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Microsoft Patterns and Practices : An overview of the Security Development Lifeycle

Pre-SDL Requirements: Security Training

SDL Practice 1: Training Requirements

All members of a software development team must receive appropriate training to stay informed about security basics and recent trends in security and privacy. Individuals in technical roles (developers, testers, and program managers) that are directly involved with the development of software programs must attend at least one unique security training class each year.

Basic software security training should cover foundational concepts such as:

  • Secure design, including the following topics:
    • Attack surface reduction
    • Defense in depth
    • Principle of least privilege
    • Secure defaults
    • Threat modeling, including the following topics:
      • Overview of threat modeling
      • Design implications of a threat model
      • Coding constraints based on a threat model
      • Secure coding, including the following topics:
        • Buffer overruns (for applications using C and C++)
        • Integer arithmetic errors (for applications using C and C++)
        • Cross-site scripting (for managed code and Web applications)
        • SQL injection (for managed code and Web applications)
        • Weak cryptography
        • Security testing, including the following topics:
          • Differences between security testing and functional testing
          • Risk assessment
          • Security testing methods
          • Privacy, including the following topics:
            • Types of privacy-sensitive data
            • Privacy design best practices
            • Risk assessment
            • Privacy development best practices
            • Privacy testing best practices

The preceding training establishes an adequate knowledge baseline for technical personnel. As time and resources permit, training in advanced concepts may be necessary. Examples include, but are not limited to, the following:

  • Advanced security design and architecture
  • Trusted user interface design
  • Security vulnerabilities in detail
  • Implementing custom threat mitigations

Phase One: Requirements

SDL Practice 2: Security Requirements

The need to consider security and privacy “up front” is a fundamental aspect of secure system development. The optimal point to define trustworthiness requirements for a software project is during the initial planning stages. This early definition of requirements allows development teams to identify key milestones and deliverables, and permits the integration of security and privacy in a way that minimizes any disruption to plans and schedules. Security and privacy requirements analysis is performed at project inception and includes specification of minimum security requirements for the application as it is designed to run in its planned operational environment and specification and deployment of a security vulnerability/work item tracking system.

SDL Practice 3: Quality Gates/Bug Bars

Quality gates and bug bars are used to establish minimum acceptable levels of security and privacy quality. Defining these criteria at the start of a project improves the understanding of risks associated with security issues and enables teams to identify and fix security bugs during development. A project team must negotiate quality gates (for example, all compiler warnings must be triaged and fixed prior to code check-in) for each development phase, and then have them approved by the security advisor, who may add project-specific clarifications and more stringent security requirements as appropriate. The project team must also illustrate compliance with the negotiated quality gates in order to complete the Final Security Review (FSR).

A bug bar is a quality gate that applies to the entire software development project. It is used to define the severity thresholds of security vulnerabilities—for example, no known vulnerabilities in the application with a “critical” or “important” rating at time of release. The bug bar, once set, should never be relaxed. A dynamic bug bar is a moving target that is likely to be poorly understood within the development organization.

SDL Practice 4: Security and Privacy Risk Assessment

Security risk assessments (SRAs) and privacy risk assessments (PRAs) are mandatory processes that identify functional aspects of the software that require deep review. Such assessments must include the following information:

  1. (Security) Which portions of the project will require threat models before release?
  2. (Security) Which portions of the project will require security design reviews before release?
  3. (Security) Which portions of the project (if any) will require penetration testing by a mutually agreed upon group that is external to the project team?
  4. (Security) Are there any additional testing or analysis requirements the security advisor deems necessary to mitigate security risks?
  5. (Security) What is the specific scope of the fuzz testing requirements?
  6. (Privacy) What is the Privacy Impact Rating? The answer to this question is based on the following guidelines:
  • P1 High      Privacy Risk. The feature, product, or service stores or transfers PII,      changes settings or file type associations, or installs software.
  • P2      Moderate Privacy Risk. The sole behavior that affects privacy in the      feature, product, or service is a one-time, user-initiated, anonymous data      transfer (for example, the user clicks on a link and the software goes out      to a Web site).
  • P3 Low      Privacy Risk. No behaviors exist within the feature, product, or service      that affect privacy. No anonymous or personal data is transferred, no PII      is stored on the machine, no settings are changed on the user’s behalf,      and no software is installed.

