Common Event FormatEdit

Common Event Format is a text-based log event format designed to normalize security-relevant events so diverse devices and applications can be ingested, parsed, and correlated by modern monitoring and defense tools. Originating with ArcSight and now widely supported across the security market, CEF aims to reduce the friction that comes with collecting logs from heterogeneous sources such as firewalls, intrusion detection systems, endpoints, and cloud services. The approach centers on a compact header that identifies the event class and a flexible extension section that carries key-value pairs describing the specifics of the event. See also Syslog and RFC 5424 for foundational logging concepts that CEF often sits alongside in deployments.

CEF is not a stand-alone data store; rather, it is a semi-structured, human-readable format that makes it easier for security information and event management (SIEM) systems to normalize and correlate events from multiple sources. The standard is widely used in environments where rapid threat detection and incident response are valued, and it is frequently parsed by dedicated platforms such as ArcSight products and other security analytics tools. In practice, analysts will often see CEF-formatted lines within a larger stream of logs that may include raw syslog messages, JSON payloads, or other vendor-specific formats. See also Security information and event management for the broader category of analytics built on this data.

Overview

CEF is designed around a two-part structure:

  • The header, which contains seven fields separated by vertical bars: Version, Device Vendor, Device Product, Device Version, Device Event Class ID, Name, and Severity.
  • The Extension, which is a flexible set of key=value pairs that carry the event’s details (for example, source IP, destination IP, username, protocol, and so on).

The standard line begins with a prefix that signals the format, for example: CEF:0|Vendor|Product|Version|DeviceEventClassID|Name|Severity|Extension. The Version field indicates the schema version in use, which helps receivers interpret the rest of the line consistently. The Device Event Class ID is a numeric or alphanumeric code that identifies the general category of event (for example, a login failure or a policy violation), while the Name provides a readable label for humans and machines. Severity is usually a numeric or mapped descriptor that signals the urgency or impact of the event.

Because the Extension carries free-form key=value pairs, the exact content can vary by device and vendor, but common keys include src (source address), dst (destination address), spt (source port), dpt (destination port), cs1, cs2, and other cs-labeled fields used for custom correlation. The combination of a stable header and a rich, extensible extension makes CEF useful for cross-vendor alerting and automated workflow.

CEF sits alongside more general logging standards. Many organizations route events into a central log system using Syslog as the transport, with CEF metadata embedded in the message body or carried in a dedicated field. In some setups, logs that originate from devices or apps that speak Syslog or RFC 5424 may be translated into CEF for consistency in a SIEM. See LEEF for a similar approach used by some platforms and RFC 5424 for a foundational syslog format.

History and development

CEF emerged in response to the fragmentation of security logs across disparate devices and platforms. By providing a common header and a flexible extension, it made it easier for security teams to write rules and alerts that could operate across heterogeneous data sources. ArcSight, a pioneer in SIEM technology, played a leading role in popularizing and promoting the format. Over time, other vendors and open-source communities adopted the approach or built compatible parsers, which helped broaden interoperability.

As the cyber security market evolved, the appeal of a widely adopted, vendor-friendly standard remained strong. That broad adoption supports quicker on-ramps for new devices and faster time-to-value for security operations centers, which is a practical advantage in markets that prize efficiency and predictable cost structures. See also Open standards and Vendor-neutral security standards for related discussions about how standards influence competition and innovation.

Structure and syntax details

  • Header fields:

    • Version: typically a numeric version of the CEF schema.
    • Device Vendor: the maker of the device or software generating the event.
    • Device Product: the specific product name.
    • Device Version: the product version.
    • Device Event Class ID: a code identifying the event type.
    • Name: a human-readable event name.
    • Severity: a numeric or descriptive indicator of impact or urgency.
  • Extension: a space-separated list of key=value pairs that carry the specifics of the event. Examples of common keys include:

    • src, dst: source and destination addresses
    • spt, dpt: source and destination ports
    • cs1, cs2, ... csN with cs1Label, cs2Label, etc., to describe custom attributes
    • act: action taken
    • dup: duplicate indicator
    • deviceSeverity or other vendor-specific fields

Because the Extension is flexible, the exact set of keys will vary by device and deployment. This flexibility is a strength for interoperability, but it can also create inconsistencies in how similar events are expressed across devices. Practitioners often rely on a catalog of known extensions to ensure that parsers and correlation rules map fields in a predictable way. See Extension (log data) and Event correlation for related concepts.

In practice, many teams use CEF as a way to get fast, cross-source visibility without needing to standardize every vendor’s internal logging model. The approach facilitates rule-based alerting, dashboards, and automated responses within SIEM platforms, while leaving room for device-specific data in the Extension.

Adoption and usage

CEF is widely used in both on-premises and cloud-based security operations. It is compatible with many leading SIEM products and can be parsed or converted by a variety of log collectors, forwarders, and analytics engines. Because the Extension is open-ended, teams can capture the detailed context their security policies require, while the header ensures a consistent starting point for parsing. Tools and integrations often provide built-in parsers or templates to map common device events into CEF fields, helping to reduce configuration time and human error.

The format’s ubiquity means it is frequently cited in incident response playbooks and threat intelligence feeds. It also plays a role in regulatory and compliance workflows where organizations need auditable, machine-readable evidence of security events. See Compliance (data protection) and Threat intelligence for related topics on governance and threat context.

Controversies and debates (from a market-driven perspective)

  • Interoperability versus vendor lock-in: Proponents argue that CEF’s widespread support lowers switching costs and promotes healthy competition by enabling organizations to standardize on a common data posture. Critics, however, point to the fact that it originated with a single vendor and remains heavily associated with proprietary tooling; large ecosystems still rely on vendor-specific extensions, which can create subtle lock-in if organizations lean too heavily on vendor-specific event attributes. See Vendor lock-in for a broader treatment of this dynamic.

  • Open standards versus proprietary extensions: The header provides a common structure, but the Extension allows vendor-specific, sometimes idiosyncratic fields. This tension—between a shared schema and flexible metadata—drives debate about whether CEF alone suffices for long-term interoperability or if teams should push toward more formal, open, fully machine-readable formats such as JSON-based schemas or event pipelines anchored in open standards. Compare with JSON logging and [OpenTelemetry] for alternative approaches.

  • Privacy and data minimization: Logs inevitably carry sensitive information, including IP addresses, usernames, and other identifiers. From a market-focused perspective, strong log governance and data minimization practices are essential to satisfy risk management goals and regulatory requirements (for example, GDPR or CCPA). Critics sometimes argue that centralized logging weights operational convenience over privacy controls, but compliance-minded shops aim to balance both through access controls, redaction, and lifecycle policies.

  • Regulation, governance, and standardization: Some observers prefer tighter, government-led standardization to ensure consistency across industries and jurisdictions. Advocates of more market-driven approaches contend that competition and innovation—rather than fiat regulation—drive better tools and more practical interoperability. The debate mirrors broader policy questions about how to best support robust cybersecurity without imposing undue burdens on innovation. See Regulatory compliance and Industry standards for related discussions.

  • Practicality and evolution: As ecosystems evolve toward cloud-native architectures and automation, many teams supplement or supplant CEF with newer formats or multi-format pipelines. While CEF remains useful for rapid integration and alerting, practitioners increasingly evaluate how CEF fits within a broader instrumentation strategy that may include JSON-based events, structured telemetry, and standardized schemas across services. See Cloud security and Telemetry for related topics.

See also