CVE-2024-21503Network attack vector

Formatting hang via docstring regex backtracking

Published Mar 19, 2024 · Updated Aug 1, 2024

Regular expression denial of service in Black before 24.3.0 allows attackers to hang formatting via crafted Python docstrings. The lines_with_leading_tabs_expanded function applies an overlapping whitespace regex to each line, causing catastrophic backtracking on long tab prefixes. The hang is reachable only when Black formats untrusted input or docstrings containing thousands of leading tabs.

CVSS severity5.3
Medium
EPSS probability0.98%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Regular expression denial of service in Black before 24.3.0 allows attackers to hang formatting via crafted Python docstrings. The lines_with_leading_tabs_expanded function applies an overlapping whitespace regex to each line, causing catastrophic backtracking on long tab prefixes. The hang is reachable only when Black formats untrusted input or docstrings containing thousands of leading tabs.

The record

CVE
CVE-2024-21503
Published
Mar 19, 2024
Updated
Aug 1, 2024
Vendor
Python Software Foundation
Product
Black
Classifications
CWE-1333, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 19, 2024CVE publishedPublication date reported by the CVE source.
  2. Aug 1, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=0 <24.3.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Python Software Foundation · Blackversion=0 <24.3.0

Published CVSS scores

5.3GitHub Advisory DatabaseCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-1333Inefficient Regular Expression Complexity
T1499.004Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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