Phase Two: Design

SDL Practice 5: Design Requirements

The optimal time to influence a project’s design trustworthiness is early in its life cycle. It is critically important to consider security and privacy concerns carefully during the design phase. Mitigation of security and privacy issues is much less expensive when performed during the opening stages of a project life cycle. Project teams should refrain from the practice of “bolting on” security and privacy features and mitigations near the end of a project’s development.

A formal exception or bug     deferral method should be considered as part of any software development     process. Many applications are based on legacy designs and code, so it may     be necessary to defer certain security or privacy measures as a result of technical     constraints.

In addition, it is crucially important for project teams to understand the distinction between “secure features” and “security features.” It is quite possible to implement security features, which are in fact, insecure. Secure features are defined as features whose functionality is well engineered with respect to security, including rigorous validation of all data before processing or cryptographically robust implementation of libraries for cryptographic services. The term security features describes program functionality with security implications, such as Kerberos authentication or a firewall.

The design requirements activity contains a number of required actions. Examples include the creation of security and privacy design specifications, specification review, and specification of minimal cryptographic design requirements. Design specifications should describe security or privacy features that will be directly exposed to users, such as those that require user authentication to access specific data or user consent before use of a high-risk privacy feature. In addition, all design specifications should describe how to securely implement all functionality provided by a given feature or function. It’s a good practice to validate design specifications against the application’s functional specification. The functional specification should:

  • Accurately and completely describe the intended use of a feature or function.
  • Describe how to deploy the feature or function in a secure fashion.

SDL Practice 6: Attack Surface Reduction

Attack surface reduction is closely aligned with threat modeling, although it addresses security issues from a slightly different perspective. Attack surface reduction is a means of reducing risk by giving attackers less opportunity to exploit a potential weak spot or vulnerability. Attack surface reduction encompasses shutting off or restricting access to system services, applying the principle of least privilege, and employing layered defenses wherever possible.

SDL Practice 7: Threat Modeling

The preferred method for threat modeling is to use the SDL     Threat Modeling Tool. The SDL Threat Modeling Tool is based on the STRIDE threat classification taxonomy.

Threat modeling is used in environments where there is meaningful security risk. It is a practice that allows development teams to consider, document, and discuss the security implications of designs in the context of their planned operational environment and in a structured fashion. Threat modeling also allows consideration of security issues at the component or application level. Threat modeling is a team exercise, encompassing program/project managers, developers, and testers, and represents the primary security analysis task performed during the software design stage.

Phase Three: Implementation

SDL Practice 8: Use Approved Tools

All development teams should define and publish a list of approved tools and their associated security checks, such as compiler/linker options and warnings. This list should be approved by the security advisor for the project team. Generally speaking, development teams should strive to use the latest version of approved tools to take advantage of new security analysis functionality and protections.

SDL Practice 9: Deprecate Unsafe Functions

Many commonly used functions and APIs are not secure in the face of the current threat environment. Project teams should analyze all functions and APIs that will be used in conjunction with a software development project and prohibit those that are determined to be unsafe. Once the banned list is determined, project teams should use header files (such as banned.h and strsafe.h), newer compilers, or code scanning tools to check code (including legacy code where appropriate) for the existence of banned functions, and replace those banned functions with safer alternatives.


Generally speaking, development teams should decide the optimal frequency for performing static analysis – to     balance productivity with adequate security coverage.

SDL Practice 10: Static Analysis

Project teams should perform static analysis of source code. Static analysis of source code provides a scalable capability for security code review and can help ensure that secure coding policies are being followed. Static code analysis by itself is generally insufficient to replace a manual code review. The security team and security advisors should be aware of the strengths and weaknesses of static analysis tools and be prepared to augment static analysis tools with other tools or human review as appropriate.

