Web filtering

What the requirement asks

The organization is to manage which external websites can be reached, so that exposure to malicious content falls. Purpose: keep malware from compromising systems and stop people reaching web resources they are not authorized to use. Guidance: reduce the risk of personnel visiting sites with illegal content or known to host viruses or phishing, for example by blocking the IP addresses or domains concerned, which some browsers and anti-malware tools do automatically or can be set to do. Identify which kinds of website personnel should and should not reach, and consider blocking sites with upload functions unless there is a valid business reason, known or suspected malicious sites distributing malware or phishing, command and control servers, malicious sites identified through threat intelligence (5.7), and sites sharing illegal content. Before deploying filtering, set rules on using online resources safely and properly, including restrictions on undesirable or inappropriate sites and web applications, and keep them current. Train personnel to use online resources safely and properly, covering the rules, the contact point for security concerns and the exception process for reaching restricted resources for legitimate business reasons, and on not overriding browser warnings that a site is insecure. Other information: web filtering can use signatures, heuristics, lists of allowed or prohibited sites or domains and bespoke configuration to keep malicious software and activity off the network and systems. In the ISMS the control is compared with the controls determined in risk treatment and recorded in the Statement of Applicability as included or excluded, with the reason and the implementation status.

What the guidance adds

Web filtering is a control with a user impact. The guidance asks for the category, the block, the exception and the review.

What good looks like

Put the control in the Statement of Applicability with an owner, a status and the risk it treats. Then keep current rules on safe, proper use of online resources and one live sample together.

Evidence an assessor asks for

• Web filteringCurrent rules on safe, proper use of online resources
• Web filteringWeb filtering or secure web gateway configuration showing blocked categories such as malicious, phishing, command and control, illegal content and upload sites
• Web filteringIntegration of threat intelligence feeds into block lists
• Web filteringThe exception request and approval process with records of approved exceptions
• Web filteringTraining records covering web use rules, reporting contacts, exceptions and not overriding browser security warnings
• Web filteringCoverage evidence that filtering applies to remote and roaming devices as well as on-site traffic

The question an assessor will ask

An assessor will ask for current rules on safe, proper use of online resources, then trace it to the system it protects. For A.8.23 the test is whether that record ties to a live sample, not whether the control is named in a policy.

Common gaps

• Filtering applies only on the office network, not to remote workers
• Upload and file-sharing sites are open to all users without a business reason
• Users routinely click through browser security warnings
• Block lists are static and not updated from threat intelligence

Before the next audit

Before the next audit, check one live sample for this gap: filtering applies only on the office network, not to remote workers.