EEF-CVE-2026-66882

## Summary

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.

## Configurations

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.