Phase Four: Verification

SDL Practice 11: Dynamic Program Analysis

Run-time verification of software programs is necessary to ensure that a program’s functionality works as designed. This verification task should specify tools that monitor application behavior for memory corruption, user privilege issues, and other critical security problems. The SDL process uses run-time tools like AppVerifier, along with other techniques such as fuzz testing, to achieve desired levels of security test coverage.

SDL Practice 12: Fuzz Testing

Fuzz testing is a specialized form of dynamic analysis used to induce program failure by deliberately introducing malformed or random data to an application. The fuzz testing strategy is derived from the intended use of the application and the functional and design specifications for the application. The security advisor may require additional fuzz tests or increases in the scope and duration of fuzz testing.

SDL Practice 13: Threat Model and Attack Surface Review

It is common for an application to deviate significantly from the functional and design specifications created during the requirements and design phases of a software development project. Therefore, it is critical to re-review threat models and attack surface measurement of a given application when it is code complete. This review ensures that any design or implementation changes to the system have been accounted for, and that any new attack vectors created as a result of the changes have been reviewed and mitigated.

Phase Five: Release

SDL Practice 14: Incident Response Plan

Every software release subject to the requirements of the SDL must include an incident response plan. Even programs with no known vulnerabilities at the time of release can be subject to new threats that emerge over time. The incident response plan should include:

  • An identified sustained engineering (SE) team, or if the team is too small to have SE resources, an emergency response plan (ERP) that identifies the appropriate engineering, marketing, communications, and management staff to act as points of first contact in a security emergency.
  • On-call contacts with decision-making authority that are available 24 hours a day, seven days a week.
  • Security servicing plans for code inherited from other groups within the organization.
  • Security servicing plans for licensed third-party code, including file names, versions, source code, third-party contact information, and contractual permission to make changes (if appropriate).

SDL Practice 15: Final Security Review

The Final Security Review (FSR) is a deliberate examination of all the security activities performed on a software application prior to release. The FSR is performed by the security advisor with assistance from the regular development staff and the security and privacy team leads. The FSR is not a “penetrate and patch” exercise, nor is it a chance to perform security activities that were previously ignored or forgotten. The FSR usually includes an examination of threat models, exception requests, tool output, and performance against the previously determined quality gates or bug bars. The FSR results in one of three different outcomes:

  • Passed FSR. All security and privacy issues identified by the FSR process are fixed or mitigated.
  • Passed FSR with exceptions. All security and privacy issues identified by the FSR process are fixed or mitigated and/or all exceptions are satisfactorily resolved. Those issues that cannot be addressed (for example, vulnerabilities posed by legacy “design level” issues) are logged and corrected in the next release.
  • FSR with escalation. If a team does not meet all SDL requirements and the security advisor and the product team cannot reach an acceptable compromise, the security advisor cannot approve the project, and the project cannot be released. Teams must either address whatever SDL requirements that they can prior to launch or escalate to executive management for a decision.

SDL Practice 16: Release/Archive

Software release to manufacturing (RTM) or release to Web (RTW) is conditional on completion of the SDL process. The security advisor assigned to the release must certify (using the FSR and other data) that the project team has satisfied security requirements. Similarly, for all products that have at least one component with a Privacy Impact Rating of P1, the project’s privacy advisor must certify that the project team has satisfied the privacy requirements before the software can be shipped.

In addition, all pertinent information and data must be archived to allow for post-release servicing of the software. This includes all specifications, source code, binaries, private symbols, threat models, documentation, emergency response plans, license and servicing terms for any third-party software and any other data necessary to perform post-release servicing tasks.

Optional Security Activities

Optional security activities are generally performed when a software application is likely to be used in critical environments or scenarios. They are often specified by a security advisor as part of a negotiated set of additional requirements to ensure a greater level of security analysis for certain software components. The practices in this section provide examples of optional security tasks and should not be considered an exhaustive list.

