CVE-2026-91121: XSS (CVSS 5)

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.

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CVSS v3.1 Score

5.0

MEDIUM

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:N/I:L/A:N

EPSS — Exploit Prediction

0.0026

Probability of exploitation

0.16%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-79 Cross-site Scripting (XSS)

References

Other References

https://github.com/discourse/discourse/commit/24cd7cc4c868f20e6037d67d801d60c3e289b278 https://github.com/discourse/discourse/commit/6e7a181909c34a059d2b0a6291335a363205b787 https://github.com/discourse/discourse/commit/9d6f5e64a1cb68e8208520e4209b361e14f7394b https://github.com/discourse/discourse/commit/af197941f7613a3845cba21d217641c77c8ef836 https://github.com/discourse/discourse/pull/42882 https://github.com/discourse/discourse/security/advisories/GHSA-34rh-wjfv-65gq

Frequently Asked Questions

What is CVE-2026-91121? +

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. It has a CVSS v3.1 base score of 5.0 (MEDIUM).

How severe is CVE-2026-91121? +

CVE-2026-91121 has a CVSS v3.1 score of 5.0 out of 10, rated MEDIUM. This is a medium-severity vulnerability that should be remediated as part of regular maintenance. The EPSS score is 0.0026, placing it in the 0th percentile for exploitation probability.

How do I check if I'm vulnerable to CVE-2026-91121? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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CVE-2026-91121