Regex Dos in Flask with Hmac Signatures

Regex Denial-of-Service (Regex Dos) occurs when an attacker provides input that causes a regular expression to exhibit catastrophic backtracking, consuming excessive CPU time. In Flask applications that use Hmac Signatures for request authentication, combining regex-based validation with signature verification can amplify risk if untrusted data influences the regex pattern or is processed before signature validation is confirmed.

Consider a Flask route that extracts a token from a header or query parameter and then applies a regex to validate its format before verifying the Hmac Signature. If the regex is poorly constructed (e.g., using nested quantifiers on unbounded input) and the attacker can supply the token or parts of it, the regex evaluation may take prohibitively long. Even though the Hmac Signature check ultimately ensures message integrity, the server may still spend considerable time on the regex step before rejecting the request, enabling a denial-of-service vector.

A concrete scenario: a Flask application uses a regex to ensure a signature token matches ^[a-zA-Z0-9\-_]+$ and then computes an Hmac over the request payload. If the token contains a long repetitive string like A+ (e.g., A...A), a regex such as (a+)+ on user-controlled data can cause exponential backtracking. Although the Hmac verification would fail, the CPU cost is incurred during the regex phase. In more complex cases, the regex might be applied to header values or URL segments that the developer assumes are safe, while the Hmac Signature is computed over a broader set of parameters. If the regex is applied to attacker-influenced data, the DoS surface exists even when signatures protect integrity.

OpenAPI/Swagger spec analysis can surface paths where regex patterns are declared for parameters or request bodies. If those parameters are used in authentication logic alongside Hmac Signatures, the scan can flag regex complexity issues and remind developers to validate input before performing cryptographic work. This is important because the scan operates unauthenticated and tests the exposed attack surface, which can include endpoints where regex validation occurs prior to signature verification.

To mitigate, ensure regex patterns avoid nested quantifiers on untrusted input, keep patterns simple and bounded, and consider moving lightweight format checks before expensive Hmac computations only when safe. Prefer deterministic validation logic for known formats (e.g., fixed-length strings, base64url patterns without ambiguous quantifiers). Remember that middleBrick detects and reports these risks, providing remediation guidance, while tools like the CLI (middlebrick scan <url>) or Web Dashboard can track findings over time to reduce exposure.

Remediation focuses on safe regex design and secure Hmac verification patterns in Flask. Avoid regexes with nested quantifiers on untrusted input and ensure signature verification is performed using constant-time comparison to prevent side-channel leaks. Below are concrete, working examples demonstrating secure handling.

Example 1: Safe regex validation before Hmac verification

Use a simple, non-backtracking regex for format validation and compute the Hmac only if the format is acceptable. Keep the regex bounded and avoid user-controlled quantifiers.

import re
import hmac
import hashlib
from flask import Flask, request, abort

app = Flask(__name__)
SECRET_KEY = b'super-secret-key'

# Safe regex: no nested quantifiers, bounded character class
SAFE_TOKEN_REGEX = re.compile(r'^[A-Za-z0-9\-_]{1,128}$')

def verify_hmac_signature(data: bytes, signature: str) -> bool:
    expected = hmac.new(SECRET_KEY, data, hashlib.sha256).hexdigest()
    # Use hmac.compare_digest for constant-time comparison
    return hmac.compare_digest(expected, signature)

@app.route('/webhook')
def webhook():
    token = request.args.get('token')
    if not token:
        abort(400, 'Missing token')
    # Validate format first with a safe regex
    if not SAFE_TOKEN_REGEX.match(token):
        abort(400, 'Invalid token format')
    # Verify Hmac signature over the request body
    signature = request.headers.get('X-Signature')
    if not signature:
        abort(400, 'Missing signature')
    if not verify_hmac_signature(request.data, signature):
        abort(403, 'Invalid signature')
    return 'OK', 200

Example 2: Hmac verification without regex for simple tokens

If token format validation is unnecessary, skip regex entirely and verify the Hmac directly. This reduces complexity and eliminates Regex Dos risk.

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

app = Flask(__name__)
SECRET_KEY = b'super-secret-key'

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

@app.route('/api/action', methods=['POST'])
def api_action():
    signature = request.headers.get('X-Signature')
    if not signature:
        abort(400, 'Missing signature')
    if not verify_hmac_signature(request.data, signature):
        abort(403, 'Invalid signature')
    # Process the verified request
    return 'Accepted', 200

Best practices summary

• Design regex patterns to avoid nested quantifiers and unbounded repetition on untrusted input.
• Use constants for maximum lengths and character classes; prefer re.compile for reuse.
• Use hmac.compare_digest for signature comparison to prevent timing attacks.
• Consider whether regex validation is necessary; if the token is an Hmac, format checks can often be omitted.
• Apply these patterns across endpoints that use Hmac Signatures, and use the middleBrick Web Dashboard or CLI to monitor findings and ensure remediation guidance is followed.