CVE-2021-24591Network attack vector

Authenticated stored XSS via unsanitized CustomCSS

Published Sep 6, 2021 · Updated Aug 3, 2024

Stored cross-site scripting in QuantumCloud Highlight before 0.9.3 allows remote authenticated users to inject script through CustomCSS. The plugin saves the CustomCSS setting without sanitizing it and later renders the stored value as sitewide custom CSS. A high-privilege WordPress account can plant browser-executed content even when unfiltered_html is disabled; execution requires a visitor to load an affected page.

CVSS severity5.4
Medium
EPSS probability0.62%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Stored cross-site scripting in QuantumCloud Highlight before 0.9.3 allows remote authenticated users to inject script through CustomCSS. The plugin saves the CustomCSS setting without sanitizing it and later renders the stored value as sitewide custom CSS. A high-privilege WordPress account can plant browser-executed content even when unfiltered_html is disabled; execution requires a visitor to load an affected page.

The record

CVE
CVE-2021-24591
Published
Sep 6, 2021
Updated
Aug 3, 2024
Vendor
QuantumCloud
Product
Highlight Sitewide Notice, Text, Button Menu
Classifications
CWE-79, T1203
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Sep 6, 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?

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

  1. Affected versionversion=0.9.3 <0.9.3

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

QuantumCloud · Highlight Sitewide Notice, Text, Button Menuversion=0.9.3 <0.9.3

Published CVSS scores

3.5NVDAV:N/AC:M/Au:S/C:N/I:P/A:N
5.4NVDCVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N

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
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
T1203Exploitation for Client Execution

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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