1. Introduction
A Web Application Firewall (WAF) has been detected during a scan. A WAF protects web applications by monitoring and filtering HTTP(S) traffic using rules to block common attacks. This detection means the scanner’s requests may have been blocked, leading to incomplete vulnerability coverage. Impact on confidentiality is possible if a WAF rule incorrectly blocks legitimate access; integrity could be affected if malicious traffic bypasses the WAF; availability might be reduced due to false positives blocking users.
2. Technical Explanation
The presence of a WAF indicates that an application owner has implemented security controls to inspect incoming web requests. Exploitation isn’t directly related to the WAF itself, but rather to whether it’s correctly configured and allows traffic from vulnerability scanners. A typical scenario involves the scanner being blocked due to its IP address not being whitelisted in the WAF ruleset. For example, an attacker could attempt to exploit a known web application vulnerability while the WAF is misconfigured or bypassed.
- Root cause: The scanner’s traffic isn’t permitted by the WAF configuration.
- Exploit mechanism: An attacker sends malicious requests to the web application, potentially exploiting vulnerabilities that the WAF fails to detect due to incorrect rules or bypass techniques.
- Scope: Any web application protected by a WAF is affected.
3. Detection and Assessment
- Quick checks: Review vulnerability scan reports for errors like 403 Forbidden or connection timeouts.
- Scanning: Tenable Nessus and other scanners may report blocked requests during a web application scan. These are indicators, not definitive proof.
- Logs and evidence: Examine the WAF logs for entries related to the scanner’s IP address. Look for denied requests or blocked URLs.
# Example command placeholder:
# No specific command is available as this relies on reviewing scan/WAF logs.
4. Solution / Remediation Steps
Ensure the vulnerability scanner’s traffic is allowed by the WAF to get accurate results. These steps involve configuring the WAF and re-running the scan.
4.1 Preparation
- Dependencies: Access to the WAF management interface is required. A roll back plan involves restoring the previous WAF configuration.
- Change window: Coordinate with application owners if necessary, especially in production environments.
4.2 Implementation
- Step 1: Identify the IP address ranges used by your Tenable scanner. Refer to Tenable documentation for these ranges.
- Step 2: Log in to the WAF management interface.
- Step 3: Add rules to allow traffic from the identified scanner IP address ranges. The exact method varies depending on your WAF vendor.
- Step 4: Save and apply the new WAF configuration.
4.3 Config or Code Example
Before
# Example: No rule exists to allow scanner IP addresses.
# (This is illustrative, actual config varies by WAF)
After
# Example: Rule added to allow traffic from Tenable scanner IPs.
# Allow requests from 10.10.10.0/24
# (This is illustrative, actual config varies by WAF)
4.4 Security Practices Relevant to This Vulnerability
Practices that improve the overall security of your web application are relevant here.
- Practice 1: Least privilege – only allow necessary traffic through the WAF, reducing the attack surface.
- Practice 2: Input validation – ensure the WAF ruleset effectively blocks malicious input to prevent attacks like SQL injection and cross-site scripting.
4.5 Automation (Optional)
If your WAF supports API access, you could automate rule updates using scripts.
# Example: PowerShell script to update WAF rules via API (illustrative only).
# Requires knowledge of the specific WAF API and authentication methods.
# $wafApiUrl = "https://your-waf-api-url"
# $apiKey = "your-api-key"
# # Add scanner IPs to allowlist using API calls...
5. Verification / Validation
Confirm the fix by re-running the scan and checking for successful access. A smoke test verifies application functionality remains intact.
- Post-fix check: Review WAF logs to confirm traffic from scanner IPs is now allowed, with no blocked requests.
- Re-test: Re-run the vulnerability scan and verify that all previously blocked resources are now assessed.
- Smoke test: Access key application pages (e.g., login page, product listing) to ensure functionality isn’t impacted by the WAF changes.
# Example: Check WAF logs for allowed scanner IP addresses.
# grep "scanner_ip_address" /var/log/waf/access.log
6. Preventive Measures and Monitoring
Regularly review your WAF configuration to ensure it remains effective.
- Baselines: Update security baselines or policies to include approved scanner IP address ranges.
- Pipelines: Integrate WAF rule validation into CI/CD pipelines to prevent misconfigurations.
- Asset and patch process: Review WAF rules periodically (e.g., quarterly) to ensure they are up-to-date with current threat intelligence.
7. Risks, Side Effects, and Roll Back
Adding scanner IPs could potentially introduce a risk if those IPs were compromised. A roll back involves restoring the previous WAF configuration.
- Risk or side effect 1: Allowing scanner IPs increases the attack surface slightly. Mitigate by regularly reviewing allowed IP addresses and ensuring scanners are securely managed.
8. References and Resources
Link only to sources that match this exact vulnerability.
- Vendor advisory or bulletin: https://docs.tenable.com/vulnerability-management/Content/Settings/Sensors/CloudSensors.htm
- NVD or CVE entry: Not applicable, as this is a configuration issue not a specific vulnerability.
- Product or platform documentation relevant to the fix: Refer to your WAF vendor’s documentation for instructions on managing IP address allowlists.