Password Spraying in Fastapi with Hmac Signatures

Password spraying is an authentication attack where an adversary uses a small number of common passwords against many accounts to avoid account lockouts. When Fastapi endpoints rely on Hmac Signatures for request authentication without additional protections, the attack surface can enable password spraying against the user identifier or username portion of the signed payload.

In a typical Fastapi implementation using Hmac Signatures, the client computes a signature over a canonical string that often includes a user identifier (such as username or user ID), a timestamp, and a secret key. The server recomputes the signature using the stored secret for that user and compares it. If the endpoint does not enforce strict rate limiting or account enumeration defenses, an attacker can iterate over many usernames while keeping each password attempt constant, effectively performing a password spray across accounts.

Because Hmac Signatures provide integrity and authenticity of the request, the attacker can still submit validly signed requests for different usernames with the same password guess. Without protections like per-user rate limiting, request throttling, or monotonic nonce/timestamp validation, the server may process these requests independently, allowing the attacker to test credentials across the user base without triggering account lockout on any single account.

Consider an endpoint that expects a JSON body with username, timestamp, and signature fields. If the server only validates the Hmac signature and does not correlate requests across users with the same password guess, a single password can be tried against many usernames within the time window where timestamp and nonce validation are lax. This behavior aligns with findings from authentication bypass and authorization flaws (BOLA/IDOR) and privilege escalation checks in scanner coverage, where logical flaws in authorization and authentication controls are prioritized.

To detect such patterns, scanning should include checks for missing per-user rate limiting, weak or missing nonce/Replay protection, and whether Hmac-based authentication leaks information about valid usernames through timing or error messages. The presence of unprotected endpoints using Hmac Signatures in Fastapi can significantly lower the security risk score if password spraying is feasible across the authenticated surface.

Remediation focuses on making password spraying harder by binding the Hmac signature to a per-user secret, enforcing strict replay protection, and applying rate limits per user and globally. Below are concrete Fastapi code examples that implement these measures.

First, ensure each user has a unique secret stored securely (for example, in a database). Use a constant-time comparison for signature verification to avoid timing leaks. Here is a minimal, realistic Fastapi route that validates Hmac Signatures with per-user secrets and nonce/timestamp checks:

import time
import hmac
import hashlib
import secrets
from fastapi import FastAPI, HTTPException, Header, status
from pydantic import BaseModel
import constant_time_compare  # hypothetical helper using hmac.compare_digest

app = FastAPI()

# Example user store; in production use a secure database with proper secret management
USERS = {
    "alice": {"password_hash": "<hash>", "api_secret": secrets.token_hex(32)},
    "bob": {"password_hash": "<hash>", "api_secret": secrets.token_hex(32)},
}

class SignedRequest(BaseModel):
    username: str
    timestamp: int
    nonce: str
    signature: str
    password: str

def verify_hmac_signature(payload: dict, provided_signature: str, secret: str) -> bool:
    canonical = f"{payload['username']}{payload['timestamp']}{payload['nonce']}{payload.get('extra', '')}"
    expected = hmac.new(secret.encode('utf-8'), canonical.encode('utf-8'), hashlib.sha256).hexdigest()
    return hmac.compare_digest(expected, provided_signature)

@app.post("/login")
async def login(request: SignedRequest, x_request_id: str = Header(None)):
    user = USERS.get(request.username)
    if not user:
        # Return a generic error to avoid username enumeration
        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="Invalid credentials")
    
    # Replay protection: ensure timestamp is within acceptable window (e.g., 5 minutes)
    now = int(time.time())
    if abs(now - request.timestamp) > 300:
        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="Request expired")
    
    # Verify Hmac signature using per-user secret
    if not verify_hmac_signature(request.dict(exclude={'password'}), request.signature, user['api_secret']):
        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="Invalid signature")
    
    # Constant-time password check (pseudo)
    if not constant_time_compare.check_password(request.password, user['password_hash']):
        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="Invalid credentials")
    
    # At this point, authentication is successful; issue session token...
    return {"status": "ok", "username": request.username}

Second, enforce per-user and global rate limits to mitigate spraying. Fastapi applications can integrate middleware or use a shared cache (e.g., Redis) to track attempts. Here is an example using a simple in-memory approach for illustration:

from fastapi.middleware import Middleware
from fastapi.middleware.base import BaseHTTPMiddleware
import time
from collections import defaultdict

class RateLimitMiddleware(BaseHTTPMiddleware):
    def __init__(self, app, max_attempts=5, window=60):
        super().__init__(app)
        self.max_attempts = max_attempts
        self.window = window
        self.attempts = defaultdict(list)  # key: username or IP

    async def dispatch(self, request, call_next):
        if request.url.path == "/login":
            # Extract identifier safely; in production use a consistent key
            username = request._body.decode('utf-8').split('"username":"')[1].split('"')[0] if 'username' in request._body.decode('utf-8') else request.client.host
            now = time.time()
            attempts = [t for t in self.attempts[username] if now - t < self.window]
            if len(attempts) >= self.max_attempts:
                raise HTTPException(status_code=status.HTTP_429_TOO_MANY_REQUESTS, detail="Rate limit exceeded")
            self.attempts[username].append(now)
        response = await call_next(request)
        return response

Finally, ensure that error messages do not reveal whether a username exists, and prefer constant-time comparison for password checks. Combine these measures with the platform’s continuous monitoring (available in the Pro plan) and CI/CD integration (GitHub Action) to fail builds if risk scores degrade due to exposed authentication logic. These Hmac Signatures-specific fixes reduce the feasibility of password spraying and align with scanner findings related to Authentication, BOLA/IDOR, and BFLA/Privilege Escalation checks.