Stored HTML injection via unencoded mention URL
Published Sep 24, 2026 · Updated Sep 24, 2026
Discourse before 2026.1.8, 2026.6.3, 2026.7.2, and 2026.8.0 allows authenticated remote users to inject stored HTML through topic actions and activity logs. The small-action and nested-activity-log components interpolate the free-form action_code_who display name into mention-link href attributes without URL encoding, so a quote can terminate the attribute and insert attacker-controlled elements even though visible mention text is escaped. Exploitation requires creating a quote-bearing display name and having another user view the affected topic action or activity log, allowing persistent markup injection in rendered pages.
Summary
What happened
Discourse before 2026.1.8, 2026.6.3, 2026.7.2, and 2026.8.0 allows authenticated remote users to inject stored HTML through topic actions and activity logs. The small-action and nested-activity-log components interpolate the free-form action_code_who display name into mention-link href attributes without URL encoding, so a quote can terminate the attribute and insert attacker-controlled elements even though visible mention text is escaped. Exploitation requires creating a quote-bearing display name and having another user view the affected topic action or activity log, allowing persistent markup injection in rendered pages.
The record
- CVE
- CVE-2026-91119
- Published
- Sep 24, 2026
- Updated
- Sep 24, 2026
- Vendor
- Discourse
- Product
- Discourse
- Classifications
- T1189
- Attack vector
- network
- Privileges
- authenticated
Timeline
How it unfolded
- Sep 24, 2026CVE publishedPublication date reported by the CVE source.
- Sep 24, 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.
- Affected versionversion=>= 2026.1.0-latest, < 2026.1.8
- Affected versionversion=>= 2026.6.0-latest, < 2026.6.3
- Affected versionversion=>= 2026.7.0-latest, < 2026.7.2
- Affected versionversion=< 2026.8.0
What conditions does exploitation require?
What is affected?
Published CVSS scores
No CVSS assessment is available in this record.
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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.
Activity data is delayed. The latest report is Sep 28, 2026.
Weakness, pattern, technique
Public exploit references
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Technologies
Your stack
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Your stack
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