Webhooks cannot target explicitly trusted private services in Production

Issue Summary

Self-hosted Ghost instances running in production cannot deliver webhooks to explicitly trusted services on the same private Docker network.

Ghost rejects webhook targets whose hostnames resolve to private IP addresses:
URL_PRIVATE_INVALID: URL resolves to a non-permitted private IP block

Please consider supporting an configuration controlled allowlist for exact private webhook hostnames and ports.

Steps to Reproduce

1. Create a private Docker network:

docker network create ghost-network

2. Start a MySQL 8.4 container:

docker run -d \
    --name ghost-mysql \
    --network ghost-network \
    -e MYSQL_ROOT_PASSWORD=ghost-root-password \
    -e MYSQL_DATABASE=ghost \
    -e MYSQL_USER=ghost \
    -e MYSQL_PASSWORD=ghost-password \
    mysql:8.4

3. Start a webhook receiver on the same network:

docker run -d \
    --name webhook-receiver \
    --network ghost-network \
    example/webhook-receiver:latest

The receiver listens on port 3000 and exposes: POST /api/webhooks/posts

4. Start Ghost in production mode on the same network:

docker run -d \
    --name ghost \
    --network ghost-network \
    -p 2368:2368 \
    -e NODE_ENV=production \
    -e url=http://localhost:2368 \
    -e database__client=mysql \
    -e database__connection__host=ghost-mysql \
    -e database__connection__port=3306 \
    -e database__connection__user=ghost \
    -e database__connection__password=ghost-password \
    -e database__connection__database=ghost \
    ghost:6.63.0

5. Verify that Docker DNS resolves the receiver from the Ghost container:

docker exec ghost node -e \
    'require("dns").lookup("webhook-receiver", console.log)'
The hostname resolves to a private Docker network address such as:

null 172.18.0.3 4

6. Verify that the receiver is reachable from the Ghost container:

docker exec ghost node -e '
const http = require("http");

const request = http.request(
    "http://webhook-receiver:3000/api/webhooks/posts",
    {
        method: "POST",
        headers: {
            "Content-Type": "application/json"
        }
    },
    response => {
        console.log("Status:", response.statusCode);
        response.resume();
    }
);

request.on("error", console.error);
request.end("{}");
'

7. In Ghost Admin, create a custom integration and configure a webhook with this target URL:
   http://webhook-receiver:3000/api/webhooks/posts

8. Trigger the webhook by performing the configured event, such as editing or publishing a post.

9. Observe that Ghost rejects the destination because it resolves to a private Docker network address.

The validation occurs in errorIfHostnameResolvesToPrivateIp():
https://github.com/TryGhost/Ghost/blob/main/ghost/core/core/server/lib/request-external.js

Ghost Version

6.63.0

Node.js Version

22.23.2

How did you install Ghost?

Official ghost:6.63.0 Docker image running on a self-hosted Docker host.

Database type

MySQL 8

Browser & OS version

Not applicable.

Relevant log / error output

Triggering webhook for "post.edited" with url "http://webhook-receiver:3000/api/webhooks/posts"

ERROR [WEBHOOK_DELIVERY_FAILURE]
url=http://webhook-receiver:3000/api/webhooks/posts
status=none
error_code=URL_PRIVATE_INVALID
message=URL resolves to a non-permitted private IP block

RequestError: URL resolves to a non-permitted private IP block
    at Request._beforeError
    at Request.flush
    at errorIfHostnameResolvesToPrivateIp
    at async Request._makeRequest

Code of Conduct

• I agree to be friendly and polite to people in this repository

Start with errorIfHostnameResolvesToPrivateIp() in ghost/core/core/server/lib/request-external.js and trace how webhook requests reach that validation. Define how an exact hostname-and-port allowlist should be configured and applied without permitting other private destinations. Done means the documented Docker webhook target is accepted while non-allowlisted private targets remain rejected.