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

Slack's own API docs require &, <, > to be replaced with &, <, > 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

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/[email protected]'s RenderOptions.escape):

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 [email protected] package confirmed the default (pre-bug) output for the same string would have been evil2<https://attacker.example/urgent-rollback|Click here to view incident> — i.e. the single global assignment is solely responsible for the unescaped output observed live.