CVE-2026-55674: Discourse

Critical severity, CVSS 9.3. EPSS: 0.6% chance of exploitation in the next 30 days.

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.

Affected products

• Discourse Discourse: before 2026.1.6 (fixed in 2026.1.6); from 2026.5.0-latest, before 2026.5.2 (fixed in 2026.5.2); from 2026.6.0-latest, before 2026.6.1 (fixed in 2026.6.1)

Published 2026-08-17. Last modified 2026-09-18.