Webhooks and insecure internal web services

If you have non-GitLab web services running on your GitLab server or within its local network, these may be vulnerable to exploitation via Webhooks.

With Webhooks, you and your project maintainers and owners can set up URLs to be triggered when specific changes occur in your projects. Normally, these requests are sent to external web services specifically set up for this purpose, that process the request and its attached data in some appropriate way.

Things get hairy, however, when a Webhook is set up with a URL that doesn’t point to an external, but to an internal service, that may do something completely unintended when the webhook is triggered and the POST request is sent.

Because Webhook requests are made by the GitLab server itself, these have complete access to everything running on the server (http://localhost:123) or within the server’s local network (http://192.168.1.12:345), even if these services are otherwise protected and inaccessible from the outside world.

If a web service does not require authentication, Webhooks can be used to trigger destructive commands by getting the GitLab server to make POST requests to endpoints like http://localhost:123/some-resource/delete.

To prevent this type of exploitation from happening, starting with GitLab 10.6, all Webhook requests to the current GitLab instance server address and/or in a private network will be forbidden by default. That means that all requests made to 127.0.0.1, ::1 and 0.0.0.0, as well as IPv4 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16 and IPv6 site-local (ffc0::/10) addresses won’t be allowed.

This behavior can be overridden by enabling the option “Allow requests to the local network from web hooks and services” in the “Outbound requests” section inside the Admin area under Settings (/admin/application_settings/network):

Outbound requests admin settings

Note: System hooks are enabled to make requests to local network by default since they are set up by administrators. However, you can turn this off by disabling the Allow requests to the local network from system hooks option.

Whitelist for local requests

Introduced in GitLab 12.2

You can allow certain domains and IP addresses to be accessible to both system hooks and webhooks even when local requests are not allowed by adding them to the whitelist. Navigate to Admin Area > Settings > Network (/admin/application_settings/network) and expand Outbound requests:

Outbound local requests whitelist

The whilelist entries can be separated by semicolons, commas or whitespaces (including newlines) and be in different formats like hostnames, IP addresses and/or IP ranges. IPv6 is supported. Hostnames that contain unicode characters should use IDNA encoding.

The whitelist can hold a maximum of 1000 entries. Each entry can be a maximum of 255 characters.

Example:

example.com;gitlab.example.com
127.0.0.1,1:0:0:0:0:0:0:1
127.0.0.0/8 1:0:0:0:0:0:0:0/124
Note: Wildcards (*.example.com) and ports (127.0.0.1:3000) are not currently supported.