CVE-2019-11763Public exploitNetwork attack vector

Cross-site scripting via null-byte entity parsing

Published Jan 8, 2020 · Updated Aug 4, 2024

Cross-site scripting bypass in Firefox before 70, Firefox ESR before 68.2, and Thunderbird before 68.2 allows remote attackers to evade filters. The HTML tokenizer mishandles U+0000 null bytes after comment openers and within or after entity markers, causing comment text to become markup and concealing dangerous characters from sanitizers. A web application must accept attacker-controlled markup, filter it differently from Mozilla's parser, and deliver it to an affected client; successful use executes script in that application's origin.

CVSS severity6.1
Medium
EPSS probability0.99%
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 bypass in Firefox before 70, Firefox ESR before 68.2, and Thunderbird before 68.2 allows remote attackers to evade filters. The HTML tokenizer mishandles U+0000 null bytes after comment openers and within or after entity markers, causing comment text to become markup and concealing dangerous characters from sanitizers. A web application must accept attacker-controlled markup, filter it differently from Mozilla's parser, and deliver it to an affected client; successful use executes script in that application's origin.

The record

CVE
CVE-2019-11763
Published
Jan 8, 2020
Updated
Aug 4, 2024
Vendor
Mozilla
Product
Firefox
Classifications
CWE-79, T1203
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jan 8, 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=before 70

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mozilla · Firefoxversion=before 70
Mozilla · Thunderbirdversion=before 68.2
Mozilla · Firefox ESRversion=before 68.2

Published CVSS scores

6.1NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
4.3NIST NVDAV:N/AC:M/Au:N/C:N/I:P/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
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

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

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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