diff --git a/pkg/modules/avatar/avatar.go b/pkg/modules/avatar/avatar.go
index afd5b7b34..7c453542e 100644
--- a/pkg/modules/avatar/avatar.go
+++ b/pkg/modules/avatar/avatar.go
@@ -18,6 +18,7 @@ package avatar
 
 import (
 	"errors"
+	"image"
 	"io"
 	"strings"
 
@@ -146,6 +147,19 @@ func StoreUploadedAvatar(s *xorm.Session, u *user.User, src io.ReadSeeker) error
 		return err
 	}
 
+	// The mimetype sniff above accepts image types we cannot actually store
+	// (e.g. SVG, WebP) because upload.StoreAvatarFile decodes via image.Decode,
+	// which only has the decoders registered process-wide by the imaging package
+	// (png, jpeg, gif, tiff, bmp). image.DecodeConfig uses those same decoders, so
+	// validating here rejects undecodable images with a 400 instead of failing
+	// deeper in storage with a 500.
+	if _, _, err := image.DecodeConfig(src); err != nil {
+		return ErrNotAnImage
+	}
+	if _, err := src.Seek(0, io.SeekStart); err != nil {
+		return err
+	}
+
 	u.AvatarProvider = "upload"
 	if err := upload.StoreAvatarFile(s, u, src); err != nil {
 		return err
diff --git a/pkg/routes/api/v2/avatar_upload.go b/pkg/routes/api/v2/avatar_upload.go
index 80893a53b..52b6038f6 100644
--- a/pkg/routes/api/v2/avatar_upload.go
+++ b/pkg/routes/api/v2/avatar_upload.go
@@ -31,9 +31,9 @@ import (
 )
 
 type avatarUploadInput struct {
-	// Broad allow-list because Huma's MimeTypeValidator rejects the part pre-handler; octet-stream covers programmatic clients. The real gate is the byte-level image check in avatar.StoreUploadedAvatar.
+	// Allow-list mirrors the image formats avatar.StoreUploadedAvatar can actually decode (the decoders registered process-wide by the imaging package: png, jpeg, gif, tiff, bmp); octet-stream covers programmatic clients. Huma's MimeTypeValidator rejects the part pre-handler, so anything advertised here must also pass the byte-level image check in avatar.StoreUploadedAvatar. Formats without a registered decoder (e.g. svg, webp) are intentionally excluded.
 	RawBody huma.MultipartFormFiles[struct {
-		Avatar huma.FormFile `form:"avatar" contentType:"image/png,image/jpeg,image/gif,image/webp,image/svg+xml,application/octet-stream" required:"true" doc:"The avatar image to upload. Must be an image; it is resized server-side and re-encoded as PNG."`
+		Avatar huma.FormFile `form:"avatar" contentType:"image/png,image/jpeg,image/gif,image/tiff,image/bmp,application/octet-stream" required:"true" doc:"The avatar image to upload. Must be a decodable raster image (PNG, JPEG, GIF, TIFF or BMP); it is resized server-side and re-encoded as PNG."`
 	}]
 }
 
diff --git a/pkg/webtests/huma_avatar_upload_test.go b/pkg/webtests/huma_avatar_upload_test.go
index 326d5a4ba..790039c5a 100644
--- a/pkg/webtests/huma_avatar_upload_test.go
+++ b/pkg/webtests/huma_avatar_upload_test.go
@@ -148,6 +148,29 @@ func TestAvatarUpload(t *testing.T) {
 		assert.NotEqual(t, "upload", u.AvatarProvider)
 	})
 
+	t.Run("Undecodable image (SVG) rejected with 400", func(t *testing.T) {
+		// SVG sniffs as image/svg+xml, passing the mimetype prefix check, but
+		// image.Decode/DecodeConfig has no SVG decoder. Sent as octet-stream it
+		// bypasses the part content-type allow-list, so the byte-level decode
+		// validation in StoreUploadedAvatar is what must reject it. This must be
+		// a 400 (ErrNotAnImage), not a 500 from a failed decode deeper in storage.
+		e, err := setupTestEnv()
+		require.NoError(t, err)
+		token := humaTokenFor(t, &testuser1)
+
+		svg := []byte(`<?xml version="1.0" encoding="UTF-8"?>` +
+			`<svg xmlns="http://www.w3.org/2000/svg" width="8" height="8"><rect width="8" height="8" fill="#abc"/></svg>`)
+		body, contentType := multipartAvatarBody(t, "avatar", "avatar.svg", svg)
+		rec := uploadAvatarRequest(t, e, body, contentType, token)
+		require.Equal(t, http.StatusBadRequest, rec.Code, "body: %s", rec.Body.String())
+
+		s := db.NewSession()
+		defer s.Close()
+		u, err := user.GetUserByID(s, testuser1.ID)
+		require.NoError(t, err)
+		assert.NotEqual(t, "upload", u.AvatarProvider)
+	})
+
 	t.Run("Unauthenticated", func(t *testing.T) {
 		e, err := setupTestEnv()
 		require.NoError(t, err)