CVE-2026-30913

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

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)

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

Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.

CVE-2026-30913 has a CVSS score of 4.6 (Medium). The vector is network-reachable, low privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.8.3); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-30913? CVE-2026-30913 is a medium-severity cross-site scripting (XSS) vulnerability in flarum/nicknames (composer), affecting versions < 1.8.3. It is fixed in 1.8.3. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
2. How severe is CVE-2026-30913? CVE-2026-30913 has a CVSS score of 4.6 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of flarum/nicknames are affected by CVE-2026-30913? flarum/nicknames (composer) versions < 1.8.3 is affected.
4. Is there a fix for CVE-2026-30913? Yes. CVE-2026-30913 is fixed in 1.8.3. Upgrade to this version or later.
5. Is CVE-2026-30913 exploitable, and should I be worried? Whether CVE-2026-30913 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-30913 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-30913? Upgrade flarum/nicknames to 1.8.3 or later.