Guide: Setting up geo-blocking for specific attack vectors.

While geo-blocking is often dismissed as a blunt instrument, it remains a highly cost-effective and efficient first layer of defense when applied with surgical precision. The key is to move beyond the simplistic "block entire continents" approach and instead correlate geographic ingress points with specific, recurring attack patterns. This minimizes false positives and avoids blocking legitimate traffic from regions where you may have a user base.

Based on my analysis of firewall log patterns across several clients, I've identified common correlations. Implementing blocks at the country or autonomous system (AS) level for these scenarios can reduce noisy, resource-consuming attacks before they ever hit your core WAF rules.

* **Credential Stuffing & Layer 7 DDoS:** These attacks frequently originate from specific countries with high concentrations of compromised devices or booter services. If you have zero legitimate users in, for example, Vietnam or Egypt, and you see a massive spike in POST requests to `/login` and `/api/v1/auth` from these regions, a temporary or permanent country block is justifiable. The cost of processing these requests (compute cycles, WAF rule evaluations) far outweighs the benefit.
* **Scanner & Vulnerability Probe Traffic:** Aggressive, non-stealthy scanners often hail from a narrow set of ASNs. You can identify these by looking for user agents like `masscan` or `zgrab` combined with paths targeting `phpMyAdmin`, `wp-admin`, or known exploit endpoints. Blocking at the AS level here is more precise than a country block.
* **Bot-driven Carding Attacks:** E-commerce sites often see carding traffic originate from specific geographic clusters where stolen card details are aggregated. If you see a high volume of failed transactions on `/checkout` from a country you don't ship to, a geo-fence is a logical first step.

The implementation strategy within Imperva is straightforward but requires careful staging.

1. **Analyze:** Use the Imperva Security Analytics or raw logs to identify the top countries and ASNs for requests that result in specific security events (e.g., 403s, 400s from failed JSON parsing, POST floods). Don't guess; use data.
2. **Create a Custom Rule:** Navigate to the relevant security policy and create a new custom rule. Set the match condition to `Country` (or `ASN` if your plan supports it) and select the identified countries.
3. **Refine with Path/Endpoint:** Add a second, AND condition to target the specific attack vector. For example: `Country is in [List]` **AND** `Request URI Path contains "/api/login"`. This ensures you only block traffic targeting that vulnerable endpoint from those regions.
4. **Action:** Set the action to `Block` or `Challenge` (if you're being cautious). Always start with a `Challenge` (like a CAPTCHA) for a short period to monitor for false positives before moving to a hard block.
5. **Monitor & Iterate:** Review the rule's activity logs daily. Adjust the country list or targeted paths as the attack vectors shift. This is not a set-and-forget rule.

A critical financial consideration: while geo-blocking reduces attack traffic, remember that DDoS protection fees are often calculated on a per-cleaned-Gbps basis. If your primary motive is mitigating volumetric DDoS costs, geo-blocking at the edge can lead to direct savings by dropping traffic before it enters Imperva's scrubbing network. However, for application-layer attacks, the savings are more about preserving origin server capacity and reducing the computational cost of evaluating complex WAF rule sets against malicious traffic. Always model the cost of the attack traffic (in terms of compute, bandwidth, and WAF capacity units) against the risk of blocking a legitimate user.

-- Liam