Kanidm: Stored HTML injection in "passkey-enrolment" partial via displayname → htmx-driven authenticated request forgery

Summary

### Summary

The kanidmd web UI renders the WebAuthn passkey-registration challenge as raw JSON inside an inline `<script id="data">` element using the Askama `|safe` filter. The challenge embeds the account's `displayname`, which `serde_json` serialises without escaping `<`/`>`. A `displayname` containing `</script>` therefore terminates the script element early and injects arbitrary HTML into the credential-update page. Because the page is htmx-driven and the server's CSP allows `'unsafe-eval'`, injected `hx-*` attributes can issue authenticated same-origin API requests with the viewer's bearer cookie.

### Impact

An authenticated attacker who is a member of `idm_people_admins` can write the `displayname` of any `Person` entry — including high-privilege persons — because `idm_acp_people_pii_manage` carries no high-privilege exclusion filter. When the targeted high-privilege user later opens **Add Passkey** on their own credential-update page (`/ui/reset`), the injected markup is swapped into the DOM and htmx fires attacker-chosen same-origin requests authenticated as the victim. This allows a helpdesk-tier operator to escalate to `idm_admins` (e.g. by POSTing themselves into the group) or otherwise act with the victim's session. The self-write path (`idm_people_self_name_write`) is self-XSS only and is not counted toward impact. Even without the htmx vector, the breakout permits `<meta http-equiv='refresh'>` open-redirect and arbitrary defacement of the credential page.

### Details

- https://github.com/kanidm/kanidm/blob/master/server/core/templates/credential_update_add_passkey_partial.html#L3 — the `|safe` sink - https://github.com/kanidm/kanidm/blob/master/server/core/src/https/views/reset.rs#L506-L509 — `serde_json::to_string` of the challenge - https://github.com/kanidm/kanidm/blob/master/server/lib/src/idm/credupdatesession.rs#L2453-L2460 — `displayname` flows into `start_passkey_registration`

### Affected versions

All releases shipping the htmx credential-update views