XSS prevention for Ruby on Rails

This is a cross-site scripting (XSS) prevention cheat sheet by Semgrep, Inc. It contains code patterns of potential XSS in an application. Instead of scrutinizing code for exploitable vulnerabilities, the recommendations in this cheat sheet pave a safe road for developers that mitigate the possibility of XSS in your code. By following these recommendations, you can be reasonably sure your code is free of XSS.

Mitigation summary

In general, always let Rails render ERB template files rather than constructing them in code. If HTML escaping is needed, use html_safe() in Ruby code and review each individual usage carefully. Once reviewed, mark the line with # nosem. Beware of putting data in dangerous locations in templates. And as always, run a security checker continuously on your code.

Check your project using Semgrep

The following command runs an optimized set of rules for your project:

semgrep --config p/default

1. Unescaped variable enters template engine in Ruby code

1.A. Using html_safe()

html_safe() marks the supplied string as "safe for HTML rendering." This bypasses HTML escaping and potentially creates XSS vulnerabilities.

Example:

html = "<div>#{name}</div>".html_safe

References

• Brakeman scanner - Cross-site scripting
• Preventing XSS in Ruby on Rails

Mitigation

Ban html_safe(). Alternatively, If needed, review each usage and exempt with # nosem.

Semgrep rule

ruby.rails.security.audit.xss.avoid-html-safe.avoid-html-safe

1.B. Using content_tag()

content_tag()'s escaping behavior has changed between Rails 2 and 3. In Rails 2, no supplied content is escaped. In Rails 2 and 3, attribute names are not escaped. Further, the returned value is marked as "safe," the same as if html_safe() had been used. This confusing behavior makes it difficult to use content_tag() properly; improper use can create XSS vulnerabilities in your application.

Example:

content_tag :p, "Hello, #{name}"

References

• Brakeman scanner - Content tag

Mitigation

Ban content_tag(). Alternatively, If necessary, prefer html_safe() due to its straightforward behavior.

Semgrep rule

ruby.rails.security.audit.xss.avoid-content-tag.avoid-content-tag

1.C. Using raw()

raw() disables HTML escaping for the returned content. This permits raw HTML to be rendered in a template, which could create a XSS vulnerability.

Example:

raw @user.name

References

• raw() documentation
• Preventing XSS in Ruby on Rails

Mitigation

Ban raw(). Alternatively, Prefer html_safe() if necessary.

Semgrep rule

ruby.rails.security.audit.xss.avoid-raw.avoid-raw

1.D. Disabling of ActiveSupport#escape_html_entities_in_json

ActiveSupport#escape_html_entities_in_json is a setting which determines whether Hash#to_json() will escape HTML characters. Disabling this could create XSS vulnerabilities.

Example:

config.active_support.escape_html_entities_in_json = false

References

• escape_html_entities_in_json documentation
• Brakeman scanner - Cross-site scripting (JSON)
• How to disable HTML escaping for JSON, but keep enabled for views?

Mitigation

Ban disabling of ActiveSupport#escape_html_entities_in_json. Alternatively, If HTML is needed in JSON, use JSON.generate() and review each usage carefully. Exempt each case with # nosem.

Semgrep rule

ruby.lang.security.json-entity-escape.json-entity-escape

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2. Bypassing the template engine

2.A. Manually creating an ERB template

Manually creating an ERB template could create a server-side template injection (SSTI) vulnerability if it is created with user input. (This could also result in XSS.) Due to the severity of this type of vulnerability, it is better to use a template file instead of creating templates in code.

Example:

ERB.new("<div>#{@user.name}</div>").result

References

• Brakeman scanner - Template injection
• Ruby ERB template injection

Mitigation

Ban template creation in code. Alternatively, Use ERB template files.

Semgrep rule

ruby.rails.security.audit.xss.manual-template-creation.manual-template-creation

2.B. Rendering an inline template with render inline

render inline: is the same as creating a template manually and is therefore susceptible to the same vulnerabilities as manually creating an ERB template. This can result in a SSTI or XSS vulnerability.

Example:

render inline: "<div>#{@user.name}</div>"

References

• Zen Rails Security Checklist
• Inline renders - even worse than XSS!

Mitigation

Ban render inline:. Alternatively, Use ERB template files.

Semgrep rule

ruby.rails.security.audit.xss.avoid-render-inline.avoid-render-inline

2.C. Using render text:

render text: unintuitively sets the Content-Type to text/html. This means anything rendered through render text: will be interpreted as HTML. Templates rendered in this manner could create a XSS vulnerability.

Example:

render text: "<div>#{@user.name}</div>"

References

• Inline renders - even worse than XSS!

Mitigation

Ban render text:. Alternatively, Use ERB template files.

Semgrep rule

ruby.rails.security.audit.xss.avoid-render-text.avoid-render-text

---

3. Templates: Variable explicitly unescaped

3.A. Using html_safe()

html_safe() marks the supplied string as "safe for HTML rendering." This bypasses HTML escaping and potentially creates XSS vulnerabilities.

