This affects all versions of package node-pdf-generator. Due to lack of user input validation and sanitization done to the content given to node-pdf-generator, it is possible for an attacker to craft a url that will be passed to an external server allowing an SSRF attack.
GitLab CE/EE, versions 8.18 up to 11.x before 11.3.11, 11.4 before 11.4.8, and 11.5 before 11.5.1, are vulnerable to an SSRF vulnerability in webhooks.