CVE-2026-55674Network attack vectorHinoki check available

Stored XSS via unescaped color-scheme cookie

Published Aug 17, 2026 · Updated Aug 18, 2026

Cross-site scripting in Discourse before 2026.1.6, 2026.5.2, and 2026.6.1 allows remote attackers to run JavaScript via crafted color-scheme cookies. The GitHub advisory states that ApplicationHelper accepted color_scheme_id and dark_scheme_id from cookies and rendered the selected scheme into a color-scheme tag without escaping, so an attacker could break out of the attribute, inject HTML, and bypass the nonce-based Content Security Policy. Because anonymous-page caching did not vary on those cookies, a single request could poison cached responses for later anonymous visitors with the same User-Agent, turning the bug into persistent stored XSS until cache refresh.

CVSS severity9.3
Critical
EPSS probability0.36%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkAvailable
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Summary

What happened

Cross-site scripting in Discourse before 2026.1.6, 2026.5.2, and 2026.6.1 allows remote attackers to run JavaScript via crafted color-scheme cookies. The GitHub advisory states that ApplicationHelper accepted color_scheme_id and dark_scheme_id from cookies and rendered the selected scheme into a color-scheme tag without escaping, so an attacker could break out of the attribute, inject HTML, and bypass the nonce-based Content Security Policy. Because anonymous-page caching did not vary on those cookies, a single request could poison cached responses for later anonymous visitors with the same User-Agent, turning the bug into persistent stored XSS until cache refresh.

The record

CVE
CVE-2026-55674
Published
Aug 17, 2026
Updated
Aug 18, 2026
Vendor
Discourse
Product
Discourse
Classifications
CWE-79, T1059.007
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 17, 2026CVE publishedPublication date reported by the CVE source.
  2. Aug 18, 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=< 2026.1.6
  2. Affected versionversion=>= 2026.5.0-latest, < 2026.5.2
  3. Affected versionversion=>= 2026.6.0-latest, < 2026.6.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Discourse · Discourseversion=< 2026.1.6; version=>= 2026.5.0-latest, < 2026.5.2; version=>= 2026.6.0-latest, < 2026.6.1

Published CVSS scores

9.3GitHub, Inc.CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/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 exploitUnconfirmed

Weakness, pattern, technique

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
T1059.007Command and Scripting Interpreter: JavaScript

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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