Phlex vulnerable to Cross-site Scripting (XSS) via maliciously formed HTML attribute names and values

There is a potential cross-site scripting (XSS) vulnerability that can be exploited via maliciously crafted user data.

The reason these issues were not detected before is the escapes were working as designed. However, their design didn’t take into account just how recklessly permissive browser are when it comes to executing unsafe JavaScript via HTML attributes.

Impact

If you render an <a> tag with an href attribute set to a user-provided link, that link could potentially execute JavaScript when clicked by another user.

a(href: user_profile) { "Profile" }

If you splat user-provided attributes when rendering any HTML or SVG tag, malicious event attributes could be included in the output, executing JavaScript when the events are triggered by another user.

h1(**JSON.parse(user_attributes))

Patches

Patches are available on RubyGems for all minor versions released in the last year.

• 1.10.2
• 1.9.3

If you are on main, it has been patched since da8f943

Workarounds

Configuring a Content Security Policy that does not allow unsafe-inline would effectively prevent this vulnerability from being exploited.

References

In addition to upgrading to a patched version of Phlex, we strongly recommend configuring a Content Security Policy header that does not allow unsafe-inline. Here’s how you can configure a Content Security Policy header in Rails. https://guides.rubyonrails.org/security.html#content-security-policy-header