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Introduction to Safe Coding

At Google, we have developed a practical approach called "safe coding" to prevent security vulnerabilities. Safe coding is:

  • Scalable – It eliminates entire classes of security vulnerabilities, such as cross-site scripting (XSS), which have traditionally been notoriously difficult to avoid in large-scale development1.
  • Proactive – It prevents vulnerabilities from ever being introduced during development.

The two elements of safe coding are:

Inherently safe APIs

Client code using these APIs can under no (reasonable) circumstances suffer from the vulnerability in question

Enforcing safe coding

Ensures that inherently safe APIs are used correctly and comprehensively throughout a code base

Benefits of safe coding

The benefits of the safe coding approach are twofold:

  • Confidence that a particular class of vulnerabilities does not exist in an application and will not be introduced as the code base evolves.
  • In some scenarios, a drastic reduction in actual bugs as safe coding style is adopted.

Confined, limited potential for bugs, and improved reviewability

Inherently safe APIs are designed to ensure that application code written on top of the API cannot have certain security vulnerabilities. For this reason, comprehensive use of safe APIs removes the potential for such bugs in application code. The only circumstances under which such a bug can occur, is if there is a flaw in the implementation of the API itself or of some other lower-level library or framework.

What does this mean for code reviews?

A code reviewer does not have to inspect application code in order to assess an application with respect to the vulnerabilities in question. Instead, the only code that needs to be reviewed are the implementations of the APIs (HTML template systems, etc), and any uses of reviewed exceptions of unsafe API usage.

Actual reduction in bugs

Adoption of "XSS-proof" safe APIs has resulted in a very significant reduction of actual bugs in large-scale applications. Several projects have refactored code and templates to adhere to an XSS-proof coding style, and noted a substantial drop in discovered XSS vulnerabilities which has held stable over some years. The few remaining vulnerabilities were not due to flaws in application code, but rather in low-level library and framework code.

Further reading

Footnotes

  1. In 2014, XSS accounted for ca. 65% of vulnerabilities reported through Google's Vulnerability Reward ("Bug Bounty") Program. ↩