diff --git a/custom/conf/app.example.ini b/custom/conf/app.example.ini
index a7218a082c..cf257d83f6 100644
--- a/custom/conf/app.example.ini
+++ b/custom/conf/app.example.ini
@@ -1707,6 +1707,9 @@ LEVEL = Info
 ;;
 ;; convert links of attached images to inline images. Only for images hosted in this gitea instance.
 ;BASE64_EMBED_IMAGES = false
+;;
+;; The maximum size of an image attachment to be embedded in the email.
+;BASE64_EMBED_IMAGES_MAX_SIZE = 5242880
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
diff --git a/modules/setting/mailer.go b/modules/setting/mailer.go
index 638c442884..48a8e965a6 100644
--- a/modules/setting/mailer.go
+++ b/modules/setting/mailer.go
@@ -13,22 +13,23 @@ import (
 
 	"code.gitea.io/gitea/modules/log"
 
-	shellquote "github.com/kballard/go-shellquote"
+	"github.com/kballard/go-shellquote"
 )
 
 // Mailer represents mail service.
 type Mailer struct {
 	// Mailer
-	Name                 string              `ini:"NAME"`
-	From                 string              `ini:"FROM"`
-	EnvelopeFrom         string              `ini:"ENVELOPE_FROM"`
-	OverrideEnvelopeFrom bool                `ini:"-"`
-	FromName             string              `ini:"-"`
-	FromEmail            string              `ini:"-"`
-	SendAsPlainText      bool                `ini:"SEND_AS_PLAIN_TEXT"`
-	SubjectPrefix        string              `ini:"SUBJECT_PREFIX"`
-	OverrideHeader       map[string][]string `ini:"-"`
-	Base64EmbedImages    bool                `ini:"BASE64_EMBED_IMAGES"`
+	Name                                  string              `ini:"NAME"`
+	From                                  string              `ini:"FROM"`
+	EnvelopeFrom                          string              `ini:"ENVELOPE_FROM"`
+	OverrideEnvelopeFrom                  bool                `ini:"-"`
+	FromName                              string              `ini:"-"`
+	FromEmail                             string              `ini:"-"`
+	SendAsPlainText                       bool                `ini:"SEND_AS_PLAIN_TEXT"`
+	SubjectPrefix                         string              `ini:"SUBJECT_PREFIX"`
+	OverrideHeader                        map[string][]string `ini:"-"`
+	Base64EmbedImages                     bool                `ini:"BASE64_EMBED_IMAGES"`
+	Base64EmbedImagesMaxSizePerAttachment int64               `ini:"BASE64_EMBED_IMAGES_MAX_SIZE_PER_ATTACHMENT"`
 
 	// SMTP sender
 	Protocol             string `ini:"PROTOCOL"`
@@ -152,6 +153,7 @@ func loadMailerFrom(rootCfg ConfigProvider) {
 	sec.Key("SENDMAIL_CONVERT_CRLF").MustBool(true)
 	sec.Key("FROM").MustString(sec.Key("USER").String())
 	sec.Key("BASE64_EMBED_IMAGES").MustBool(false)
+	sec.Key("BASE64_EMBED_IMAGES_MAX_SIZE_PER_ATTACHMENT").MustInt64(5 * 1024 * 1024)
 
 	// Now map the values on to the MailService
 	MailService = &Mailer{}
diff --git a/services/mailer/mail.go b/services/mailer/mail.go
index 8d231e08fa..d8227fc6e1 100644
--- a/services/mailer/mail.go
+++ b/services/mailer/mail.go
@@ -21,7 +21,9 @@ import (
 
 	activities_model "code.gitea.io/gitea/models/activities"
 	issues_model "code.gitea.io/gitea/models/issues"
+	access_model "code.gitea.io/gitea/models/perm/access"
 	repo_model "code.gitea.io/gitea/models/repo"
+	"code.gitea.io/gitea/models/unit"
 	user_model "code.gitea.io/gitea/models/user"
 	"code.gitea.io/gitea/modules/base"
 	"code.gitea.io/gitea/modules/emoji"
@@ -421,6 +423,7 @@ func Base64InlineImages(body string, ctx *MailCommentContext) (string, error) {
 }
 
 func AttachmentSrcToBase64DataURI(attachmentPath string, ctx *MailCommentContext) (string, error) {
+	maxSizePerImageAttachment := setting.MailService.Base64EmbedImagesMaxSizePerAttachment
 	if !strings.HasPrefix(attachmentPath, setting.AppURL) { // external image
 		return "", fmt.Errorf("external image")
 	}
@@ -435,6 +438,16 @@ func AttachmentSrcToBase64DataURI(attachmentPath string, ctx *MailCommentContext
 		return "", err
 	}
 
+	// "Doer" is theoretically not the correct permission check (as Doer created the action on which to send), but as this is batch processed the receipants can't be accessed.
+	// Therefore we check the Doer, with which we counter leaking information as a Doer brute force attack on attachments would be possible.
+	perm, err := access_model.GetUserRepoPermission(ctx, ctx.Issue.Repo, ctx.Doer)
+	if err != nil {
+		return "", err
+	}
+	if !perm.CanRead(unit.TypeIssues) {
+		return "", fmt.Errorf("no permission")
+	}
+
 	fr, err := storage.Attachments.Open(attachment.RelativePath())
 	if err != nil {
 		return "", err
@@ -446,7 +459,16 @@ func AttachmentSrcToBase64DataURI(attachmentPath string, ctx *MailCommentContext
 		return "", err
 	}
 
+	if len(content) > int(maxSizePerImageAttachment) {
+		return "", fmt.Errorf("image too large (%d bytes) of max %d bytes", len(content), maxSizePerImageAttachment)
+	}
+
 	mimeType := http.DetectContentType(content)
+
+	if !strings.HasPrefix(mimeType, "image/") {
+		return "", fmt.Errorf("not an image")
+	}
+
 	encoded := base64.StdEncoding.EncodeToString(content)
 	dataURI := fmt.Sprintf("data:%s;base64,%s", mimeType, encoded)