CVE-2020-4070Network attack vector

Cross-site scripting via unescaped exception message

Published Jun 22, 2020 · Updated Aug 4, 2024

Cross-site scripting in W3C CSS Validator through commit 54d68a1 allows remote attackers to execute script via crafted validator links. StyleSheetParser.parseURL escapes the submitted URI but passes an exception with an unescaped message to CssError, allowing that message to enter the HTML error response. Exploitation requires a user to click the crafted link, after which attacker-supplied script runs in the validator page’s browser context.

CVSS severity4.6
Medium
EPSS probability0.55%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Cross-site scripting in W3C CSS Validator through commit 54d68a1 allows remote attackers to execute script via crafted validator links. StyleSheetParser.parseURL escapes the submitted URI but passes an exception with an unescaped message to CssError, allowing that message to enter the HTML error response. Exploitation requires a user to click the crafted link, after which attacker-supplied script runs in the validator page’s browser context.

The record

CVE
CVE-2020-4070
Published
Jun 22, 2020
Updated
Aug 4, 2024
Vendor
World Wide Web Consortium (W3C)
Product
CSS Validator
Classifications
CWE-79, T1204.001
Attack vector
network
Privileges
Unavailable

Timeline

How it unfolded

  1. Jun 22, 2020CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 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=<= 54d68a1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesUnavailable in this record.

What is affected?

World Wide Web Consortium (W3C) · CSS Validatorversion=<= 54d68a1

Published CVSS scores

5.4NIST NVDCVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N
3.5NIST NVDAV:N/AC:M/Au:S/C:N/I:P/A:N
4.6GitHub, Inc.CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/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
PrivilegesUnavailable
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
T1204.001User Execution: Malicious Link

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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