Arp Spoofing in Fastapi with Jwt Tokens

Arp spoofing is a Layer 2 attack where an attacker sends falsified Address Resolution Protocol messages on a local network to associate their MAC address with the IP address of a legitimate host, typically the gateway or another API server. In a Fastapi application that relies on JWT tokens for authentication, arp spoofing does not directly compromise the cryptographic integrity of the token, but it creates conditions that undermine the trust assumptions used by token-based flows.

When a client (for example, a browser or a mobile app) submits a JWT in an Authorization header to a Fastapi endpoint over HTTP (not HTTPS), an attacker on the same network can use arp spoofing to position themselves as the man-in-the-middle. The client may still send the JWT, but the attacker can intercept, inspect, and replay it. Even if the communication is HTTPS, arp spoofing can be used to redirect traffic to a malicious proxy that terminates TLS, allowing the attacker to perform SSL stripping or present a rogue certificate if client certificate validation is absent. In such scenarios, the JWT becomes exposed as it traverses the compromised path.

Fastapi applications often use JWT tokens for stateless session management, typically validating the signature and claims in middleware or dependency functions. If the deployment environment allows Layer 2 attacks (for example, shared or poorly segmented networks), arp spoofing exposes the token in transit. Attackers can capture tokens and reuse them (session hijacking) unless additional protections such as strict Transport Layer Security, token binding, or short expirations are in place. Moreover, if the Fastapi service accepts unauthenticated endpoints that return sensitive data before token validation, an attacker can chain arp spoofing with other request manipulations to escalate exposure.

The combination is particularly risky when development or staging environments skip HTTPS, rely on self-signed certificates, or do not enforce HSTS. In such cases, arp spoofing makes it trivial to capture JWTs and replay them against the Fastapi application. The scanner’s LLM/AI Security checks highlight unauthenticated LLM endpoints and unsafe consumption patterns that could further expose token handling logic, while the input validation and authentication checks surface weak points where token validation is incomplete.

middleBrick’s OpenAPI/Swagger spec analysis helps identify whether JWT validation is consistently applied across paths and whether definitions reference secure schemes. By correlating spec definitions with runtime behavior, the scan can flag missing security requirements and inconsistent use of securitySchemes that make token handling more vulnerable to network-level attacks like arp spoofing.

Remediation centers on ensuring JWT validation is robust, transport is secured, and tokens are never exposed in unsafe contexts. Below are concrete Fastapi code examples that implement strong JWT handling.

First, always use HTTPS in production and development. Enforce secure cookies and HSTS if cookies are used. For token-based APIs, require HTTPS for all endpoints.

from fastapi import Fastapi, Depends, HTTPException, status
from fastapi.security import HTTPBearer, HTTPAuthorizationCredentials
from jose import JWTError, jwt
from pydantic import BaseModel
import os

app = Fastapi()
security_scheme = HTTPBearer()

SECRET_KEY = os.getenv("JWT_SECRET_KEY", "super-secret-change-in-production")
ALGORITHM = "RS256"

class TokenData(BaseModel):
    sub: str | None = None

async def get_current_user(credentials: HTTPAuthorizationCredentials = Depends(security_scheme)):
    if credentials.scheme.lower() != "bearer":
        raise HTTPException(
            status_code=status.HTTP_401_UNAUTHORIZED,
            detail="Invalid authentication scheme",
            headers={"WWW-Authenticate": "Bearer"}
        )
    token = credentials.credentials
    try:
        payload = jwt.decode(token, SECRET_KEY, algorithms=[ALGORITHM])
        username: str = payload.get("sub")
        if username is None:
            raise HTTPException(status_code=401, detail="Invalid token payload")
        return {"username": username}
    except JWTError:
        raise HTTPException(status_code=401, detail="Invalid token")

@app.get("/secure-data")
async def read_secure_data(user: dict = Depends(get_current_user)):
    return {"message": f"Hello {user['username']}, this is protected"}

This example uses the HTTP Bearer scheme and validates the JWT on each request. It avoids accepting tokens from URL query parameters or non-Bearer schemes, reducing the risk of token leakage via logs or browser history.

Second, configure token expiration and revocation strategies. Short-lived access tokens paired with secure refresh token handling limit the window for replay after arp spoofing.

from datetime import datetime, timedelta
import pytz

def create_access_token(data: dict, expires_delta: timedelta | None = None):
    to_encode = data.copy()
    if expires_delta:
        expire = datetime.now(pytz.UTC) + expires_delta
    else:
        expire = datetime.now(pytz.UTC).replace(tzinfo=pytz.UTC) + timedelta(minutes=15)
    to_encode.update({"exp": expire})
    encoded = jwt.encode(to_encode, SECRET_KEY, algorithm=ALGORITHM)
    return encoded

Third, ensure your security scheme definition in OpenAPI matches the runtime enforcement. middleBrick’s spec analysis confirms that securitySchemes and path requirements are consistent, which helps prevent accidental token bypass.

Finally, adopt defense-in-depth: segment networks to reduce arp spoofing feasibility, enable HSTS, pin certificates where applicable, and monitor for anomalous token reuse patterns that may indicate interception. The Pro plan’s continuous monitoring can alert you if security scores degrade due to configuration drift.