fix(channels): harden all channels against unbounded read DoS

Systematic sweep across all channel implementations to cap unbounded
io.ReadAll and io.Copy calls that allow remote memory or disk
exhaustion.

Inbound webhooks (4 MB cap, matching existing aibot.go pattern):
- wecom/bot.go handleMessageCallback — io.LimitReader + 413 reject
- wecom/app.go handleMessageCallback — io.LimitReader + 413 reject

Outbound API responses (1 MB cap):
- wecom/bot.go sendWeComMessage — error + success response reads
- wecom/app.go uploadToWeComMedia, sendWeComMessage, refreshAccessToken
- wecom/aibot.go postToResponseURL — error response read
- line/line.go sendLineMessage — error response read

Media downloads (50 MB cap, prevents disk exhaustion):
- utils/media.go DownloadFile — io.CopyN replaces io.Copy, protects
  all 6 channels using this utility (telegram, slack, discord, line,
  feishu, onebot)
- matrix/matrix.go downloadMedia — size check after DownloadBytes
- feishu/feishu_64.go downloadResource — io.CopyN replaces io.Copy

Fixes #1405

pkg/channels/feishu/feishu_64.go

@@ -636,7 +636,9 @@ func (c *FeishuChannel) downloadResource(
 		return ""
 	}
 
-	if _, copyErr := io.Copy(out, resp.File); copyErr != nil {
+	maxSize := int64(utils.MaxMediaDownloadSize)
+	written, copyErr := io.CopyN(out, resp.File, maxSize)
+	if copyErr != nil && copyErr != io.EOF {
 		out.Close()
 		os.Remove(localPath)
 		logger.ErrorCF("feishu", "Failed to write resource to file", map[string]any{
@@ -644,6 +646,14 @@ func (c *FeishuChannel) downloadResource(
 		})
 		return ""
 	}
+	if written >= maxSize {
+		out.Close()
+		os.Remove(localPath)
+		logger.ErrorCF("feishu", "Resource exceeds size limit, download aborted", map[string]any{
+			"limit_mb": maxSize / (1 << 20),
+		})
+		return ""
+	}
 	out.Close()
 
 	ref, err := store.Store(localPath, media.MediaMeta{

pkg/channels/line/line.go

@@ -654,7 +654,7 @@ func (c *LINEChannel) callAPI(ctx context.Context, endpoint string, payload any)
 	defer resp.Body.Close()
 
 	if resp.StatusCode != http.StatusOK {
-		respBody, err := io.ReadAll(resp.Body)
+		respBody, err := io.ReadAll(io.LimitReader(resp.Body, 1<<20))
 		if err != nil {
 			return channels.ClassifySendError(resp.StatusCode, fmt.Errorf("reading LINE API error response: %w", err))
 		}

pkg/channels/matrix/matrix.go

@@ -726,10 +726,18 @@ func (c *MatrixChannel) downloadMedia(
 	reqCtx, cancel := context.WithTimeout(dlCtx, 20*time.Second)
 	defer cancel()
 
+	// Reject oversized media after download. The mautrix SDK only exposes
+	// DownloadBytes (no streaming variant), so we cannot prevent the initial
+	// allocation. The 20s context timeout bounds the practical damage, and
+	// this check ensures oversized payloads are not persisted to disk.
+	const maxMediaSize = 50 << 20 // 50 MB
 	data, err := c.client.DownloadBytes(reqCtx, parsed)
 	if err != nil {
 		return "", err
 	}
+	if len(data) > maxMediaSize {
+		return "", fmt.Errorf("matrix media exceeds %d MB size limit", maxMediaSize/(1<<20))
+	}
 
 	// Encrypted attachments put URL in msgEvt.File and require client-side decryption.
 	if msgEvt != nil && msgEvt.File != nil && msgEvt.URL == "" {

pkg/channels/wecom/aibot.go

@@ -793,7 +793,7 @@ func (c *WeComAIBotChannel) sendViaResponseURL(responseURL, content string) erro
 		return nil
 	}
 
-	respBody, err := io.ReadAll(resp.Body)
+	respBody, err := io.ReadAll(io.LimitReader(resp.Body, 1<<20))
 	if err != nil {
 		return fmt.Errorf("reading response_url body: %w: %w", channels.ErrTemporary, err)
 	}

pkg/channels/wecom/app.go

@@ -320,8 +320,9 @@ func (c *WeComAppChannel) uploadMedia(ctx context.Context, accessToken, mediaTyp
 	}
 	defer resp.Body.Close()
 
+	const maxRespSize = 1 << 20 // 1 MB
 	if resp.StatusCode != http.StatusOK {
-		respBody, readErr := io.ReadAll(resp.Body)
+		respBody, readErr := io.ReadAll(io.LimitReader(resp.Body, maxRespSize))
 		if readErr != nil {
 			return "", channels.ClassifySendError(
 				resp.StatusCode,
@@ -379,8 +380,9 @@ func (c *WeComAppChannel) sendWeComMessage(ctx context.Context, accessToken stri
 	}
 	defer resp.Body.Close()
 
+	const maxSendRespSize = 1 << 20 // 1 MB
 	if resp.StatusCode != http.StatusOK {
-		respBody, readErr := io.ReadAll(resp.Body)
+		respBody, readErr := io.ReadAll(io.LimitReader(resp.Body, maxSendRespSize))
 		if readErr != nil {
 			return channels.ClassifySendError(
 				resp.StatusCode,
@@ -393,7 +395,7 @@ func (c *WeComAppChannel) sendWeComMessage(ctx context.Context, accessToken stri
 		)
 	}
 
