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+# Security
+### Resources
+
+[Mozilla’s HTTP Observatory CLI][observatory-cli] and the
+[Qualys SSL Labs Server Test][qualys-ssl] are good resources for finding
+potential problems and ensuring compliance with security best practices.
+
+<!-- Uncomment these sections when CSP/SRI are implemented.
+### Content Security Policy (CSP)
+
+Content Security Policy is a web standard that intends to mitigate certain
+forms of Cross-Site Scripting (XSS) as well as data injection.
+
+Content Security Policy rules should be taken into consideration when
+implementing new features, especially those that may rely on connection with
+external services.
+
+GitLab's CSP is used for the following:
+
+- Blocking plugins like Flash and Silverlight from running at all on our pages.
+- Blocking the use of scripts and stylesheets downloaded from external sources.
+- Upgrading `http` requests to `https` when possible.
+- Preventing `iframe` elements from loading in most contexts.
+
+Some exceptions include:
+
+- Scripts from Google Analytics and Piwik if either is enabled.
+- Connecting with GitHub, Bitbucket, GitLab.com, etc. to allow project importing.
+- Connecting with Google, Twitter, GitHub, etc. to allow OAuth authentication.
+
+We use [the Secure Headers gem][secure_headers] to enable Content
+Security Policy headers in the GitLab Rails app.
+
+Some resources on implementing Content Security Policy:
+
+- [MDN Article on CSP][mdn-csp]
+- [GitHub’s CSP Journey on the GitHub Engineering Blog][github-eng-csp]
+- The Dropbox Engineering Blog's series on CSP: [1][dropbox-csp-1], [2][dropbox-csp-2], [3][dropbox-csp-3], [4][dropbox-csp-4]
+
+### Subresource Integrity (SRI)
+
+Subresource Integrity prevents malicious assets from being provided by a CDN by
+guaranteeing that the asset downloaded is identical to the asset the server
+is expecting.
+
+The Rails app generates a unique hash of the asset, which is used as the
+asset's `integrity` attribute. The browser generates the hash of the asset
+on-load and will reject the asset if the hashes do not match.
+
+All CSS and JavaScript assets should use Subresource Integrity.
+
+Some resources on implementing Subresource Integrity:
+
+- [MDN Article on SRI][mdn-sri]
+- [Subresource Integrity on the GitHub Engineering Blog][github-eng-sri]
+
+-->
+
+### Including external resources
+
+External fonts, CSS, and JavaScript should never be used with the exception of
+Google Analytics and Piwik - and only when the instance has enabled it. Assets
+should always be hosted and served locally from the GitLab instance. Embedded
+resources via `iframes` should never be used except in certain circumstances
+such as with ReCaptcha, which cannot be used without an `iframe`.
+
+### Avoiding inline scripts and styles
+
+In order to protect users from [XSS vulnerabilities][xss], we will disable
+inline scripts in the future using Content Security Policy.
+
+While inline scripts can be useful, they're also a security concern. If
+user-supplied content is unintentionally left un-sanitized, malicious users can
+inject scripts into the web app.
+
+Inline styles should be avoided in almost all cases, they should only be used
+when no alternatives can be found. This allows reusability of styles as well as
+readability.
+
+
+[observatory-cli]: https://github.com/mozilla/http-observatory-cli
+[qualys-ssl]: https://www.ssllabs.com/ssltest/analyze.html
+[secure_headers]: https://github.com/twitter/secureheaders
+[mdn-csp]: https://developer.mozilla.org/en-US/docs/Web/Security/CSP
+[github-eng-csp]: http://githubengineering.com/githubs-csp-journey/
+[dropbox-csp-1]: https://blogs.dropbox.com/tech/2015/09/on-csp-reporting-and-filtering/
+[dropbox-csp-2]: https://blogs.dropbox.com/tech/2015/09/unsafe-inline-and-nonce-deployment/
+[dropbox-csp-3]: https://blogs.dropbox.com/tech/2015/09/csp-the-unexpected-eval/
+[dropbox-csp-4]: https://blogs.dropbox.com/tech/2015/09/csp-third-party-integrations-and-privilege-separation/
+[mdn-sri]: https://developer.mozilla.org/en-US/docs/Web/Security/Subresource_Integrity
+[github-eng-sri]: http://githubengineering.com/subresource-integrity/
+[xss]: https://en.wikipedia.org/wiki/Cross-site_scripting