Fix Feishu webhook signature verification (go-gitea#34788)

# Fix Feishu Webhook Signature Verification

This PR implements proper signature verification for Feishu (Lark)
webhooks according to the [official
documentation](https://open.feishu.cn/document/client-docs/bot-v3/add-custom-bot).

## Changes

- Implemented the `GenSign` function based on Feishu's official Go
sample code
- Modified the webhook request creation to include timestamp and
signature in the payload when a secret is configured
- Fixed the signature generation algorithm to properly use HMAC-SHA256
with the correct string format

## Implementation Details

The signature verification works as follows:
1. When a webhook secret is provided, a timestamp is generated
2. The signature string is created using `timestamp + "\n" + secret`
3. The HMAC-SHA256 algorithm is applied to an empty string using the
signature string as the key
4. The result is Base64 encoded to produce the final signature
5. Both timestamp and signature are added to the payload

According to Feishu's documentation, the timestamp must be within 1 hour
(3600 seconds) of the current time to be considered valid.

## Security Note

Feishu emphasizes the importance of keeping webhook URLs secure. Do not
disclose them on GitHub, blogs, or any public sites to prevent
unauthorized use.

## References

- [Feishu Custom Bot
Documentation](https://open.feishu.cn/document/client-docs/bot-v3/add-custom-bot)

---------

Co-authored-by: hiifong <[email protected]>
Co-authored-by: wxiaoguang <[email protected]>

Author: 3 people

services/webhook/feishu.go

@@ -5,9 +5,13 @@ package webhook
 
 import (
 	"context"
+	"crypto/hmac"
+	"crypto/sha256"
+	"encoding/base64"
 	"fmt"
 	"net/http"
 	"strings"
+	"time"
 
 	webhook_model "code.gitea.io/gitea/models/webhook"
 	"code.gitea.io/gitea/modules/git"
@@ -16,10 +20,12 @@ import (
 )
 
 type (
-	// FeishuPayload represents
+	// FeishuPayload represents the payload for Feishu webhook
 	FeishuPayload struct {
-		MsgType string `json:"msg_type"` // text / post / image / share_chat / interactive / file /audio / media
-		Content struct {
+		Timestamp int64  `json:"timestamp,omitempty"` // Unix timestamp for signature verification
+		Sign      string `json:"sign,omitempty"`      // Signature for verification
+		MsgType   string `json:"msg_type"`            // text / post / image / share_chat / interactive / file /audio / media
+		Content   struct {
 			Text string `json:"text"`
 		} `json:"content"`
 	}
@@ -178,9 +184,29 @@ func (feishuConvertor) WorkflowJob(p *api.WorkflowJobPayload) (FeishuPayload, er
 	return newFeishuTextPayload(text), nil
 }
 
+// feishuGenSign generates a signature for Feishu webhook
+// https://open.feishu.cn/document/client-docs/bot-v3/add-custom-bot
+func feishuGenSign(secret string, timestamp int64) string {
+	// key="{timestamp}\n{secret}", then hmac-sha256, then base64 encode
+	stringToSign := fmt.Sprintf("%d\n%s", timestamp, secret)
+	h := hmac.New(sha256.New, []byte(stringToSign))
+	return base64.StdEncoding.EncodeToString(h.Sum(nil))
+}
+
 func newFeishuRequest(_ context.Context, w *webhook_model.Webhook, t *webhook_model.HookTask) (*http.Request, []byte, error) {
-	var pc payloadConvertor[FeishuPayload] = feishuConvertor{}
-	return newJSONRequest(pc, w, t, true)
+	payload, err := newPayload(feishuConvertor{}, []byte(t.PayloadContent), t.EventType)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	// Add timestamp and signature if secret is provided
+	if w.Secret != "" {
+		timestamp := time.Now().Unix()
+		payload.Timestamp = timestamp
+		payload.Sign = feishuGenSign(w.Secret, timestamp)
+	}
+
+	return prepareJSONRequest(payload, w, t, false /* no default headers */)
 }
 
 func init() {

services/webhook/feishu_test.go

@@ -168,6 +168,7 @@ func TestFeishuJSONPayload(t *testing.T) {
 		URL:        "https://feishu.example.com/",
 		Meta:       `{}`,
 		HTTPMethod: "POST",
+		Secret:     "secret",
 	}
 	task := &webhook_model.HookTask{
 		HookID:         hook.ID,
@@ -183,10 +184,13 @@ func TestFeishuJSONPayload(t *testing.T) {
 
 	assert.Equal(t, "POST", req.Method)
 	assert.Equal(t, "https://feishu.example.com/", req.URL.String())
-	assert.Equal(t, "sha256=", req.Header.Get("X-Hub-Signature-256"))
 	assert.Equal(t, "application/json", req.Header.Get("Content-Type"))
 	var body FeishuPayload
 	err = json.NewDecoder(req.Body).Decode(&body)
 	assert.NoError(t, err)
 	assert.Equal(t, "[test/repo:test] \r\n[2020558](http://localhost:3000/test/repo/commit/2020558fe2e34debb818a514715839cabd25e778) commit message - user1\r\n[2020558](http://localhost:3000/test/repo/commit/2020558fe2e34debb818a514715839cabd25e778) commit message - user1", body.Content.Text)
+	assert.Equal(t, feishuGenSign(hook.Secret, body.Timestamp), body.Sign)
+
+	// a separate sign test, the result is generated by official python code, so the algo must be correct
+	assert.Equal(t, "rWZ84lcag1x9aBFhn1gtV4ZN+4gme3pilfQNMk86vKg=", feishuGenSign("a", 1))
 }

services/webhook/payloader.go

@@ -92,7 +92,10 @@ func newJSONRequest[T any](pc payloadConvertor[T], w *webhook_model.Webhook, t *
 	if err != nil {
 		return nil, nil, err
 	}
+	return prepareJSONRequest(payload, w, t, withDefaultHeaders)
+}
 
+func prepareJSONRequest[T any](payload T, w *webhook_model.Webhook, t *webhook_model.HookTask, withDefaultHeaders bool) (*http.Request, []byte, error) {
 	body, err := json.MarshalIndent(payload, "", "  ")
 	if err != nil {
 		return nil, nil, err