Blind POST SSRF via Web Push Notification Endpoint

Summary

A registered user can register an arbitrary URL as their Web Push notification endpoint. phpBB stores this URL without validation and later uses the Minishlink WebPush library to send HTTP POST requests to it. This allows the attacker to force the phpBB server to make outbound HTTP requests to internal network services, cloud metadata endpoints, or other attacker-controlled destinations.

This is the same class of vulnerability as HackerOne #1018568 (SSRF in Jabber settings), but requires only a registered user rather than admin access. The Jabber feature was removed in phpBB 4.0 via migration; Web Push was added as its replacement.

Severity

Medium — CVSS 3.1: 5.0 (AV:N/AC:L/PR:L/UI:N/S:C/C:L/I:N/A:N)

Affected Version

phpBB 4.0.0-alpha1 (commit 4a57f1ff3c)