Stack Overflow in Flask with Hmac Signatures

A Stack Overflow in a Flask application that uses Hmac Signatures typically occurs when user-controlled data influences the size or depth of data structures (e.g., deeply nested JSON, large arrays, or recursive objects) before the request is authenticated. If the server deserializes this data to validate the Hmac Signature—such as hashing a canonical representation of a JSON payload—an attacker can craft a payload that causes excessive memory or CPU usage during deserialization or canonicalization, leading to a denial of service.

Consider a Flask route that expects a JSON body and an X-Signature header computed as Hmac-SHA256(secret, canonical_json). If the endpoint first parses the JSON and then canonicalizes it for signing, an attacker can submit deeply nested or very large JSON that triggers a stack overflow during parsing or canonicalization. Even if the Hmac verification itself is implemented safely, the preprocessing step becomes the attack surface.

When OpenAPI specs are involved, recursive or oneOf/anyOf definitions can amplify this risk: an unauthenticated scan can detect schema constructs that enable deeply nested payloads. middleBrick’s Input Validation and Property Authorization checks identify such risky structures in the spec and runtime behavior, highlighting where uncontrolled nesting may lead to resource exhaustion before signature validation occurs.

Because the attack is unauthenticated, middleBrick’s black-box scanning can safely probe these scenarios. The LLM/AI Security module further ensures that system prompts used for API documentation or schema generation are not leaked during probing, which could otherwise aid an attacker in refining payloads. Note that middleBrick detects and reports these conditions but does not fix or block them; it provides prioritized findings with severity and remediation guidance.

In practice, this combination does not introduce a new vulnerability class but exposes existing risks more readily: Flask apps that deserialize user input before Hmac verification can suffer from Stack Overflow–style DoS when input validation and schema constraints are weak. Using middleBrick’s CLI (middlebrick scan <url>) or GitHub Action helps surface these issues in CI/CD before deployment.

To mitigate Stack Overflow risks when using Hmac Signatures in Flask, ensure input is validated and bounded before any deserialization or canonicalization used for signing. Use strict schema validation and size limits, and avoid processing untrusted data in ways that can grow unboundedly.

Example: Safe Hmac verification with bounded JSON parsing in Flask

import json
import hmac
import hashlib
from flask import Flask, request, abort, jsonify

app = Flask(__name__)
SECRET = b'super-secret-key'
MAX_JSON_LENGTH = 1024 * 128  # 128 KiB limit

def verify_signature(data: bytes, signature: str) -> bool:
    expected = hmac.new(SECRET, data, hashlib.sha256).hexdigest()
    return hmac.compare_digest(expected, signature)

@app.route('/api/submit', methods=['POST'])
def submit():
    sig = request.headers.get('X-Signature')
    if not sig:
        abort(400, description='Missing signature')
    # Limit payload size before parsing to prevent resource exhaustion
    if len(request.data) > MAX_JSON_LENGTH:
        abort(413, description='Payload too large')
    try:
        payload = json.loads(request.data)
    except json.JSONDecodeError:
        abort(400, description='Invalid JSON')
    # Canonicalize safely using the parsed object; ensure no uncontrolled nesting
    canonical = json.dumps(payload, separators=(',', ':'), sort_keys=True)
    if not verify_signature(canonical.encode('utf-8'), sig):
        abort(401, description='Invalid signature')
    # Process payload...
    return jsonify({'status': 'ok'})

Remediation checklist

• Set a maximum request body size at the Flask app or WSGI level to prevent large payloads.
• Validate and constrain JSON schema before canonicalization; reject deeply nested structures or arrays exceeding reasonable limits.
• Use json.dumps(..., separators=(',', ':'), sort_keys=True) for deterministic canonicalization to avoid signature mismatches.
• Always use hmac.compare_digest to prevent timing attacks during signature verification.
• If using OpenAPI specs, ensure recursive models are reviewed and constrained; middleBrick’s Pro plan provides continuous monitoring for such risky definitions across your API inventory.

For teams with many endpoints, the middleBrick CLI (middlebrick scan <url>) or GitHub Action can be integrated to fail builds when risky input validation patterns are detected, complementing these code-level fixes.