Discourse 2026.x XSS via color_scheme_id Cookie (before 2026.7.0)
CVE-2026-55674 Published on August 17, 2026
Discourse: Cache poisoning/XSS via color scheme cookies
Discourse is an open-source discussion platform. Prior to 2026.1.6, 2026.5.2, 2026.6.1, and 2026.7.0, an unauthenticated attacker could send a single request with a crafted color_scheme_id (or dark_scheme_id) cookie to inject arbitrary HTML into a Discourse page. Because the cookie value was rendered into a color scheme tag without escaping, the attacker could break out of the attribute and inject a tag that bypassed Discourse's nonce-based Content Security Policy, resulting in arbitrary JavaScript execution in visitors' browsers. This issue is fixed in versions 2026.1.6, 2026.5.2, 2026.6.1, and 2026.7.0.
Vulnerability Analysis
CVE-2026-55674 is exploitable with network access, requires user interaction. This vulnerability is considered to have a low attack complexity. The potential impact of an exploit of this vulnerability is considered to have a high impact on confidentiality and integrity, and no impact on availability.
Weakness Type
What is a XSS Vulnerability?
The software does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.
CVE-2026-55674 has been classified to as a XSS vulnerability or weakness.
Products Associated with CVE-2026-55674
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Affected Versions
discourse:- Version < 2026.1.6 is affected.
- Version >= 2026.5.0-latest, < 2026.5.2 is affected.
- Version >= 2026.6.0-latest, < 2026.6.1 is affected.