Summary

When the flarum/nicknames extension is enabled, a registered user can set their nickname to a string that email clients interpret as a hyperlink. The nickname is inserted verbatim into plain-text notification emails, and recipients may be misled into visiting attacker-controlled domains.

Affected package

• flarum/nicknames — permissive display name driver that allows special characters; affected since initial release on the 1.x branch

Any third-party display name driver that permits special characters would be equally affected.

Variants

1. Domain autolink — a nickname such as nasty.com is automatically converted to a clickable hyperlink by virtually all email clients (Gmail, Outlook, Apple Mail, Thunderbird).
2. Markdown link syntax — a nickname such as [CLICK](https://evil.com) is rendered as a clickable hyperlink by email clients that auto-render markdown in plain-text emails (e.g. Apple Mail, Thunderbird).

Steps to reproduce

Variant 1 (autolink — affects all email clients)

1. Enable flarum/nicknames, set nickname to nasty.com
2. Trigger a notification email to another user (e.g. follow them, mention them)
3. The nickname appears as a clickable link in the received email

Variant 2 (markdown — affects markdown-rendering email clients)

1. Enable flarum/nicknames, set nickname to [CLICK](https://evil.com)
2. Trigger a notification email to another user
3. In a markdown-rendering email client (e.g. Apple Mail), the nickname appears as a clickable link

Impact

Phishing / social engineering: victims may be misled into visiting attacker-controlled URLs via links appearing to originate from a trusted platform notification email. Variant 1 is exploitable against virtually all email clients without any special conditions.

• CVSS v3.1: AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:L/A:N — 4.6 Medium

Root cause

The default username-based display name driver constrains values to [a-zA-Z0-9_-]+, making it immune. flarum/nicknames introduced permissive validation (min/max length and an optional admin-configured regex) that allows arbitrary characters including those meaningful in URL and markdown contexts. This has been the case since the first commit of the extension.

Proposed fix

• Add validation in flarum/nicknames to reject or sanitize nicknames containing characters that email clients may interpret as URLs or markdown links
• Alternatively, sanitize the display name before insertion into notification email bodies so that legitimate nicknames like Jane.Smith are preserved but rendered safely

References

• Bug bounty submission: SBB-L4ZVAFH8 (Intigriti)