Example:

<%= name.html_safe %>

References

• Brakeman scanner - Cross-site scripting
• Preventing XSS in Ruby on Rails

Mitigation

Ban html_safe(). Alternatively, Prefer using html_safe() in Ruby code instead of templates.

Semgrep rule

ruby.rails.security.audit.xss.templates.avoid-html-safe.avoid-html-safe

3.B Using content_tag()

content_tag()'s escaping behavior has changed between Rails 2 and 3. In Rails 2, no supplied content is escaped. In Rails 2 and 3, attribute names are not escaped. Further, the returned value is marked as "safe," the same as if html_safe() had been used. This confusing behavior makes it difficult to use content_tag() properly; improper use can create XSS vulnerabilities in your application.

Example:

<%= content_tag :p, "Hello, #{name}" %>

References

• Brakeman scanner - Content tag

Mitigation

Ban content_tag(). Alternatively, If necessary, prefer html_safe() in Ruby code due to its straightforward behavior.

Semgrep rule

ruby.rails.security.audit.xss.templates.avoid-content-tag.avoid-content-tag

3.C. Using raw()

raw() disables HTML escaping for the returned content. This permits raw HTML to be rendered in a template, which could create a XSS vulnerability.

Example:

<%= raw @user.name =>

References

• raw() documentation
• Preventing XSS in Ruby on Rails

Mitigation

Ban raw(). Alternatively, Prefer html_safe() in Ruby code if necessary.

Semgrep rule

ruby.rails.security.audit.xss.templates.avoid-raw.avoid-raw

3.D. Using <%== ... %>, which is an alias for html_safe()

The double-equals == is an ERB alias for html_safe(). This will mark the contents as "safe for rendering" and may introduce an XSS vulnerability.

Example:

<%== @user.name %>

References

• Alias for html_safe()
• Raw vs. html_safe

Mitigation

Ban <%== ... %>, which is an alias for html_safe(). Alternatively, Prefer html_safe() in Ruby code if necessary.

Semgrep rule

ruby.rails.security.audit.xss.templates.alias-for-html-safe.alias-for-html-safe

---

4. Templates: Variable in dangerous location

4.A Unquoted variable in HTML attribute

Unquoted template variables rendered into HTML attributes is a potential XSS vector because an attacker could inject JavaScript handlers which do not require HTML characters. An example handler might look like: onmouseover=alert(1). HTML escaping will not mitigate this. The variable must be quoted to avoid this.

Example:

<div class=<%= classes %></div>

References:

• Flask cross-site scripting considerations - unquoted variable in HTML attribute

Mitigation

Flag unquoted HTML attributes ERB expressions. Alternatively, Always use quotes around HTML attributes.

Semgrep rule

ruby.rails.security.audit.xss.templates.unquoted-attribute.unquoted-attribute

4.B. Variable in href attribute

Template variables in a href value could still accept the javascript: URI. This could be a XSS vulnerability. HTML escaping will not prevent this. Use link_to beginning with a literal forward slash to generate links.

Example:

<a href="<%= link %>"></a>

References

• Flask cross-site scripting considerations - variable in href

Mitigation

Flag template variables in href attributes. Alternatively, Use url_for to generate links.

Semgrep rule

ruby.rails.security.audit.xss.templates.var-in-href.var-in-href

4.C. Using link_to with unrestricted URL scheme

Detected a template variable used in 'link_to'. This will generate dynamic data in the 'href' attribute. This allows a malicious actor to input the 'javascript:' URI and is subject to cross- site scripting (XSS) attacks. If using a relative URL, start with a literal forward slash and concatenate the URL, like this: <%= link_to "Here", "/"+@link %>. You may also consider setting the Content Security Policy (CSP) header.

Example:

<%= link_to "Here", @link %>

References

• OWASP Cheatsheet - Ruby on Rails XSS
• Brakeman scanner - link_to

Mitigation

Flag link_to in templates. Alternatively, If you must use this, add a literal forward-slash at the beginning to create a relative url.

Semgrep rule

ruby.rails.security.audit.xss.templates.dangerous-link-to.dangerous-link-to

4.D. Variable in <script> block

Template variables placed directly into JavaScript or similar are now directly in a code execution context. Normal HTML escaping will not prevent the possibility of code injection because code can be written without HTML characters. This creates the potential for XSS vulnerabilities, or worse.

Example:

<script>var name = <%= name %>;</script>

References

• Template engines: Why default encoders are not enough
• Protecting against XSS in Rails - JavaScript contexts
• escape_javascript documentation

Mitigation

Ban template variables in <script> blocks. Alternatively, If necessary, use the the escape_javascript function or its alias, j. Review each usage carefully and exempt with # nosem.

Semgrep rule

ruby.rails.security.audit.xss.templates.var-in-script-tag.var-in-script-tag