This whitepaper explains how xml external entity and server-side request forgery vulnerabilities work and how they can be combined to perform attacks.
You can download it from the following link: https://packetstormsecurity.com/files/download/161943/URL-Exploiting-XXE-to-SSRF.pdf
Source: https://packetstormsecurity.com/files/161943/URL-Exploiting-XXE-To-SSRF.html