safevalues

safevalues

Safevalues is a library to help you prevent Cross-Site Scripting vulnerabilities in TypeScript (and JavaScript). It is meant to be used together with tsec to provide strong security guarantees and help you deploy Trusted Types and other CSP restrictions in your applications. Google has used these components together to reduce DOM XSS (paper), and we hope it will be useful in your codebase.

Features

Policy definition for building safe-by-construction Trusted Types

Trusted Types is a browser API that enables developers to control the values that can be assigned to XSS sinks. Developers need to define a Trusted Type policy to build these values, and then the Trusted Type API constrains these policies.

The Trusted Types API is not opinionated on what should be considered safe. It only acts as a tool for developers to mark values they can trust.

safevalues in contrast, defines functions that make security decisions on what is safe (by construction, via escaping or sanitization), so that developers who are not security experts don't need to.

safevalues produces Trusted Types (through its own policy) when available.

Additional types and functions for sinks not covered by Trusted Types

Some DOM APIs are not covered by Trusted Types, but can also be abused; leading to XSS or other security issues. Alternative security mechanisms such as the unsafe-inline CSP protection can help to secure these APIs, but not all browsers or apps support them.

safevalues defines additional types, builders, and setters to help protect these sinks.

DOM sink wrappers

To build a Trusted Type-compatible app and surface potential violations at compile time, we recommend that you compile your code with tsec. tsec bans certain DOM APIs. safevalues defines wrappers around these APIs which lets you assign Trusted Types with them.

Some wrappers don't require a particular type, but sanitize the argument they get before they assign it to the DOM sink (e.g. safeLocation.setHref).

Trusted Type polyfills

Whenever possible, safevalues uses Trusted Types to build its values, in order to benefit from the runtime protection of Trusted Types. When Trusted Types is not available, safevalues transparently defines its own types and your app will continue to work.

Known issues

ReferenceError: Can't find variable: process

When using a bundler that performs dead-code elimination, you must ensure that process.env.NODE_ENV is declared globally with either a value of development or production. This is done in Webpack by specifying a mode, in Terser using the --define flag and in Rollup using the rollup-plugin-define plugin. See (#212).

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Disclaimer

This is not an officially supported Google product.