-	respBody, err := io.ReadAll(resp.Body)
+	respBody, err := io.ReadAll(io.LimitReader(resp.Body, maxSendRespSize))
 	if err != nil {
 		return fmt.Errorf("failed to read response: %w", err)
 	}
@@ -550,13 +552,18 @@ func (c *WeComAppChannel) handleMessageCallback(ctx context.Context, w http.Resp
 		return
 	}
 
-	// Read request body
-	body, err := io.ReadAll(r.Body)
+	// Read request body (limit to 4 MB to prevent memory exhaustion).
+	const maxBodySize = 4 << 20 // 4 MB
+	body, err := io.ReadAll(io.LimitReader(r.Body, maxBodySize+1))
 	if err != nil {
 		http.Error(w, "Failed to read body", http.StatusBadRequest)
 		return
 	}
 	defer r.Body.Close()
+	if len(body) > maxBodySize {
+		http.Error(w, "Request body too large", http.StatusRequestEntityTooLarge)
+		return
+	}
 
 	// Parse XML to get encrypted message
 	var encryptedMsg struct {
@@ -697,7 +704,7 @@ func (c *WeComAppChannel) refreshAccessToken() error {
 	}
 	defer resp.Body.Close()
 
-	body, err := io.ReadAll(resp.Body)
+	body, err := io.ReadAll(io.LimitReader(resp.Body, 1<<20))
 	if err != nil {
 		return fmt.Errorf("failed to read response: %w", err)
 	}

pkg/channels/wecom/bot.go

@@ -253,13 +253,18 @@ func (c *WeComBotChannel) handleMessageCallback(ctx context.Context, w http.Resp
 		return
 	}
 
-	// Read request body
-	body, err := io.ReadAll(r.Body)
+	// Read request body (limit to 4 MB to prevent memory exhaustion).
+	const maxBodySize = 4 << 20 // 4 MB
+	body, err := io.ReadAll(io.LimitReader(r.Body, maxBodySize+1))
 	if err != nil {
 		http.Error(w, "Failed to read body", http.StatusBadRequest)
 		return
 	}
 	defer r.Body.Close()
+	if len(body) > maxBodySize {
+		http.Error(w, "Request body too large", http.StatusRequestEntityTooLarge)
+		return
+	}
 
 	// Parse XML to get encrypted message
 	var encryptedMsg struct {
@@ -452,8 +457,9 @@ func (c *WeComBotChannel) sendWebhookReply(ctx context.Context, userID, content
 	}
 	defer resp.Body.Close()
 
+	const maxRespSize = 1 << 20 // 1 MB
 	if resp.StatusCode != http.StatusOK {
-		body, readErr := io.ReadAll(resp.Body)
+		body, readErr := io.ReadAll(io.LimitReader(resp.Body, maxRespSize))
 		if readErr != nil {
 			return channels.ClassifySendError(
 				resp.StatusCode,
@@ -466,7 +472,7 @@ func (c *WeComBotChannel) sendWebhookReply(ctx context.Context, userID, content
 		)
 	}
 
-	body, err := io.ReadAll(resp.Body)
+	body, err := io.ReadAll(io.LimitReader(resp.Body, maxRespSize))
 	if err != nil {
 		return fmt.Errorf("failed to read response: %w", err)
 	}

pkg/utils/media.go

@@ -48,12 +48,17 @@ func SanitizeFilename(filename string) string {
 	return base
 }
 
+// MaxMediaDownloadSize is the upper bound for media file downloads (50 MB).
+// Prevents disk exhaustion from oversized or malicious attachments.
+const MaxMediaDownloadSize = 50 << 20
+
 // DownloadOptions holds optional parameters for downloading files
 type DownloadOptions struct {
 	Timeout      time.Duration
 	ExtraHeaders map[string]string
 	LoggerPrefix string
 	ProxyURL     string
+	MaxSize      int64 // 0 = use MaxMediaDownloadSize default
 }
 
 // DownloadFile downloads a file from URL to a local temp directory.
@@ -134,7 +139,12 @@ func DownloadFile(urlStr, filename string, opts DownloadOptions) string {
 	}
 	defer out.Close()
 
-	if _, err := io.Copy(out, resp.Body); err != nil {
+	maxSize := opts.MaxSize
+	if maxSize <= 0 {
+		maxSize = MaxMediaDownloadSize
+	}
+	written, err := io.CopyN(out, resp.Body, maxSize)
+	if err != nil && err != io.EOF {
 		out.Close()
 		os.Remove(localPath)
 		logger.ErrorCF(opts.LoggerPrefix, "Failed to write file", map[string]any{
@@ -142,6 +152,15 @@ func DownloadFile(urlStr, filename string, opts DownloadOptions) string {
 		})
 		return ""
 	}
+	// Check if there is more data beyond the limit (oversized file).
+	if written >= maxSize {
+		out.Close()
+		os.Remove(localPath)
+		logger.ErrorCF(opts.LoggerPrefix, "File exceeds size limit, download aborted", map[string]any{
+			"limit_mb": maxSize / (1 << 20),
+		})
+		return ""
+	}
 
 	logger.DebugCF(opts.LoggerPrefix, "File downloaded successfully", map[string]any{
 		"path": localPath,