Vulnerability assessment scans can have two approaches on an asset: null session or authenticated with credentials. Each approach provides a unique view of the vulnerabilities present on an asset, so organizations should choose carefully which one they use (or both) to assess and measure for risks. In many cases, regulatory compliance, like PCI DSS, requires authenticated scans inside the perimeter, but null session is acceptable for public-facing resources as a part of certification. With these in mind, consider how each one works and what risks will be captured with each – there is a huge difference.
Null Session Assessments
Null session (with no credentials) vulnerability assessments will provide administrators an anonymous perspective of the risk profile for an asset. It is akin to a threat actor targeting an asset from the network with no foreknowledge of the resource. While this is valuable, it will only document vulnerability findings from a network or network-facing application perspective. Any vulnerability that does not have a network-facing service will be undetectable. If you consider that a modern scanner can detect less than 5% of an asset’s vulnerabilities with a null session scan, the results will be disturbingly skewed from the actual risks if this is the sole source of information.
It is therefore only recommended to run a null session scan to get a hacker’s perspective of resources and privileged assessments for dictating actual remediation workflows; for example, which external resources or services are susceptible to WannaCry. Null session scans will be a subset of these scan results anyways; barring any false positives. With these characteristics in mind, here are a few best practices for null session scans:
- Targeting external assets with null session scans forms the basis for regulatory compliance initiatives like PCI DSS
- Null session scans can help find vulnerable assets quickly that are remote-susceptible to worms and certain bots
- Null session scans are much faster than their credentialed counterparts since they will only apply a portion of the vulnerability signature database to a scan job
- Null session scans are ideal for identifying rogue assets and rogue network services and applications like FTP, SMTP, VNC, RDP, etc
Credentialed (Authenticated) Assessments
Credentialed scans, regardless of target platform or application, will provide the most accurate and best-detailed vulnerability assessment results. These scans provide the ability to log in as an administrator, root, or root equivalent, to interrogate the entire operating system, registry, file system, ports, processes, services, and users for vulnerabilities. As discussed previously, using credentials requires a target to allow remote authentication and unrestricted access. If you plan to use credentials, there are some best practices you should follow for assessing resources:
- Use a dedicated privileged credential for vulnerability assessments. This account should not be shared with any interactive users or services.
- Monitor for privileged activity using dedicated assessment credentials and escalate if they are being used outside of scan windows.
- Ensure pre-requisites are met for remote access. If they fail, review assessment report findings to determine which services, like remote registry access, are not enabled for a target.
- If hosts are typically hardened and do not allow remote access, consider using one (or all) of the following techniques to get privileged access:
- Install, enable, or configure a second management network that has the proper services enabled for a credentialed scan. This management network should have strong access control lists prohibiting any forward-facing access or network bridging.
- Enable settings or group policy that allows for credentialed scans on a time basis and reference a scan window.
- Consider alternative approaches for a credential scan using local or dissolvable agents.
- Clone the environment, exactly. This is highly viable for virtualized environments where images can be cloned to a lab, unhardened, and a credentialed assessment performed. This approach is common in mission-critical high-availability environments and sensitive government installations.
Credentialed Assessments Preferred – BeyondtTrust Can Help with Fully-Automated Credentialed Scanning
Based on the necessity to measure risk, all resources should be subject to a credentialed assessment at some time, not just servers nor assets with crown jewels. Hopefully, we have already made this case based on ransomware attacks and threats to infrastructure, cloud, and IoT. If a team tries to justify why they should not be subjected to some form credentialed assessment and only rely on null session assessments, they are wrong and the risks should be explained to them. This is true for even regulatory scans. Assess them with null session from the Internet but perform credentialed scans on them internally. That way, you can truly measure your risk.
Retina and BeyondSaaS can perform internal and external vulnerability assessments for your organization and provide the reports and mitigation guidance to ensure you always stay protected.
