Cross-site scripting in Discourse - #VU166159
Published: November 13, 2026
Vulnerability identifier: #VU166159
CSH Severity: Low
CVSS v4: 5.1 [CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N]
CVE-ID: N/A
CWE-ID: CWE-79
Exploitation vector: Remote access
Exploit availability:
No public exploit available
Vulnerability details
The vulnerability allows a remote user to inject arbitrary HTML into posts.
The vulnerability exists due to unsafe reparsing of sanitized HTML in Discourse's code-block highlighting component when highlighting Unicode bidirectional characters with Nokogiri 1.19.4 or earlier. A remote user can craft a post containing bidirectional characters in attribute values that become HTML during reparsing to bypass sanitization and inject arbitrary markup.
Affected software
Discourse
Remediation
Install security update from vendor's website.
Discourse - addressed in versions 2026.7.5, 2026.9.2, 2026.10.1