CVE-2026-63466: Unleash: Global Mustache.escape override disables HTML escaping process-wide, enabling Slack/Teams link-injection via unrestricted username

Vulnerability Details

File: src/lib/addons/feature-event-formatter-md.ts Line: 355 (in v8.0.1; format() method)

Root Cause

FeatureEventFormatterMd.format() does:

ts Mustache.escape = (text) => text; const text = Mustache.render(action, context);

mustache (pinned ^4.2.0, confirmed installed 4.2.0) keeps escape as a module-level singleton (mustache.js: mustache.escape = escapeHtml;), read by every Mustache.render() call in the process unless a per-call config.escape override is passed (var escape = this.getConfigEscape(config) || mustache.escape;). Node's module cache guarantees every import Mustache from 'mustache' in the process — feature-event-formatter-md.ts, email-service.ts, webhook.ts, datadog.ts, new-relic.ts — shares the same object instance.

This assignment therefore permanently disables HTML escaping for every other Mustache.render() call in the same Node process (including email-service.ts templates) from the moment any single notification addon (Webhook, Slack legacy, Microsoft Teams, Datadog, New Relic) first formats any event, for the remaining lifetime of the process.

feature-event-formatter-md-events.ts (EVENTMAP) confirms the blast radius: nearly every event's action template interpolates attacker-controlled values with single-mustache (intended-to-be-escaped) syntax, most importantly {{user}}, which is event.createdBy — the acting account's username (or email if set; src/lib/util/extract-user.ts: extractUsernameFromUser). Neither username nor name have any charset/length validation anywhere in the codebase (create-user-schema.ts, create-invited-user-schema.ts, user-service.ts:289 only does Joi.assert(name, Joi.string(), 'Name') — a type check, nothing more).

Slack's own API docs require &, <, > to be replaced with &amp;, &lt;, &gt; before sending user-generated text, specifically so Slack's mrkdwn parser does not interpret it as <url|label> link syntax. Mustache's default escapeHtml happens to produce exactly those entities, so this was (likely unintentionally) the application's only defense against link-injection in chat notifications — and it is unconditionally switched off by the same code path that depends on it.

Attack Scenario 1. Admin has a Webhook, Slack (legacy), Microsoft Teams, Datadog, or New Relic integration configured (a very common production setup for flag-change notifications). 2. Attacker has (or self-registers, if public signup is enabled — POST /invite/:token/signup is permission: NONE) any Editor-level account and sets username to e.g. evil<https://attacker.example/urgent-rollback|Click here to view incident>. 3. Attacker performs any ordinary write action (create/update/toggle a feature flag — routine, no special privilege beyond Editor on one project). 4. The configured addon's handleEvent() calls this.msgFormatter.format(event), which mutates the global escape function and immediately renders the {{user}}-containing template with escaping disabled. 5. The resulting message — containing the attacker's raw <url|label> Slack link syntax — is POSTed to the team's Slack/Teams channel or webhook endpoint and rendered as a real, clickable, attacker-labeled hyperlink inside a trusted automated notification feed.

Vulnerable Code ts Mustache.escape = (text) => text;

const text = Mustache.render(action, context); const url = path ? ${this.unleashUrl}${Mustache.render(path, context)} : undefined;

Impact - Stored markdown/link-injection (phishing-link injection) into any configured outbound notification channel (Slack legacy, MS Teams, Webhook default markdown, Datadog, New Relic), using an attacker-controlled username — no admin privilege required, only Editor on a single project, and potentially reachable through public self-signup. - Secondary: loss of HTML escaping for any other reachable Mustache single-mustache placeholder process-wide until restart (increases severity of any other currently-unreached or future Mustache sink, e.g. email templates). - Tertiary: a custom Webhook bodyTemplate that interpolates raw event/user fields directly into a JSON string literal (rather than the pre-escaped eventJson field the code already provides for this purpose) can have its JSON structure broken by an attacker-controlled " character once the global escape function is neutered.

Recommended Fix Never mutate the shared Mustache.escape global. Pass a local escape function via Mustache's per-call render option instead (supported and typed in @types/mustache@4.2.6's RenderOptions.escape):

ts const renderConfig = { escape: (text: string) => text }; const text = Mustache.render(action, context, undefined, renderConfig); const url = path ? ${this.unleashUrl}${Mustache.render(path, context, undefined, renderConfig)} : undefined;

Verification Dynamically confirmed on v8.0.1 in a local Docker lab (official unleashorg/unleash-server:8.0.1 image + Postgres 15): - Created a Webhook addon with the addon UI's own placeholder bodyTemplate ({{event.createdBy}} etc.), pointed at a local listener. - Created an Editor-role user with username = evil2<https://attacker.example/urgent-rollback|Click here to view incident> (accepted with HTTP 201, no sanitization). - Logged in as that user and created a feature flag (ordinary Editor action). - Captured webhook payload: "createdBy": "evil2<https://attacker.example/urgent-rollback|Click here to view incident>" — <, >, | completely unescaped, live Slack link-injection syntax. - Control test with the same pinned mustache@4.2.0 package confirmed the default (pre-bug) output for the same string would have been evil2&lt;https:&#x2F;&#x2F;attacker.example&#x2F;urgent-rollback|Click here to view incident&gt; — i.e. the single global assignment is solely responsible for the unescaped output observed live.