CVE-2026-67446: Mailpit: Thumbnail generation decodes unbounded image dimensions before scaling

Mailpit’s thumbnail endpoint decodes attacker-supplied image attachments into a full raster before checking any decoded-pixel, dimension, or memory budget. A remote client that can store an email and reach the default web API can supply a compact high-dimension image, then request /api/v1/message/{id}/part/{partID}/thumb to force server-side memory and CPU work far larger than the encoded attachment size before Mailpit returns a 180x120 thumbnail.