Manual Code Review

Manual code review is an optional task in the SDL and is usually performed by highly skilled individuals on the application security team and/or the security advisor. While analysis tools can do much of the work of finding and flagging vulnerabilities, they are not perfect. As a result, manual code review is usually focused on the “critical” components of an application. Most often it is used where sensitive data, such as personally identifiable information (PII), is processed or stored. It is also used to examine other critical functionality such as cryptographic implementations.


Penetration Testing

Any issues identified during penetration     testing must be addressed and resolved before the project is approved for     release.

Penetration testing is a white box security analysis of a software system performed by skilled security professionals simulating the actions of a hacker. The objective of a penetration test is to uncover potential vulnerabilities resulting from coding errors, system configuration faults, or other operational deployment weaknesses. Penetration tests are often performed in conjunction with automated and manual code reviews to provide a greater level of analysis than would ordinarily be possible.

Vulnerability Analysis of Similar Applications

Many reputable sources of information about software vulnerabilities can be found on the Internet. In some cases, the analysis of vulnerabilities found in analogous software applications can shed light on potential design or implementation issues in software under development.

Other Process Requirements

Root Cause Analysis

While not traditionally a part of the software development process, root cause analysis plays an important part in ensuring software security. Upon discovery of a previously unknown vulnerability, an investigation should be performed to ascertain precisely where the security processes failed. These vulnerabilities can be attributed to a variety of causes, including human error, tool failure, and policy failure. The goal of root cause analysis is to understand the precise nature of the failure. This information helps to ensure that errors of the same type are accounted for in future revisions of the SDL.

Periodic Process Updates

Software threats are not static. As a result, the process used to secure software cannot be static. Organizations should take the knowledge learned from practices such as root cause analysis, policy changes, and improvements in technology and automation, and apply them to the SDL on a predictable schedule. Generally speaking, a yearly update schedule should suffice. The exception to this rule is when new, previously unknown vulnerability types are identified. This phenomenon requires immediate, out-of-cycle revision of the SDL to ensure proper mitigations are in place going forward.

Application Security Verification Process

Organizations developing secure software will naturally want a means to verify that the processes outlined in the Microsoft SDL have been followed. Access to centralized development and test data helps decision-making in a number of important scenarios, such as the Final Security Review, SDL requirement exception handling, and security audits. The process of verifying application security involves a number of different processes and actors:

  • A specially designated application should be used to track compliance with the SDL. This application serves as the central repository for all SDL process artifacts, including (but not limited to) design and implementation notes, threat models, tool log uploads, and other process attestations. As with any critical application, it should use access controls to ensure:
  • Only authorized personnel can use the application.
  • Strong separation between roles. For example, a developer may be able to use the application and upload data, but should be prohibited from accessing functionality reserved for the security and privacy advisors, security team leads, and testers.
    • The security and privacy team leads are responsible for ensuring that the data necessary for an objective judgment is properly categorized and entered into the tracking application.
    • The information entered into the tracking application is used by the security and privacy advisors to provide the analysis framework for the Final Security Review.
    • The security and privacy advisors are responsible for reviewing the data entered into the tracking application (including the FSR results and other additional security tasks assigned by the advisors) and certifying that all requirements are met and/or all exceptions are satisfactorily resolved.

This document focuses on the Advanced level of the SDL Optimization Model, where rudimentary tracking processes are (in most instances) insufficient to the task. However, organizations with less sophisticated processes or smaller resource pools—those who fit within the Basic or Standardized levels of the SDL Optimization Model—can likely make do with a simpler tracking process.

It is very important that the tracking and verification process accurately capture:

  • The security and privacy requirements of the organization (for example, no known critical vulnerabilities at release).
  • The functional and technical requirements of the application under development.
  • The application’s operational context.

For example, if a development team creates a process control application to run in a critical environment, the proper investment of time and resources must be allocated to the creation and maintenance of the tracking process to enable objective analyses by the organization’s security and privacy principals, executive leadership, and relevant third parties such as compliance auditors or evaluators.

Put differently, skimping on the tracking process inevitably leads to problems later, usually during an emergency. Ensure that reliable systems are in place to answer critical questions at critical times.

Resources :

Home page of the SDL – http://www.microsoft.com/security/sdl/default.aspx

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