Arp Spoofing in Fastapi with Basic Auth

Arp spoofing is a link-layer attack where an attacker sends falsified Address Resolution Protocol replies to associate their MAC address with the IP address of a legitimate target, typically the default gateway. In a Fastapi application that relies on HTTP Basic Auth, this attack can undermine transport integrity even when credentials are transmitted as base64-encoded values. Because Basic Auth does not encrypt the payload, an attacker who successfully ARP-spoofs a session can intercept and decode credentials locally on the network segment, provided they can also see or redirect the resulting HTTP traffic.

Fastapi itself does not introduce protocol-specific weaknesses, but deployments that listen on HTTP (not HTTPS) and use Basic Auth over shared or untrusted local networks are at risk when ARP spoofing is possible. In such environments, the static mapping between IP and MAC can be poisoned so that the client sends frames to the attacker instead of the router or gateway. Because Fastapi routes typically validate credentials per request, an intercepted Authorization header can be reused while it remains valid. Compounding this, if session tokens, cookies, or API keys are also transmitted in headers without additional protections, the same ARP-spoofed path can expose multiple secrets.

Operational context matters: containers, cloud VPCs, and virtualized networking can alter ARP behavior, but misconfigured Layer 2 segments or flat networks increase exposure. middleBrick’s unauthenticated scan checks for weak transport configurations and flags missing encryption as a finding; it tests whether an endpoint exposes credentials or tokens over HTTP and surfaces the risk alongside related checks such as Data Exposure and Encryption. This helps identify environments where Basic Auth over HTTP is unsafe due to the practical risk of interception via ARP spoofing or other layer-2 attacks.

Note that ARP spoofing is a local-network threat model; it does not directly exploit application code, but it interacts with transport choices. Fastapi applications should avoid Basic Auth over HTTP and instead use strong transport encryption and modern authentication mechanisms to reduce exposure. The scanner’s checks on Authentication, Data Exposure, and Encryption are designed to surface these classes of risk without making assumptions about internal infrastructure.

To mitigate ARp spoofing risks when using Fastapi, replace HTTP Basic Auth over plaintext HTTP with HTTPS and avoid sending credentials on each request when possible. Below are concrete remediation patterns, including secure Basic Auth usage over TLS and migration to token-based flows.

1. Basic Auth over HTTPS (minimum safe usage)

Ensure TLS termination at the load balancer or reverse proxy and configure Fastapi to require HTTPS. Even with Basic Auth, encryption in transit prevents ARP spoofing from revealing credentials. Use an explicit HTTPS redirect and secure cookies where applicable.

from fastapi import Fastapi, Depends, HTTPException, status
from fastapi.security import HTTPBasic, HTTPBasicCredentials
import uvicorn
from starlette.responses import RedirectResponse

app = Fastapi()
security = HTTPBasic()

def verify_credentials(credentials: HTTPBasicCredentials):
    # Replace with secure credential verification (e.g., constant-time compare)
    correct_username = "admin"
    correct_password = "S3cur3P@ss!"
    if credentials.username != correct_username or credentials.password != correct_password:
        return False
    return True

@app.get("/secure-endpoint")
def secure_route(credentials: HTTPBasicCredentials = Depends(security)):
    if not verify_credentials(credentials):
        raise HTTPException(
            status_code=status.HTTP_401_UNAUTHORIZED,
            detail="Invalid credentials",
            headers={"WWW-Authenticate": "Basic"},
        )
    return {"message": "Authenticated"}

# In production, terminate TLS at the proxy and use a redirect to HTTPS
@app.get("/insecure-redirect")
def redirect_to_https(): 
    return RedirectResponse(url="https://yourdomain.com/secure-endpoint", status_code=301)

if __name__ == "__main__":
    # For local testing only; use proper TLS termination in production
    uvicorn.run(app, host="0.0.0.0", port=8000)

2. Migrate to token-based authentication (recommended)

Use short-lived access tokens (e.g., JWT) over HTTPS and avoid sending passwords on every request. Pair with secure cookies and CSRF protections when using cookies.

from fastapi import Fastapi, Depends, HTTPException, status
from fastapi.security import HTTPBearer, HTTPAuthorizationCredentials
import jwt
from datetime import datetime, timedelta

SECRET_KEY = "your-secure-secret"
ALGORITHM = "HS256"
app = Fastapi()
security = HTTPBearer()

def create_token(username: str) -> str:
    payload = {
        "sub": username,
        "exp": datetime.utcnow() + timedelta(minutes=30),
        "iat": datetime.utcnow(),
    }
    return jwt.encode(payload, SECRET_KEY, algorithm=ALGORITHM)

def verify_token(credentials: HTTPAuthorizationCredentials = Depends(security)):
    try:
        payload = jwt.decode(credentials.credentials, SECRET_KEY, algorithms=[ALGORITHM])
        return payload.get("sub")
    except jwt.ExpiredSignatureError:
        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="Token expired")
    except jwt.InvalidTokenError:
        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="Invalid token")

@app.get("/token-secure")
def token_route(user: str = Depends(verify_token)):
    return {"message": f"Hello {user}"}

3. Defense-in-depth measures

• Use HTTP Strict Transport Security (HSTS) headers when serving HTTPS.
• Set the Secure and HttpOnly flags on any cookies used for session management.
• Implement rate limiting to reduce brute-force opportunities around authentication endpoints.
• Apply input validation on credentials and avoid logging sensitive values.

middleBrick’s Pro plan supports continuous monitoring and CI/CD integration (GitHub Action) so that changes to authentication routes can be validated before deployment. Its scans include checks for Authentication weaknesses and Transport Encryption to highlight configurations where Basic Auth over HTTP could be exposed to ARP spoofing or other layer-2 attacks.