Stack Overflow in Fastapi with Hmac Signatures

When a Fastapi service uses Hmac Signatures to authenticate requests but also supports high-throughput, public endpoints (such as a Stack Overflow–style question feed), the interaction between rate-limited public paths and Hmac verification can expose the unauthenticated attack surface. If the API exposes a read‑only listing endpoint without requiring a signature, an attacker can probe it at high volume to enumerate valid resources or infer rate‑limit behavior. Because middleBrick scans the unauthenticated attack surface, it can test how the endpoint behaves when called rapidly with varying parameters, revealing whether Stack Overflow–like query parameters (e.g., tag filters, pagination) are subject to Business Logic Abuse (BOLA/IDOR) or excessive data exposure.

Specifically, if the Fastapi route does not enforce strict input validation on query parameters and does not enforce rate limiting, an attacker can send many requests with crafted query strings to trigger server‑side issues or data leakage. middleBrick’s 12 checks run in parallel and include Rate Limiting, Input Validation, and BOLA/IDOR, so it can detect whether the Stack Overflow–style endpoint allows enumeration or reflects sensitive information in error messages. Even when Hmac Signatures protect write operations, a misconfigured public read path can become the weak link, and the scanner will flag findings such as missing authorization on listing endpoints or missing parameter sanitization.

In practice, a Fastapi endpoint like /questions that accepts tag, page, and sort should validate and scope access even when no authentication is required. If parameter handling is inconsistent with the application’s data model, this can lead to IDOR-like behavior or information exposure that middleBrick identifies through its Property Authorization and Data Exposure checks. The scanner also highlights whether responses contain excessive data (e.g., full user objects) when only a subset should be returned to unauthenticated clients.

To secure Fastapi endpoints that use Hmac Signatures while still supporting public read patterns (such as Stack Overflow–style listing), implement strict input validation, consistent parameter handling, and explicit allow‑lists. Below are concrete, working examples that demonstrate how to structure verification and validation so that unauthenticated endpoints remain safe and scan findings can be addressed.

Example 1: Hmac signature verification for authenticated writes

import time
import hashlib
import hmac
from fastapi import FastAPI, Header, HTTPException, status
from pydantic import BaseModel

app = FastAPI()

SHARED_SECRET = b"your-secure-secret-used-for-hmac"

def verify_hmac_signature(payload: bytes, signature_header: str) -> bool:
    """Verify that the signature matches the payload using Hmac-SHA256."""
    expected_signature = hmac.new(SHARED_SECRET, payload, hashlib.sha256).hexdigest()
    return hmac.compare_digest(expected_signature, signature_header)

@app.post("/submit")
async def submit_question(
    payload: bytes,
    x_signature: str = Header(..., alias="X-Signature")
):
    if not verify_hmac_signature(payload, x_signature):
        raise HTTPException(
            status_code=status.HTTP_401_UNAUTHORIZED,
            detail="Invalid signature"
        )
    # process the validated payload
    return {"status": "accepted"}

Example 2: Public read endpoint with strict input validation and no authentication

from fastapi import FastAPI, Query, HTTPException
from typing import Optional
import re

app = FastAPI()

# Allowlist tags to prevent injection via tag parameter
ALLOWED_TAGS = {"python", "javascript", "fastapi", "security", "api"}

def safe_tag_filter(tag: str) -> bool:
    if tag not in ALLOWED_TAGS:
        return False
    # Basic pattern safety: prevent unusual characters
    if re.search(r"[^\w\-\.]", tag):
        return False
    return True

@app.get("/questions")
async def list_questions(
    tag: Optional[str] = Query(None, description="Filter by tag"),
    page: int = Query(1, ge=1, description="Page number"),
    page_size: int = Query(10, ge=1, le=100, description="Page size")
):
    if tag is not None and not safe_tag_filter(tag):
        raise HTTPException(status_code=400, detail="Invalid tag")
    # Return a limited, safe response shape to avoid Data Exposure
    return {
        "page": page,
        "page_size": page_size,
        "results": []  # replace with safe, scoped data access
    }

Remediation checklist aligned with middleBrick findings

• Validate and normalize all query parameters; reject unexpected keys.
• Enforce allow‑lists for enumerations such as tags, status filters, and sort options.
• Apply consistent rate limiting on public endpoints to reduce enumeration risk.
• Ensure error messages do not leak stack traces or internal identifiers.
• Scope data access so that even public endpoints return only necessary fields (mitigates Data Exposure).
• Use hmac.compare_digest for signature verification to avoid timing attacks.

By combining Hmac‑protected write paths with rigorously validated public reads, you reduce the attack surface that middleBrick would surface under its BOLA/IDOR, Rate Limiting, and Data Exposure checks.