# What is Content Filtering?

> Content filtering is the policy-based inspection of application data or user-requested material to decide whether it should be allowed, blocked, quarantined, transformed, warned about, or recorded.

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## Content

It can be applied to web responses, email bodies and attachments, uploads, downloads, messaging, or other supported content at gateways, services, applications, and endpoints.

Decisions may use file type and structure, keywords, labels, malware scan results, data classification, reputation, or user and device context. Effective controls define the content and actions in scope, place enforcement where it has adequate visibility, and provide review paths for consequential decisions.

### Key points

- **Policy design:** Match rules and actions to data sensitivity, user role, destination, legal requirements, and business purpose instead of treating every content match alike.
- **Enforcement coverage:** Coordinate gateway, application, cloud-service, and endpoint controls so uploads, downloads, encrypted sessions, and unmanaged routes do not create unexplained gaps.
- **Privacy and resilience:** Minimize inspected and retained data, restrict access to inspection records, use Transport Layer Security (TLS) interception only when justified after evaluating alternatives and its trust, privacy, and failure effects, and define fail-open or fail-closed behavior.
- **Important limitation:** Classification and detection produce false positives and false negatives; encryption, encoding, transformations, and unsupported protocols reduce visibility. Allowed content is not necessarily safe, and blocked content is not necessarily malicious.

### Related terms

[Uniform Resource Locator (URL) filtering](<https://yellowcube.eu/glossary/uniform-resource-locator-filtering/>) · [Secure web gateway (SWG)](<https://yellowcube.eu/glossary/secure-web-gateway/>) · [Deep packet inspection (DPI)](<https://yellowcube.eu/glossary/deep-packet-inspection/>) · [Data loss prevention (DLP)](<https://yellowcube.eu/glossary/data-loss-prevention/>) · [Malware](<https://yellowcube.eu/glossary/malware/>) · [Spam filtering](<https://yellowcube.eu/glossary/spam-filtering/>)

### Sources

[NIST SP 800-215, Guide to a Secure Enterprise Network Landscape](https://csrc.nist.gov/pubs/sp/800/215/final) · [CISA, Continuous Diagnostics and Mitigation Program Technical Capabilities Volume Two: Requirements Catalog, Version 2.5](https://www.cisa.gov/sites/default/files/2023-08/CDM_Tech%20Volume2_v2.5.pdf) · [UK NCSC, What to do before building a ZTNA architecture](https://www.ncsc.gov.uk/collection/zero-trust/zero-trust-network-access-ztna/what-to-do-before-building-a-ztna-architecture)

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