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Response snapshot Oct 27, 2026, 04:06 UTC

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VulnerabilityCVE-2026-113671

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

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.

Published Updated

At a glance

Exploited
Not known
Severity
4.3 Medium (CVSS 3.1)
Chance of exploitation
0.3% in the next 30 days
Sources
1 source

What to do

GitHub lists first patched versions: @apostrophecms/ai-helper: 1.0.0-beta.13. Confirm the affected range and upgrade guidance in the exact advisory.

Read GitHub-reviewed advisory GHSA-527g-ff96-x6wj (opens a new tab)

What happened

## 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`.

Continue reading

## 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/ .

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 ` `. 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/ 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/ .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/ .png", "http://169.254.169.254") -> server fetches: http://169.254.169.254/uploads/ai-helper-images/ .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.

#

Affected package ranges reported by GitHub: @apostrophecms/ai-helper: >= 1.0.0-beta.10, < 1.0.0-beta.13

Scores and how they were measured

CVSS base severity

4.3

Version 3.1As of

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

0.3%

Scored Percentile 25.8%

Model and measurement source (opens a new tab)

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-527g-ff96-x6wj (opens a new tab)

    The public advisory supplies the description, affected package/version ranges, and any listed CVSS metric. GitHub review is not local review.

    Published Checked

  2. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-527g-ff96-x6wj, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  3. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-527g-ff96-x6wj, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  4. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-527g-ff96-x6wj, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  5. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-527g-ff96-x6wj, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  6. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-527g-ff96-x6wj, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  7. Primary source

    FIRST measured EPSS score (opens a new tab)

    Measured EPSS probability and percentile for CVE-2026-113671, scored 2026-10-26. This is a model estimate, not proof of exposure.

    Published Checked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, October 20). ApostropheCMS: Host header controls server-side image fetch origin in @apostrophecms/ai-helper, enabling authenticated SSRF (CVE-2026-113671). https://kevwire.com/record/CVE-2026-113671

MLA

"ApostropheCMS: Host header controls server-side image fetch origin in @apostrophecms/ai-helper, enabling authenticated SSRF (CVE-2026-113671)." KEVWire, 20 Oct. 2026, kevwire.com/record/CVE-2026-113671.

Chicago

KEVWire. "ApostropheCMS: Host header controls server-side image fetch origin in @apostrophecms/ai-helper, enabling authenticated SSRF (CVE-2026-113671)." Last modified October 20, 2026. https://kevwire.com/record/CVE-2026-113671.

Evidence summary (plain text)

ApostropheCMS: Host header controls server-side image fetch origin in @apostrophecms/ai-helper, enabling authenticated SSRF (Oct 20, 2026).
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`.

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.
CVSS 4.3 (version 3.1; evidence as of Oct 20, 2026; https://github.com/advisories/GHSA-527g-ff96-x6wj).
EPSS 0.3% for the following 30 days, scored Oct 26, 2026 (https://api.first.org/data/v1/epss?cve=CVE-2026-113671&date=2026-10-26).
Sources:
GitHub-reviewed advisory GHSA-527g-ff96-x6wj: https://github.com/advisories/GHSA-527g-ff96-x6wj
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMVQyMToyODowOVrOAAfv1g%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMVQyMToyNDoyMFrOAAfv1Q%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMVQyMDo0NTowN1rOAAfvzQ%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMVQyMDozMDoyNlrOAAfvxw%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMVQyMDowMzo1N1rOAAfvxQ%3D%3D
FIRST measured EPSS score: https://api.first.org/data/v1/epss?cve=CVE-2026-113671&date=2026-10-26
Record ID: CVE-2026-113671.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Oct 27, 2026, 01:40 UTC. Ingestion is not an independent editorial investigation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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