CVE-2026-91121 Discourse: Chat upload filenames rendered as raw HTML in excerpts
Attackers can inject HTML into chat excerpts via malicious filenames, altering displayed content in various UI elements.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-91121?
CVE-2026-91121 is a medium-severity cross-site scripting vulnerability in discourse, published on 24 Sep 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official description
Discourse is an open-source discussion platform. Prior to 2026.1.8, 2026.6.3, 2026.7.2, and 2026.8.0, attacker-controlled upload filenames used in chat message excerpts were rendered as unescaped HTML. A user able to upload a file and send chat content could place markup in a filename that was then interpreted by chat channel lists, chat summary emails, pinned message bars, reply previews, thread previews, and other excerpt renderers. The issue allowed trusted-HTML injection that altered rendered excerpt content, but JavaScript execution was not demonstrated with default Content Security Policy settings. The advisory states that no confidentiality, integrity, or availability impact was identified, while its structured scoring metadata separately assigns low integrity impact. Sites that disable or relax the default CSP have increased exposure. This issue is fixed in versions 2026.1.8, 2026.6.3, 2026.7.2, and 2026.8.0.
Weakness: CWE-79
Which versions are affected by CVE-2026-91121?
1 open source package ship an affected version, including discourse.
| Package | Affected | Fixed in |
|---|---|---|
discourse Bitnami | All versions | 2026.8.0 |
How do I fix CVE-2026-91121?
Upgrade discourse to 2026.8.0 or later.
Escape output for the context it lands in and set a strict Content Security Policy. Upgrading the affected component is the real fix. More about Cross-site scripting.
Is CVE-2026-91121 being exploited?
There is no known exploitation of CVE-2026-91121 in the wild and no public exploit code.
Sources
Also known as BIT-discourse-2026-91121, GHSA-34rh-wjfv-65gq.