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

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

Unleash is an open-source feature management platform. Prior to 8.0.3, FeatureEventFormatterMd.format in src/lib/addons/feature-event-formatter-md.ts assigns Mustache.escape to an identity function before rendering action and path templates. Because Mustache.escape is process-wide, the assignment disables escaping for subsequent Mustache.render calls in email-service.ts, webhook.ts, datadog.ts, and new-relic.ts. An editor-level user can place Slack or Microsoft Teams link syntax in an unrestricted username, trigger a feature event, and inject an attacker-labeled link into a trusted outbound notification channel, while other Mustache sinks remain unescaped until restart. This issue is fixed in version 8.0.3.

Source published

21 Aug 2026 18:21:46 UTC

Source modified

21 Aug 2026 21:43:35 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: cve-program · CVE Program Terms of Use. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · 4.1 CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:N/I:L/A:N

Unleash is an open-source feature management platform. Prior to 8.0.3, FeatureEventFormatterMd.format in src/lib/addons/feature-event-formatter-md.ts assigns Mustache.escape to an identity function before rendering action and path templates. Because Mustache.escape is process-wide, the assignment disables escaping for subsequent Mustache.render calls in email-service.ts, webhook.ts, datadog.ts, and new-relic.ts. An editor-level user can place Slack or Microsoft Teams link syntax in an unrestricted username, trigger a feature event, and inject an attacker-labeled link into a trusted outbound notification channel, while other Mustache sinks remain unescaped until restart. This issue is fixed in version 8.0.3.

Source published

Aug 21, 2026, 6:21:46 PM (timezone not stated)

Source modified

Aug 21, 2026, 9:43:35 PM (timezone not stated)

Observed here

9 Nov 2026 14:13:08 UTC

Source terms

ENISA EUVD · source terms and attribution apply

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: euvd · ENISA EUVD · source terms and attribution apply. Source text is retained; field grouping and change labels are provided by ChangeIntel.

EUVD CVSS 3.1 · 4.1 CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:N/I:L/A:N

Unleash is an open-source feature management platform. Prior to 8.0.3, FeatureEventFormatterMd.format in src/lib/addons/feature-event-formatter-md.ts assigns Mustache.escape to an identity function before rendering action and path templates. Because Mustache.escape is process-wide, the assignment disables escaping for subsequent Mustache.render calls in email-service.ts, webhook.ts, datadog.ts, and new-relic.ts. An editor-level user can place Slack or Microsoft Teams link syntax in an unrestricted username, trigger a feature event, and inject an attacker-labeled link into a trusted outbound notification channel, while other Mustache sinks remain unescaped until restart. This issue is fixed in version 8.0.3.

Source published

21 Aug 2026 19:17:32 UTC

Source modified

14 Oct 2026 19:10:00 UTC

Observed here

25 Oct 2026 15:18:45 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 4.1 CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:N/I:L/A:N

## 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` (`EVENT_MAP`) 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/[email protected]`'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 `[email protected]` 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.

Source published

21 Aug 2026 19:14:52 UTC

Source modified

21 Aug 2026 19:40:58 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:N/I:L/A:N

Publisher rating · MODERATE