/tag/24.3.0'
  - url: >-
      https://github.com/pypa/advisory-database/tree/main/vulns/black/PYSEC-2024-48.yaml
  - url: 'https://security.snyk.io/vuln/SNYK-PYTHON-BLACK-6256273'
tags:
  - osv
  - pip
epss: 0.00979
epssPercentile: 0.60679
ingestedAt: '2026-09-12T03:13:01.692Z'
---

## Overview

Versions of the package black before 24.3.0 are vulnerable to Regular Expression Denial of Service (ReDoS) via the lines_with_leading_tabs_expanded function in the strings.py file. An attacker could exploit this vulnerability by crafting a malicious input that causes a denial of service.

Exploiting this vulnerability is possible when running Black on untrusted input, or if you habitually put thousands of leading tab characters in your docstrings.

## Affected packages

- `black < 24.3.0`

## Remediation

Upgrade to a patched release:

- `black 24.3.0`