CVE-2021-23364Public exploitNetwork attack vector

Service interruption via backtracking query regexes

Published Apr 28, 2021 · Updated Sep 17, 2024

Regular expression denial of service in Browserslist 4.0.0 through 4.16.4 allows attackers to interrupt query processing. Six query-parser regular expressions use an ambiguous numeric pattern that backtracks excessively on long malformed numeric queries. The consequence is CPU exhaustion in a process that passes attacker-controlled queries to the library or CLI; installation alone is not remotely exploitable.

CVSS severity5.3
Medium
EPSS probability2.45%
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 Browserslist 4.0.0 through 4.16.4 allows attackers to interrupt query processing. Six query-parser regular expressions use an ambiguous numeric pattern that backtracks excessively on long malformed numeric queries. The consequence is CPU exhaustion in a process that passes attacker-controlled queries to the library or CLI; installation alone is not remotely exploitable.

The record

CVE
CVE-2021-23364
Published
Apr 28, 2021
Updated
Sep 17, 2024
Vendor
Browserslist
Product
Browserslist
Classifications
CWE-1333, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 28, 2021CVE publishedPublication date reported by the CVE source.
  2. Sep 17, 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=4.0.0 <unspecified
  2. Affected versionversion=unspecified <4.16.5

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Browserslist · Browserslistversion=4.0.0 <unspecified; version=unspecified <4.16.5

Published CVSS scores

5.3SnykCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
5.0NIST NVDAV:N/AC:L/Au:N/C:N/I:N/A:P

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

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

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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