CVE-2026-33675: Vikunja has SSRF via Todoist/Trello Migration File Attachment URLs that Allows Reading Internal Network Resources

The migration helper functions DownloadFile and DownloadFileWithHeaders in pkg/modules/migration/helpers.go make arbitrary HTTP GET requests without any SSRF protection. When a user triggers a Todoist or Trello migration, file attachment URLs from the third-party API response are passed directly to these functions, allowing an attacker to force the Vikunja server to fetch internal network resources and return the response as a downloadable task attachment.