GHSA-3gxm-wfjx-m847

During code logic analyis, an area that may lead to unintended behavior under specific conditions was discovered.

## Overview - Verified Version: `80cd21554124da07d17a4f962c7d770a4f70d0f2` - Vulnerability Type: Stored XSS - Affected Location: `beetsplug/web/templates/index.html:42` - Trigger Scenario: Metadata fields such as `title`, `lyrics`, or `comments` are rendered with raw template interpolation and inserted into DOM via `.html(...)`.

## Root Cause The bundled web UI uses Underscore template interpolation mode `<%= ... %>` for untrusted metadata fields. In this runtime, `<%= ... %>` is raw insertion and HTML escaping is only performed by `<%- ... %>`. Rendered output is then inserted with `.html(...)`, allowing attacker-controlled markup to become active DOM.

## Source-to-Sink Chain 1. Source (attacker-controlled input) - Item metadata values (for example `title`, `lyrics`, `comments`) can contain attacker HTML payload.

2. Data flow - Templates in `beetsplug/web/templates/index.html:42-46,87-91` render metadata with `<%= ... %>`. - Underscore runtime defines `<%= ... %>` as raw interpolation (`beetsplug/web/static/underscore.js:890-907`).

3. Sink (security-sensitive action) - Frontend inserts rendered template output into DOM via `$(this.el).html(this.template(this.model.toJSON()));` in `beetsplug/web/static/beets.js:182,208,220`.

## Exploitation Preconditions 1. Victim opens the web UI page that renders attacker-controlled metadata. 2. Metadata includes executable HTML/JS payload.

## Risk Stored payload executes in the web UI context and can perform actions available to that origin.

## Impact Attacker can run arbitrary JavaScript in the victim browser, exfiltrate viewable data, and perform UI-driven actions as the victim session.

## Remediation 1. Replace raw interpolation `<%= ... %>` with escaped output `<%- ... %>` for untrusted fields. 2. Avoid `.html(...)` for untrusted template output; use text-safe rendering. 3. Sanitize metadata values on ingest and before rendering, including attribute contexts.