CVE-2026-5388Network attack vector

Cross-site scripting via sanitizer and serializer bypasses

Published Aug 23, 2026 · Updated Aug 23, 2026

Cross-site scripting in JustHTML before 1.15.0 allows remote attackers to inject active HTML and JavaScript through crafted content. URL helpers accept encoded schemes and backslash hosts, while serializers and Markdown passthrough emit markup-breaking names, comments, and raw closing tags without safe neutralization. Exposure requires attacker-controlled data to reach the affected helpers, programmatic DOM construction, html_passthrough=True, or a permissive custom policy or transform pipeline.

CVSS severity9.8
Critical
EPSS probability0.34%
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 JustHTML before 1.15.0 allows remote attackers to inject active HTML and JavaScript through crafted content. URL helpers accept encoded schemes and backslash hosts, while serializers and Markdown passthrough emit markup-breaking names, comments, and raw closing tags without safe neutralization. Exposure requires attacker-controlled data to reach the affected helpers, programmatic DOM construction, html_passthrough=True, or a permissive custom policy or transform pipeline.

The record

CVE
CVE-2026-5388
Published
Aug 23, 2026
Updated
Aug 23, 2026
Vendor
Emil Stenström
Product
JustHTML
Classifications
CWE-178, CWE-79, CWE-20, CWE-755, T1059.007
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 23, 2026CVE publishedPublication date reported by the CVE source.
  2. Aug 23, 2026Record 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 <1.15.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Emil Stenström · JustHTMLversion=0 <1.15.0

Published CVSS scores

9.3VulnCheck CNACVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
9.8VulnCheck CNACVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/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 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-178Improper Handling of Case Sensitivity
CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
CWE-20Improper Input Validation
CWE-755Improper Handling of Exceptional Conditions
T1059.007Command and Scripting Interpreter: JavaScript/JScript

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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