SB2026042065 - Cross-site scripting in Flarum

Description

This security bulletin contains information about 1 vulnerability.

1) Cross-site scripting (CVE-ID: CVE-2026-30913)

CWE-ID: CWE-79 - Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

CVSSv4: 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/E:U/U:Clear

The vulnerability allows a remote user to mislead users into visiting attacker-controlled domains.

The vulnerability exists due to improper neutralization of user-controlled input in plain-text notification emails when inserting a nickname into email content. A remote user can set a specially crafted nickname and trigger a notification email to mislead users into visiting attacker-controlled domains.

The issue is present when the flarum/nicknames extension is enabled, and some email clients may render markdown link syntax while virtually all email clients autolink domain-like nicknames.

Remediation

Install update from vendor's website.

References

• https://github.com/flarum/framework/security/advisories/GHSA-3c4m-j3g4-hh25
• https://github.com/advisories/GHSA-3c4m-j3g4-hh25