CVE-2026-91121Network attack vectorHinoki check available

Chat excerpt HTML injection via unescaped filenames

Published Sep 24, 2026 · Updated Sep 24, 2026

HTML injection in Discourse before 2026.8.0 allows remote authenticated users to alter chat excerpt rendering through upload filenames. Excerpt renderers treat attacker-controlled upload filenames as trusted HTML instead of escaping markup before inserting them into channel lists, email summaries, pinned bars, replies, and thread previews. Exploitation requires file-upload and chat-posting access; default Content Security Policy prevents demonstrated JavaScript execution, while altered excerpt content remains possible.

CVSS severityUnavailable
Unscored
EPSS probability0.26%
Next 30 days · Oct 19, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkAvailable
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Summary

What happened

HTML injection in Discourse before 2026.8.0 allows remote authenticated users to alter chat excerpt rendering through upload filenames. Excerpt renderers treat attacker-controlled upload filenames as trusted HTML instead of escaping markup before inserting them into channel lists, email summaries, pinned bars, replies, and thread previews. Exploitation requires file-upload and chat-posting access; default Content Security Policy prevents demonstrated JavaScript execution, while altered excerpt content remains possible.

The record

CVE
CVE-2026-91121
Published
Sep 24, 2026
Updated
Sep 24, 2026
Vendor
Discourse
Product
Discourse
Classifications
Unavailable
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Sep 24, 2026CVE publishedPublication date reported by the CVE source.
  2. Sep 24, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=>= 2026.1.0-latest, < 2026.1.8
  2. Affected versionversion=>= 2026.6.0-latest, < 2026.6.3
  3. Affected versionversion=>= 2026.7.0-latest, < 2026.7.2
  4. Affected versionversion=< 2026.8.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Discourse · Discourseversion=>= 2026.1.0-latest, < 2026.1.8; version=>= 2026.6.0-latest, < 2026.6.3; version=>= 2026.7.0-latest, < 2026.7.2; version=< 2026.8.0

Published CVSS scores

No CVSS assessment is available in this record.

CVSS describes severity. EPSS estimates exploitation probability.

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.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 22, 2026Sep 28, 2026
Latest reporting daySep 28, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

No sourced classifications are available.

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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