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Guidelines
Risk
Likelihood Indicators
A model for determining how security controls affect risk

The goal of this document is to describe a methodology for determining what effect on the likelihood component of risk a missing security control will have

Overview

Service risk is composed both of the impact when a risk is manifested as well as the likelihood that the risk will manifest. Impact can be assessed in a Rapid Risk Assessment and is primarily based on the data which the service handles. Likelihood on the other hand is primarily driven by the presence or absence of security controls in the service.

What follows is a methodology for associating likelihood indicators with security controls.

Meaning of the likelihood indicators

The likelihood indicator for a given security control is the likelihood that a vulnerability in the service will be exploited in a calendar year due to the absence of the security control.

The indicators use the Standard levels reference and can be translated as:

  • LOW likelihood : The absence of this security control is unlikely to cause a risk to manifest. It may cause security incident response to be slower or more difficult. This causes HIGH and MAXIMUM impacts to result in MEDIUM risk.
  • MEDIUM likelihood : The absence of this security control may cause a risk to manifest in the coming year. This security control is important but with additional supporting controls may not be required. This causes MAXIMUM impacts to result in HIGH risk.
  • HIGH likelihood : The absence of this security control will probably cause a risk to manifest in the coming year. This security control is important and should only be missing for LOW impact services. This causes MEDIUM and HIGH impacts to result in HIGH risk and MAXIMUM impacts to result in MAXIMUM risk.
  • MAXIMUM likelihood : The absence of this security control will cause a risk to manifest in the coming year. This security control is required. This causes MEDIUM impacts to result in HIGH risk. This causes HIGH and MAXIMUM impacts to result in MAXIMUM risk.

Determining the likelihood indicator for a security control

When determining the likelihood indicator for a security control consider

  • How easy is it for a threat agent to determine the existence of a vulnerability
  • How easy is it for a threat agent to determine that a control to protect against the vulnerability is missing
  • How easy is it for a threat agent to exploit a vulnerability in the service when that control is missing
  • To what degree do missing security controls make it easier to discover vulnerabilities in the service
  • Are there current ongoing attacks on other services which are protected by this security control
  • How often has the service suffered an incident due to this or this type of missing control in the past?

The Reverse Engineering Method

This method involves thinking of hypothetical data being protected by the security control, calculating the risks resulting from that data's impact level and the various possible likelihood indicator levels and looking for which resulting risk matches best.

LOW likelihood

Impact Risk
LOW LOW
MEDIUM LOW
HIGH LOW
MAXIMUM MEDIUM

MEDIUM likelihood

Impact Risk
LOW LOW
MEDIUM MEDIUM
HIGH MEDIUM
MAXIMUM HIGH

HIGH likelihood

Impact Risk
LOW MEDIUM
MEDIUM HIGH
HIGH HIGH
MAXIMUM MAXIMUM

MAXIMUM likelihood

Impact Risk
LOW MEDIUM
MEDIUM HIGH
HIGH MAXIMUM
MAXIMUM MAXIMUM

Communicating the likelihood indicator

Likelihood indicators should be sent in the Service Mapper format