CVE-2026-91119 discourse CVE debrief

Who should care

Defenders managing Discourse installations, especially those with untrusted or numerous users, should assess exposure and apply patches to prevent potential stored HTML injection. This includes reviewing user display names and topic actions for potential malicious activity, and implementing Content Security Policy (CSP) to mitigate injected content. Additionally, defenders should verify Discourse installation versions and apply patches to prevent stored

Why it matters

CVE-2026-91119 is a stored HTML injection vulnerability in Discourse that allows attackers to inject malicious content into topic actions and activity logs, potentially affecting other users. Defenders should prioritize verifying exposure and applying patches.

• Potential stored HTML injection when viewing topic actions or activity logs.
• Possible injection of attacker-controlled elements into rendered markup.
• Verification of Discourse installation versions and user display names is necessary.
• Applying patches to prevent exploitation is crucial.

Technical summary

The topic small-action and nested-activity-log components in Discourse interpolated the free-form action_code_who value into mention-link href attributes without URL encoding, allowing stored HTML injection when another user viewed the affected topic action or activity log. This issue is fixed in versions 2026.1.8, 2026.6.3, 2026.7.2, and 2026.8.0. The vulnerability allows attackers to inject malicious content into topic actions and activity logs, potentially affecting other users. Defenders should prioritize verifying exposure and applying patches for Discourse installations, especially those with untrusted or numerous users, to prevent potential stored HTML injection.

Defensive priority

Defenders should prioritize verifying exposure and applying patches for Discourse installations, especially those with untrusted or numerous users, to prevent potential stored HTML injection.

Recommended defensive actions

• Verify Discourse installation versions and apply patches to prevent stored HTML injection.
• Review user display names and topic actions for potential malicious activity.
• Implement Content Security Policy (CSP) to mitigate injected content.
• Review compensating controls for exposed systems while remediation is scheduled and verified.
• Check relevant monitoring, detection, and logs for exposed assets that need extra review.
• Track exceptions, retest remediated assets, and close the item only after evidence is documented.
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up.

Evidence notes

The CVE record and NVD entry provide details on the vulnerability, including its CVSS score of 6.4 and MEDIUM severity. GitHub commits and pull requests are referenced, indicating fixes in versions 2026.1.8, 2026.6.3, 2026.7.2, and 2026.8.0.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.