CVE-2020-37168: Systempay Weak Crypto Allows Payment Signature Forgery

Executive Summary

A critical cryptographic vulnerability (CVE-2020-37168, CVSS 9.8) has been disclosed in Systempay 1.0, an ecommerce payment integration plugin. The flaw stems from the use of an insufficiently short 16-character production secret key for generating payment signatures, which can be brute forced by an attacker from data intercepted in POST requests to the payment endpoint.

A successful exploit allows an attacker to forge valid payment signatures, potentially enabling unauthorized transaction approvals, order manipulation, and payment gateway abuse. Organizations using the affected version of Systempay should patch or rotate keys immediately.

Vulnerability Overview

Affected Products

Technical Analysis

Root Cause

Systempay 1.0 uses a production secret key to generate HMAC-style signatures for payment requests submitted via POST. The key is only 16 characters in length, which provides insufficient entropy against modern brute-force techniques.

Because the payment form data and signatures are transmitted in observable POST requests — and because the signing algorithm is known — an attacker in a position to observe or intercept these requests can:

1. Capture a legitimate payment POST request containing form data and its computed signature
2. Recover the production secret key by systematically brute-forcing all possible 16-character key values until a match is found
3. Use the recovered key to generate arbitrary valid signatures for crafted payment payloads

Attack Flow

1. Attacker intercepts or observes a legitimate payment POST request
   (via network interception, compromised endpoint, or server log access)
2. Attacker extracts:
   - Payment form field values (amount, order ID, merchant ID, etc.)
   - Computed HMAC signature included in the request
3. Attacker brute-forces the 16-character key space offline:
   - Computes candidate signatures for all possible key values
   - Compares against observed signature until match found
4. Attacker now holds the valid production secret key
5. Attacker crafts arbitrary payment requests with forged valid signatures:
   - Manipulates amounts, order references, recipient accounts
   - Forged signatures pass server-side validation
6. Payment gateway processes attacker-controlled transactions as legitimate

Why 16 Characters is Insufficient

A 16-character alphanumeric key (62 possible characters per position) provides approximately 2^95 combinations — in theory. However, real-world key space is typically far smaller due to:

• Keys generated from predictable patterns or partial passphrases
• Use of lowercase-only or numeric-only subsets
• Key reuse across environments
• Modern GPUs capable of billions of hash evaluations per second

In practice, targeted brute-force against real-world generated keys of this length is feasible within hours to days using commodity hardware.

Impact Assessment

Immediate Remediation

Step 1: Rotate the Production Secret Key

Immediately generate a new production secret key of sufficient length and entropy. Best practices require at least 256 bits (32+ bytes) of cryptographically random material:

# Generate a cryptographically secure 32-byte key (256-bit)
openssl rand -hex 32
 
# Or base64-encoded
openssl rand -base64 32

Update the key in all Systempay configuration files and redeploy.

Step 2: Audit Transaction Logs

Review all recent payment transactions for anomalies:

- Unusually small or zero-value payments
- Duplicate order IDs with different amounts
- Transactions from unexpected IP ranges
- Orders completed without matching user session data

Step 3: Notify Your Payment Provider

If Systempay is integrated with a payment gateway (e.g., Lyra/Systempay by BNP Paribas), notify the provider of a potential key compromise and request a full audit of recent transactions.

Step 4: Apply Vendor Patch

Check with the Systempay plugin vendor for an updated release that uses a stronger key generation mechanism. If no patch is available:

• Replace the plugin with a patched or alternative solution
• Implement server-side signature validation with a stronger algorithm (e.g., HMAC-SHA256 with 256-bit keys)

Detection Indicators

Post-Remediation Checklist

1. Rotate the production secret key to a cryptographically secure 256-bit value
2. Audit all transactions from the past 90 days for anomalies
3. Notify payment gateway provider of potential compromise
4. Apply vendor patch or replace plugin with a secure alternative
5. Implement HMAC-SHA256 with adequate key length for all payment signatures
6. Deploy rate limiting and anomaly detection on the payment POST endpoint
7. Enable TLS everywhere to prevent passive interception of payment data
8. Monitor for unusual transaction patterns post-remediation

References

• NVD — CVE-2020-37168
• CWE-326 — Inadequate Encryption Strength
• OWASP — Cryptographic Failures
• PCI DSS — Protect Cardholder Data