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mail_attachment.go

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package api

import (

"bytes"

"encoding/base64"

"fmt"

"io"

"mime"

"mime/multipart"

"mime/quotedprintable"

"net/http"

"net/mail"

"net/textproto"

"path/filepath"

"strconv"

"strings"

)

const (

maxAttachmentBytes = 25 << 20 // decoded size cap for one download

maxMIMEDepth = 8

maxMIMEParts = 200

)

// AttachmentInfo describes one downloadable MIME part of a message.

type AttachmentInfo struct {

Index int `json:"index"`

Filename string `json:"filename"`

ContentType string `json:"contentType"`

Size int64 `json:"size"`

}

type rawAttachment struct {

info AttachmentInfo

data []byte // decoded content

}

// listAttachments walks the MIME tree of a raw message and returns every

// attachment part (disposition attachment, or a named non-text part) in a

// stable document order, with decoded bytes.

func listAttachments(raw []byte) []rawAttachment {

msg, err := mail.ReadMessage(bytes.NewReader(raw))

if err != nil {

return nil

}

var out []rawAttachment

count := 0

var walk func(hdr textproto.MIMEHeader, body io.Reader, depth int)

walk = func(hdr textproto.MIMEHeader, body io.Reader, depth int) {

if depth > maxMIMEDepth || count > maxMIMEParts {

return

}

count++

ct := hdr.Get("Content-Type")

mt, params, err := mime.ParseMediaType(ct)

if err != nil || mt == "" {

mt, params = "text/plain", nil

}

if strings.HasPrefix(mt, "multipart/") && params["boundary"] != "" {

mr := multipart.NewReader(body, params["boundary"])

for {

p, err := mr.NextRawPart()

if err != nil {

return

}

walk(p.Header, p, depth+1)

}

}

disp, dparams, _ := mime.ParseMediaType(hdr.Get("Content-Disposition"))

name := dparams["filename"]

if name == "" {

name = params["name"]

}

isAtt := disp == "attachment" || (name != "" && !strings.HasPrefix(mt, "multipart/"))

if depth == 0 && disp != "attachment" {

isAtt = false // a top-level body is not an attachment unless asked

}

if !isAtt {

return

}

data := decodePart(hdr.Get("Content-Transfer-Encoding"), body)

if data == nil {

return

}

out = append(out, rawAttachment{

info: AttachmentInfo{Index: len(out), Filename: sanitizeFilename(name, len(out)), ContentType: mt, Size: int64(len(data))},

data: data,

})

}

walk(textproto.MIMEHeader(msg.Header), msg.Body, 0)

return out

}

// decodePart decodes a transfer-encoded part, bounded by maxAttachmentBytes+1

// so an oversized part is detectable without unbounded memory.

func decodePart(cte string, r io.Reader) []byte {

switch strings.ToLower(strings.TrimSpace(cte)) {

case "base64":

r = base64.NewDecoder(base64.StdEncoding, &stripWS{r})

case "quoted-printable":

r = quotedprintable.NewReader(r)

}

b, _ := io.ReadAll(io.LimitReader(r, maxAttachmentBytes+1))

if b == nil {

b = []byte{}

}

return b

}

// stripWS drops CR/LF/space so base64 with line breaks decodes.

type stripWS struct{ r io.Reader }

func (s *stripWS) Read(p []byte) (int, error) {

n, err := s.r.Read(p)

j := 0

for i := 0; i < n; i++ {

if c := p[i]; c != '\r' && c != '\n' && c != ' ' && c != '\t' {

p[j] = c

j++

}

}

return j, err

}

// sanitizeFilename reduces a client-supplied name to a safe base name.

func sanitizeFilename(name string, idx int) string {

if dec, err := new(mime.WordDecoder).DecodeHeader(name); err == nil {

name = dec

}

name = strings.ReplaceAll(name, "\\", "/")

name = filepath.Base(name)

var sb strings.Builder

for _, r := range name {

if r < 0x20 || r == 0x7f || r == '"' || r == ';' || r == '%' {

sb.WriteRune('_')

continue

}

sb.WriteRune(r)

}

name = strings.TrimSpace(sb.String())

if len(name) > 200 {

name = name[:200]

}

if name == "" || name == "." || name == ".." || name == "/" {

return "attachment-" + strconv.Itoa(idx+1)

}

return name

}

// handleMailAttachment serves one attachment of the caller's message.

