CVE-2021-33587Network attack vector

Service interruption via superlinear attribute parsing

Published May 28, 2021 · Updated Aug 3, 2024

Inefficient attribute parsing in css-what 4.0.0 through 5.0.0 allows remote attackers to interrupt dependent Node.js services. The parseSelector routine applies the reAttr regular expression to the remaining selector text, allowing crafted attribute selectors to consume superlinear CPU time. Exposure requires a consuming application to pass attacker-controlled CSS selectors to the parser; the resulting CPU exhaustion can hang that application.

CVSS severity7.5
High
EPSS probability2.27%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Inefficient attribute parsing in css-what 4.0.0 through 5.0.0 allows remote attackers to interrupt dependent Node.js services. The parseSelector routine applies the reAttr regular expression to the remaining selector text, allowing crafted attribute selectors to consume superlinear CPU time. Exposure requires a consuming application to pass attacker-controlled CSS selectors to the parser; the resulting CPU exhaustion can hang that application.

The record

CVE
CVE-2021-33587
Published
May 28, 2021
Updated
Aug 3, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-400, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. May 28, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 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?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

7.5GitHub Advisory DatabaseCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
7.5UbuntuCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
5.0NVDAV:N/AC:L/Au:N/C:N/I:N/A:P
7.5NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

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 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-400Uncontrolled Resource Consumption
T1499.004Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

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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