Rostra's preview-rostra skill drives a signing-capable development UI with an external browser daemon. Treat the daemon, browser process, local host, page content, and development identity as security boundaries rather than ordinary test fixtures.

First-party UI routes accept legacy full RostraId and EventId path components, but generate short forms. Resolve a short RostraId only through the retained identity index. Resolve a full EventId through its short retained-event key and verify that the retained signed envelope recomputes to that exact full ID before shortening it. Never treat a matching short prefix as proof of a full EventId.

Before redirecting or serving an author-scoped resource, bind the route author to the retained envelope and, where applicable, to the materialized social-post record. GET and HEAD requests that use a legacy form return a 308 canonical URL without discarding the query string. Mutation POST routes resolve either form in place; they must not rely on a redirect to preserve the request body.

Social-media event bytes and profile avatar bytes are untrusted even when their event signatures verify. An avatar must declare and contain one of AVIF, BMP, GIF, ICO, JPEG, PNG, SVG, TIFF, or WebP. SVG is fully parsed without a document type and is limited to the existing 1 MB avatar limit. Validate locally submitted avatars and repeat validation when serving retained data; signed event compatibility still permits historical malformed declarations.

The content renderer and /media/... route only embed and serve inline a declared type when its detected bytes match the intentional passive set: those image formats, plus MP4 and WebM video. Every other generic media response, including a MIME mismatch, unknown binary data, and active content, uses application/octet-stream and Content-Disposition: attachment with the fixed rostra-media.bin filename. Never derive a download filename from an author declaration or post text.

All avatar and generic-media responses use X-Content-Type-Options: nosniff and a sandboxing Content Security Policy that denies default sources, base URLs, and form submission. Set these headers before conditional ETag handling so 304 Not Modified preserves the security policy and representation metadata, including attachment disposition.

The skill wrapper loads a known-empty configuration and clears environment settings that could silently select persistent profiles or state, remote CDP or cloud providers, proxies, extensions, init scripts, plugins, and browser arguments. Do not bypass the wrapper. Refuse a pre-existing task session because launch-time restrictions cannot be retrofitted reliably.

Agent-browser's domain allowlist accepts the IPv6 host [::1], not an exact port. It therefore permits navigation to every HTTP service on that loopback host. Inspect link targets and form actions before authenticated activation, verify the exact origin after navigation, and do not treat the allowlist as an origin boundary.

Agent-browser controls Chromium and its daemon with the invoking user's privileges. A browser, proxy, extension, init script, plugin, provider, or remote CDP endpoint can observe credentials and page data. Use only the local Chromium launched through the skill wrapper on a trusted single-user host.

Agent-browser's auth vault encrypts credentials, but stores the vault and its decryption key under the same user account. A task-scoped vault entry is a temporary duplicate of the development mnemonic, not an independent security boundary. Create one only after explicit approval, delete it immediately after the login attempt, never reuse it, and report interrupted cleanup.

Snapshots, screenshots, downloads, traces, state files, and browser profiles can contain live or secret data. The skill forbids plaintext state and persistent profiles by default. Create approved artifacts with owner-only permissions under a task-unique directory, inspect only what is needed, and remove artifacts on success and handled failure.

Every secure non-AJAX /unlock render embeds a freshly generated, unused credential so Create Account can fill the login fields without a request. This makes merely opening that page credential-bearing: browser processes, same-origin scripts, proxies, extensions, snapshots, screenshots, page source, and DOM inspection can receive the mnemonic. Sensitive response headers reduce caching and framing risks but do not prevent that exposure. Use /unlock only on the trusted local single-user host, do not inspect or capture it, and authenticate immediately when it is unavoidable. This accepted risk is inherent in the chosen in-place interaction; revisit it if /unlock gains third-party scripts, if the browser isolation model changes, or if another account-creation flow is added.

An authenticated browser grants signing authority and can start network-visible activity. Rostra's masked identity page still contains the recovery phrase in the DOM. Never open or inspect /settings/identity. Log out and verify /unlock before closing. Browser or logout failure can leave server-memory authority; close the task session, remove its vault entry, report the failure, and restart just dev before relying on cleanup.