//

// GET /api/v1/mail/attachment?id=<id>&folder=<folder>&index=<n>

func (h *MailHandler) handleMailAttachment(w http.ResponseWriter, r *http.Request) {

if r.Method != http.MethodGet {

h.sendError(w, http.StatusMethodNotAllowed, "Method not allowed")

return

}

user, ok := r.Context().Value("user").(string)

if !ok || user == "" {

h.sendError(w, http.StatusUnauthorized, "Unauthorized")

return

}

if h.mailDB == nil || h.msgStore == nil {

h.sendError(w, http.StatusServiceUnavailable, "Mail storage not available")

return

}

q := r.URL.Query()

id := q.Get("id")

if !validMessageID(id) {

h.sendError(w, http.StatusBadRequest, "Valid message id required")

return

}

idx, err := strconv.Atoi(q.Get("index"))

if err != nil || idx < 0 {

h.sendError(w, http.StatusBadRequest, "Valid attachment index required")

return

}

folder := q.Get("folder")

if _, ok := h.lookupMailboxes(user, folder); !ok {

h.sendError(w, http.StatusBadRequest, "Invalid folder")

return

}

mb, _, meta, found := h.findAnywhere(user, folder, id)

if !found || meta == nil {

h.sendError(w, http.StatusNotFound, "Email not found")

return

}

_ = mb

data, err := h.msgStore.ReadMessage(user, meta.MessageID)

if err != nil {

h.sendError(w, http.StatusInternalServerError, "Failed to read email")

return

}

atts := listAttachments(data)

if idx >= len(atts) {

h.sendError(w, http.StatusNotFound, "Attachment not found")

return

}

a := atts[idx]

if a.info.Size > maxAttachmentBytes {

h.sendError(w, http.StatusRequestEntityTooLarge, "Attachment too large")

return

}

// Always download as an opaque binary: never render sender-controlled

// content (HTML/SVG) in our origin.

w.Header().Set("Content-Type", "application/octet-stream")

w.Header().Set("X-Content-Type-Options", "nosniff")

w.Header().Set("Content-Security-Policy", "sandbox; default-src 'none'")

w.Header().Set("Cache-Control", "private, no-store")

w.Header().Set("Content-Disposition", contentDispositionAttachment(a.info.Filename))

w.Header().Set("Content-Length", strconv.Itoa(len(a.data)))

_, _ = w.Write(a.data)

}

// contentDispositionAttachment builds a header with an ASCII fallback and an

// RFC 5987 filename*, never containing quotes or control characters.

func contentDispositionAttachment(name string) string {

ascii := make([]rune, 0, len(name))

for _, r := range name {

if r < 0x20 || r > 0x7e || r == '"' || r == '\\' || r == ';' || r == '%' {

r = '_'

}

ascii = append(ascii, r)

}

return fmt.Sprintf("attachment; filename=\"%s\"; filename*=UTF-8''%s", string(ascii), pctEncode(name))

}

func pctEncode(s string) string {

var sb strings.Builder

for i := 0; i < len(s); i++ {

c := s[i]

if c >= 'a' && c <= 'z' || c >= 'A' && c <= 'Z' || c >= '0' && c <= '9' || c == '.' || c == '-' || c == '_' {

sb.WriteByte(c)

} else {

fmt.Fprintf(&sb, "%%%02X", c)

}

}

return sb.String()

}