CVE-2020-6817Public exploitNetwork attack vector

Application hang via CSS regex backtracking

Published Feb 16, 2023 · Updated Mar 19, 2025

Regular expression denial of service in Mozilla Bleach before 3.1.4 allows remote attackers to hang consuming applications via crafted HTML. The BleachSanitizerFilter.sanitize_css gauntlet expression repeats overlapping alternatives, causing catastrophic backtracking on a rejected style value. The flaw requires an application to accept untrusted input and explicitly allow a tag with the style attribute; processing then consumes CPU and interrupts that application.

CVSS severity7.5
High
EPSS probability0.72%
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 Mozilla Bleach before 3.1.4 allows remote attackers to hang consuming applications via crafted HTML. The BleachSanitizerFilter.sanitize_css gauntlet expression repeats overlapping alternatives, causing catastrophic backtracking on a rejected style value. The flaw requires an application to accept untrusted input and explicitly allow a tag with the style attribute; processing then consumes CPU and interrupts that application.

The record

CVE
CVE-2020-6817
Published
Feb 16, 2023
Updated
Mar 19, 2025
Vendor
Mozilla
Product
Bleach
Classifications
CWE-1333, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 16, 2023CVE publishedPublication date reported by the CVE source.
  2. Mar 19, 2025Record 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=unspecified <3.1.4

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mozilla · Bleachversion=unspecified <3.1.4

Published CVSS scores

7.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
7.5CISA-ADPCVSS: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 exploitPublished

Weakness, pattern, technique

CWE-1333Inefficient Regular Expression Complexity
T1499.004Endpoint Denial of Service: Application 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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