Reflected XSS in AshAuthentication confirmation and magic link interaction forms

Improper Neutralization of Input During Web Page Generation (XSS) vulnerability in team-alembic AshAuthentication allows reflected cross-site scripting via the confirmation and magic link interaction forms.

When a strategy is configured with require_interaction? set to true, AshAuthentication serves an intermediate HTML page asking the user to confirm the action by submitting a form. Both such pages embed a request parameter directly into a hidden input's value attribute without HTML escaping: lib/ash_authentication/add_ons/confirmation/confirmation_form.html.eex interpolates the confirm parameter, and lib/ash_authentication/strategies/magic_link/sign_in_form.html.eex interpolates the magic link token parameter. These templates are compiled with EEx.function_from_file/3 using plain <%= %> expressions, which perform no escaping, so the parameter is reflected verbatim.

Neither accept handler validates the value before rendering it. AshAuthentication.AddOn.Confirmation.Plug.accept/2 only checks that a confirm key is present, and AshAuthentication.Strategy.MagicLink.Plug.accept/2 reads the parameter directly, so no token signature is verified at this stage and arbitrary attacker-supplied text reaches the template. An unauthenticated attacker can therefore craft a URL whose parameter terminates the attribute and injects markup, for example a quote followed by a <script> element. Because the accept phase is served over GET, loading the crafted link is sufficient; no form submission or prior authentication is required.

The injected script executes in the origin of the application embedding AshAuthentication, giving it access to that origin's cookies, session, and same-origin responses. Since these pages are part of the authentication flow, a victim following what appears to be a legitimate confirmation or sign-in link is a plausible target.

This issue affects ash_authentication: from 4.8.0 before 4.14.2 and from 5.0.0-rc.0 before 5.0.0-rc.13.

Only strategies configured with require_interaction? set to true are affected. This option defaults to false, and when it is disabled the vulnerable accept route is not registered at all, so the affected pages cannot be reached.

The option is not obscure: AshAuthentication's own documentation for the confirmation add-on advises that applications using AshAuthenticationPhoenix should enable it, so the affected configuration is a documented and recommended deployment rather than an unusual one.

At least one of the following must be configured with require_interaction? true:

• the confirmation add-on, which serves the affected confirmation accept form; or
• the magic_link strategy, which serves the affected sign-in accept form.