CVE-2026-5389Public exploitNetwork attack vector

XSS via fixed code fence breakout

Published Aug 23, 2026 · Updated Aug 23, 2026

Cross-site scripting in JustHTML before 1.13.0 allows remote attackers to execute attacker-controlled script via crafted pre content. The to_markdown() pre serializer writes decoded attacker-controlled text into a fixed three-backtick fence, so an embedded backtick run closes the block early. Exploitation requires an application to render the generated Markdown with a CommonMark or GFM-style renderer that permits raw HTML, causing script to execute in the viewing user's browser.

CVSS severity5.1
Medium
EPSS probability0.19%
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 1.13.0 allows remote attackers to execute attacker-controlled script via crafted pre content. The to_markdown() pre serializer writes decoded attacker-controlled text into a fixed three-backtick fence, so an embedded backtick run closes the block early. Exploitation requires an application to render the generated Markdown with a CommonMark or GFM-style renderer that permits raw HTML, causing script to execute in the viewing user's browser.

The record

CVE
CVE-2026-5389
Published
Aug 23, 2026
Updated
Aug 23, 2026
Vendor
Emil Stenström
Product
JustHTML
Classifications
CWE-80, CAPEC-63, T1189
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

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

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

  1. Affected versionversion=0 <1.13.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Emil Stenström · JustHTMLversion=0 <1.13.0

Published CVSS scores

5.1VulnCheckCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N
6.1VulnCheckCVSS:3.1/AV:N/AC:L/PR:N/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 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 exploitPublished

Weakness, pattern, technique

CWE-80Improper Neutralization of Script-Related HTML Tags in a Web Page (Basic XSS)
CAPEC-63Cross-Site Scripting (XSS)
T1189Drive-by Compromise

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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