ApostropheCMS: Host header controls server-side image fetch origin in @apostrophecms/ai-helper, enabling authenticated SSRF

## Summary The `@apostrophecms/ai-helper` package builds the URL for a server-side image fetch by resolving a storage-relative path against `req.baseUrl`. When the site does not set a global base URL (the default), `req.baseUrl` is derived from the incoming HTTP `Host` header. An authenticated editor can generate a base64 AI image (which is persisted without a stored `url`) and then send an unauthorized `PATCH` accept request with an attacker-chosen `Host` header. The server then issues an outbound `GET` to the attacker-selected origin, producing a Server-Side Request Forgery primitive that reaches hosts and ports the client cannot otherwise contact. The affected code lives in `packages/ai-helper/modules/@apostrophecms/ai-helper-image/index.js`. ## Root Cause The image accept route trusts `req.baseUrl` as a safe origin for constructing an internal fetch target, but `req.baseUrl` is attacker-influenced whenever the site has no configured base URL. The URL builder concatenates a default local storage path (which is relative, for example `/uploads`) and resolves it against `req.baseUrl`; because the left operand is relative, the resulting origin is taken entirely from `req.baseUrl`. Two protections that exist elsewhere are missing on this path: the generate route enforces an edit capability, but the accept route performs no authorization or ownership check, and no allowlist or validation constrains the resolved origin before the fetch. ```text packages/ai-helper/modules/@apostrophecms/ai-helper-image/index.js aiHelperImageUrl (line 91-92): image.url || new URL(self.uploadfs.getUrl() + `/ai-helper-images/${image._id}.png`, req.baseUrl) aiHelperFetchImage (line 64-65): fetch(self.aiHelperImageUrl(req, image)) apiRoutes.patch['ai-helper/:_id'] (line 314-333): findOne({ _id }) only; no permission or ownership check packages/apostrophe/modules/@apostrophecms/i18n/index.js setPrefixUrls (line 799-820): req.baseUrl falls back to `${req.protocol}://${host}`, host from the Host header when apos.baseUrl is unset ``` ## Impact An attacker who holds a low-privileged editor account (the capability required to generate an AI image, which inserts a base64 record without a stored `url` and returns its identifier) can coerce the server into issuing outbound `GET` requests to an origin of their choosing by supplying a crafted `Host` header on the accept request. This yields SSRF: probing internal hosts and ports that are not reachable from the attacker's network position, and limited semi-blind exfiltration where an image-parseable response at the fixed path becomes a viewable attachment. The request path is fixed to `/uploads/ai-helper-images/<id>.png`, so only the origin (host, port, and `http` scheme) is attacker-controlled; arbitrary-path retrieval such as cloud metadata traversal is not reachable through this primitive. No integrity or availability primitive was demonstrated. Exploitation requires the default deployment where no global base URL is configured and local file storage is in use, together with a configured provider that returns base64 image data. ## Proof of Concept ```text Preconditions: - ApostropheCMS site with @apostrophecms/ai-helper installed and a configured base64-returning image provider. - No global base URL configured (apos.baseUrl unset), default local uploadfs. - Attacker holds an account with edit capability on at least one content type. ``` 1. Authenticate as the editor and generate a base64 AI image: `POST /api/v1/@apostrophecms/image/ai-helper` with body `{"prompt":"x"}`. The response contains the new record identifier `<_id>`. The persisted record has no `url` field. 2. Send the accept request with a malicious `Host` header: ```text PATCH /api/v1/@apostrophecms/image/ai-helper/<_id> HTTP/1.1 Host: 169.254.169.254 Content-Type: application/json {"accepted": true} ``` 3. The server derives `req.baseUrl` as `http://169.254.169.254`, builds the fetch target `http://169.254.169.254/uploads/ai-helper-images/<_id>.png`, and issues the outbound request. ```text Derivation confirmed against source logic: base64 record (no url) + Host: 169.254.169.254 -> new URL("/uploads/ai-helper-images/<_id>.png", "http://169.254.169.254") -> server fetches: http://169.254.169.254/uploads/ai-helper-images/<_id>.png control (apos.baseUrl set to https://cms.example.com): -> fetch target uses https://cms.example.com (Host ignored) control (record stores its own url): -> aiHelperImageUrl short-circuits to the stored url (no SSRF) ``` ## Attack Chain 1. **Exposure:** The `@apostrophecms/ai-helper-image` module registers `POST` and `PATCH` routes under `/api/v1/@apostrophecms/image/ai-helper`, reachable by any authenticated session on a site that installed the package. 2. **Control:** The attacker controls the HTTP `Host` header on the `PATCH` request; the global `locale` middleware copies it into `req.baseUrl` because `apos.baseUrl` is unset (`packages/apostrophe/modules/@apostrophecms/i18n/index.js:805,816-819`). 