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TI-2026-40196 Aliases CVE-2026-55674

Discourse: Cache poisoning/XSS via color scheme cookies

Priority MEDIUM 43CWE-79Published · Updated

Key facts

Attack vector
Network
Privileges required
None
User interaction
Required
Attack complexity
Low
Impact type
Cross-site scripting
Confidentiality · integrity · availability
High · High · None
Fix status
Fixed

Attack profile from CVSS 3.1 · CNA. Fix status from CNA. Impact type from CWE-79.

Summary

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.

Why it matters now

Exploitation None reported CISA SSVC · not in CISA KEV
EPSS 0.0059 48th percentileScored 2026-11-07
Fix Fixed discourse (discourse): 2026.1.6, 2026.5.2, 2026.6.1

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation
None
Automatable
No
Technical impact
Total

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

VendorProductAffected versionsFixed inStatusSource
discoursediscoursebefore 2026.1.6; 2026.5.0-latest to before 2026.5.2; 2026.6.0-latest to before 2026.6.12026.1.6, 2026.5.2, 2026.6.1Patch availableCNA
  • discourse Vendordiscourse Affected before 2026.1.6; 2026.5.0-latest to before 2026.5.2; 2026.6.0-latest to before 2026.6.1 Fixed in 2026.1.6, 2026.5.2, 2026.6.1 Patch available CNA

CPE names Threatist (inferred) · 1

Inferred from how NVD named this product before; not given by a source.

CPEVersionsRoleVendor
cpe:2.3:a:discourse:discourse:*:*:*:*:*:*:*:*——discourse
  • cpe:2.3:a:discourse:discourse:*:*:*:*:*:*:*:* Versions — — ·discourse

Remediation

  • Fix · CNA discourse (discourse) Fixed in 2026.1.6, 2026.5.2, 2026.6.1

Timeline

  1. 2026-10-02 Threatist record created from CVE-2026-55674

References

Other 1

  • TI-2022-22806 Discourse BBCode plugin vulnerable to arbitrary CSS injection CVSS 3.1CRITICAL 9.8
  • TI-2021-19656 Denial of Service in discourse CVSS 3.1MEDIUM 6.8
  • TI-2024-36150 Potential Backup file leaked via Nginx in Discourse CVSS 3.1HIGH 7.5
  • TI-2021-18002 RCE via malicious SNS subscription payload CVSS 3.1CRITICAL 10.0
  • TI-2026-24651 Discourse: Open redirect via `sso_destination_url` cookie in `enter` CVSS 4.0MEDIUM 5.1
  • TI-2022-18730 Discourse Patreon vulnerable to improper validation of email during Patreon authentication CVSS 3.1CRITICAL 9.8
See all 328 →

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences