Mailpit is an email testing tool and API for developers. Prior to 1.30.4, Mailpit decodes attacker-supplied image attachments into a full raster before checking decoded dimensions, pixel count, or memory use in the GET /api/v1/message/{id}/part/{partID}/thumb endpoint. The Thumbnail handler in server/apiv1/thumbnails.go obtains attachment bytes through storage.GetAttachmentPart(), accepts image/* content, and calls imaging.Decode() with AutoOrientation before imaging.Fill() scales the image to 180 by 120 pixels. A compact image declaring very large dimensions can therefore consume disproportionately large memory and CPU, and opening the message UI can trigger the same endpoint through server/ui-src/components/message/MessageAttachments.vue. This can degrade availability when an unauthenticated client can store the crafted attachment and reach the web API. This issue is fixed in version 1.30.4.

Affected: Go: github.com/axllent/mailpit

## Summary

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.

## Technical Details

The route is registered as `GET /api/v1/message/{id}/part/{partID}/thumb` in `server/server.go`. The handler in `server/apiv1/thumbnails.go` loads the requested attachment and accepts any part whose content type begins with `image/`:

```go a, err := storage.GetAttachmentPart(id, partID) // ... if !strings.HasPrefix(a.ContentType, "image/") { blankImage(a, w) return }

buf := bytes.NewBuffer(a.Content) img, err := imaging.Decode(buf, imaging.AutoOrientation(true)) ```

`storage.GetAttachmentPart()` reparses the stored raw email and returns the matching attacker-supplied attachment bytes. `Thumbnail()` then calls `imaging.Decode()` before any check on declared dimensions or estimated decoded bytes. The subsequent `imaging.Fill(img, 180, 120, ...)`, `imaging.Clone()`, and JPEG encode only happen after the full image has already been decoded.

The thumbnail output is fixed at 180x120, so the endpoint does not need to decode arbitrarily large rasters. The current implementation lets a small…