3. **Path:** The attacker first generates a base64 image via `POST`, which inserts a record without a `url` (`ai-helper-image/index.js:282-284`); the returned identifier is then used in the `PATCH` accept request, which calls `aiHelperFetchImage` (`:332`). 4. **Guard:** The `PATCH` handler checks only `req.body.accepted` and record existence via `findOne({ _id })` (`:318,:322`); it performs no capability check and no owner scoping, unlike the `POST` route which calls `checkPermissions` (`:176`). No validation constrains the resolved origin before the fetch. 5. **Primitive:** `aiHelperImageUrl` resolves the relative storage path against the Host-derived `req.baseUrl` and `aiHelperFetchImage` calls `fetch` on the result (`:91-92,:64-65`). 6. **Result:** The server performs an outbound `GET` to the attacker-chosen origin at a fixed path, granting internal host and port reachability and limited semi-blind response retrieval that the attacker's network position does not otherwise permit. ## Bypass Evidence The generate route's `checkPermissions` guard (`ai-helper-image/index.js:176`) is not present on the accept route, so no authorization protects the fetch. The following refutations were tested and defeated: - Default local storage returns a relative URL: `uploadfs.getUrl()` yields `/uploads` because the local backend builds `uploadsUrl = (apos.baseUrl || '') + prefix + '/uploads'`, so `new URL(relative, req.baseUrl)` takes its origin from `req.baseUrl`. Configuring a base URL or an absolute storage URL (S3/CDN) defeats the attack, which is why those are negative controls, not the default. - The base64 record has no persisted `url`: `insertOne(image)` stores the record before `image.url` is assigned on the in-memory response object (`:282-284` versus `:294`), so the `PATCH` handler re-reads a record with no `url` and the short-circuit in `aiHelperImageUrl` does not apply. - Ownership and enumeration: `GET` and `DELETE` on these records are `userId`-scoped and cuid2 identifiers are unguessable, so the practical actor is the generating editor; this bounds the required privilege to a low-privileged authenticated user rather than an anonymous or cross-tenant attacker. - Not a duplicate: the fix for the core pretty-URL SSRF advisory lives entirely in the separate `@apostrophecms/file` module and its stream proxy and never touches `packages/ai-helper`; the rich-text import SSRF advisory uses a different route with an explicit hostname allowlist. No advisory exists for `@apostrophecms/ai-helper`. This is a parallel unguarded sink, not an incomplete fix of either. ## Affected Versions - `Ecosystem: npm` - `Package: @apostrophecms/ai-helper` - `Confirmed affected range: >= 1.0.0-beta.10, <= 1.0.0-beta.12` - `Latest release checked: 1.0.0-beta.12 (npm registry, dist-tag latest)` - `Fix status: not fixed` The vulnerable pattern was introduced in `1.0.0-beta.10`: earlier releases through `1.0.0-beta.9` fetched the stored provider URL directly (`fetch(url)`) with no `req.baseUrl` resolution and no uploadfs-relative construction, so they are not affected by this pattern. Inspection of the published tarballs for `1.0.0-beta.10`, `1.0.0-beta.11`, and `1.0.0-beta.12` confirms the full chain (Host-derived base resolution, unguarded accept route, base64 insert without a stored url) is present and unchanged through the latest published release. This package publishes only on the prerelease (beta) channel, and the latest published version remains vulnerable. ## Suggested Fix Do not treat `req.baseUrl` as a trusted origin for a server-side fetch. Enforce the same authorization on the accept route that the generate route applies, and add an ownership or scope check so a record can only be accepted by a user permitted to act on it. Independently, constrain the fetch target: resolve the storage-relative path against a server-configured, trusted base (for example the configured storage base URL) rather than the request-derived `req.baseUrl`, and reject or allowlist any resulting origin before calling `fetch`. Ensure the remediation covers both the base64 (no stored url) representation and the stored-url representation, and any other caller that reaches `aiHelperImageUrl` or `aiHelperFetchImage`. As an interim mitigation without a code change, configure a global base URL so the resolved origin is fixed, and place egress controls on the server so outbound requests cannot reach sensitive internal hosts or metadata endpoints. Reported by zx (GitHub: @manus-pi).

Source published

20 Oct 2026 18:11:01 UTC

Source modified

20 Oct 2026 18:30:03 UTC

Observed here

24 Oct 2026 13:17:17 UTC

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GitHub Advisory Database · CC-BY-4.0

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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:N/S:U/C:L/I:N/A:N

Publisher rating · MODERATE