GHSA-3C4M-J3G4-HH25
Vulnerability from github – Published: 2026-03-10 00:56 – Updated: 2026-03-10 18:44Summary
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 the1.xbranch
Any third-party display name driver that permits special characters would be equally affected.
Variants
- Domain autolink — a nickname such as
nasty.comis automatically converted to a clickable hyperlink by virtually all email clients (Gmail, Outlook, Apple Mail, Thunderbird). - 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/nicknamesto 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.Smithare preserved but rendered safely
References
- Bug bounty submission: SBB-L4ZVAFH8 (Intigriti)
{
"affected": [
{
"package": {
"ecosystem": "Packagist",
"name": "flarum/nicknames"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "1.8.3"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-30913"
],
"database_specific": {
"cwe_ids": [
"CWE-79"
],
"github_reviewed": true,
"github_reviewed_at": "2026-03-10T00:56:30Z",
"nvd_published_at": "2026-03-10T17:40:15Z",
"severity": "MODERATE"
},
"details": "## Summary\n\nWhen 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.\n\n## Affected package\n\n- **`flarum/nicknames`** \u2014 permissive display name driver that allows special characters; affected since initial release on the `1.x` branch\n\nAny third-party display name driver that permits special characters would be equally affected.\n\n## Variants\n\n1. **Domain autolink** \u2014 a nickname such as `nasty.com` is automatically converted to a clickable hyperlink by virtually all email clients (Gmail, Outlook, Apple Mail, Thunderbird).\n2. **Markdown link syntax** \u2014 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).\n\n## Steps to reproduce\n\n**Variant 1 (autolink \u2014 affects all email clients)**\n1. Enable `flarum/nicknames`, set nickname to `nasty.com`\n2. Trigger a notification email to another user (e.g. follow them, mention them)\n3. The nickname appears as a clickable link in the received email\n\n**Variant 2 (markdown \u2014 affects markdown-rendering email clients)**\n1. Enable `flarum/nicknames`, set nickname to `[CLICK](https://evil.com)`\n2. Trigger a notification email to another user\n3. In a markdown-rendering email client (e.g. Apple Mail), the nickname appears as a clickable link\n\n## Impact\n\nPhishing / 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.\n\n- CVSS v3.1: `AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:L/A:N` \u2014 **4.6 Medium**\n\n## Root cause\n\nThe 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.\n\n## Proposed fix\n\n- Add validation in `flarum/nicknames` to reject or sanitize nicknames containing characters that email clients may interpret as URLs or markdown links\n- Alternatively, sanitize the display name before insertion into notification email bodies so that legitimate nicknames like `Jane.Smith` are preserved but rendered safely\n\n## References\n\n- Bug bounty submission: SBB-L4ZVAFH8 (Intigriti)",
"id": "GHSA-3c4m-j3g4-hh25",
"modified": "2026-03-10T18:44:19Z",
"published": "2026-03-10T00:56:30Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/flarum/framework/security/advisories/GHSA-3c4m-j3g4-hh25"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-30913"
},
{
"type": "WEB",
"url": "https://github.com/flarum/nicknames/commit/4dde99729abdce8f6e2a7437c86e38735fdcca28"
},
{
"type": "PACKAGE",
"url": "https://github.com/flarum/framework"
},
{
"type": "WEB",
"url": "https://github.com/flarum/nicknames/releases/tag/v1.8."
},
{
"type": "WEB",
"url": "https://github.com/flarum/nicknames/releases/tag/v1.8.3"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:L/A:N",
"type": "CVSS_V3"
}
],
"summary": "flarum/nicknames extension has display name injection in notification emails (autolink \u0026 markdown)"
}
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