Rule proposal: form-prefill-sanitize — extend checkout-checkbox-sanitize to text/email/select prefills

Start by reading the existing checkout-checkbox-sanitize and cart-addon-annotate rules and reuse their checkout URL gate. Define the new rule around DOMContentLoaded, the listed control types, hidden-input allowlist and denylist, focus checks, select field restrictions, and host kill-switch; done means covered behavior is tested without touching protected fields or non-checkout forms.

Category

New defense rule (extension of the checkout-checkbox-sanitize family)

What problem does this solve?

checkout-checkbox-sanitize clears pre-checked checkboxes on checkout pages so the agent doesn't inherit silent opt-ins. The same dark pattern — Mathur et al.'s Preselection — exists for non-checkbox inputs:

• Pre-populated <select> defaults that pick the costliest shipping speed
• Pre-filled hidden inputs carrying affiliate / referral / promo IDs that bias pricing or attribution
• Pre-filled email/phone fields nudging the user toward marketing signup
• Pre-selected radio defaults on tipping prompts ("18% / 20% / 22% / Custom" with 22% pre-checked)

The agent is then expected to re-select what it actually wants, same contract as the existing checkbox rule.

Proposed solution

On the same checkout-shaped URL gate, walk form controls and:

• Clear pre-populated <input type="text|email|tel|number"> values when the page state suggests pre-fill (value present without user input — detectable on initial DOM).
• Reset <select> to its first option (or a blank option if one exists) when the default differs from the natural-first choice.
• Reset <input type="radio"> groups to unchecked when a default is server-supplied.
• Clear value on <input type="hidden"> whose name matches a curated affiliate/referral/promo pattern (*aff*, *ref*, *promo*, *utm_*, coupon_id, discount_code) — but only on payment/order pages.

Out of scope for v1: role="textbox" / contenteditable widgets, complex date pickers, anything inside a payment-processor iframe.

Alternatives considered

• Bundle into checkout-checkbox-sanitize instead of a new rule. Reasonable; the toggle becomes a bigger hammer. Splitting keeps user control finer-grained because text-field clearing has higher FP risk than checkbox clearing.
• Synthetic blur/change event after clearing. Some sites recalc totals on input events; consider v2 if sites silently re-prefill on input without it.

Controlling false positives

• Strict URL gate. Payment/order paths only (/cart, /checkout, /basket, /bag, /payment, /order) — never login or arbitrary forms. Reuse the exact gate of cart-addon-annotate and checkout-checkbox-sanitize so the surface is identical to a rule that's already in production.
• Hard denylist for safety-critical hidden inputs. Never clear <input type="hidden"> whose name matches CSRF/cart/session shapes: _csrf, csrf_token, authenticity_token, cart_id, order_id, session*, nonce, state, _token, anything containing signature. Failure mode here is the form silently rejecting submit, which is worse than the original dark pattern.
• Allowlist for hidden-input clearing. Only clear hidden inputs whose name matches the affiliate/promo pattern set above. Anything outside the allowlist is preserved.
• <select>: only reset when the default value is NOT the first option. If the first option is selected (natural default — common pattern is <option value="">Select…</option> or <option>Standard shipping</option> first), the rule does nothing. Avoids clobbering legitimate first-option defaults.
• Timing. Run on DOMContentLoaded, before browser autofill. Browser autofill that runs after the rule is preserved — meaning the user's password manager and saved addresses still work for inputs the rule has already cleared. Document the ordering explicitly so the contract is clear.
• Geofencing-aware select defaults. Country/state preselection based on geo is usually legitimate. Mitigation: only reset <select> when the default deviates from the first option AND the field name is in a sneaking-prone set (shipping speed, tip percent, delivery slot, donation amount, insurance plan). Don't touch country/state/region selects.
• Per-host kill-switch. Loyalty-program checkouts where saved details reappear by design (Amazon 1-Click, Apple Pay flows) can be denylisted by host. These flows are the user's intent.
• No-op on inputs the user has actually focused. If document.activeElement or :focus-visible matched a field at any point, skip clearing it on rescan.
• Graceful-failure mode. The worst user-facing outcome is "I need to retype" — recoverable. Document this so a user can clearly attribute the friction to the rule and toggle it off.

Prior art / references

• Mathur et al. (CSCW 2019), Dark Patterns at Scale. — Preselection category; already cited.
• Brignull, deceptive.design — Preselection.
• WHATWG HTML autocomplete attribute spec — informs which fields are legitimate autofill targets vs. server-pushed prefills.
• Adjacent OSS: Consent-O-Matic sets values to opt out on consent forms — same mechanic, different intent.

Tagged Impact L–M / Complexity M.