fix(actions): make artifact signature payloads unambiguous (#37707)

This PR hardens artifact URL signing by encoding signature inputs in an
unambiguous binary payload before computing the HMAC.

What it changes:

- replace direct concatenation-style signing inputs with explicit
payload builders
- encode string fields with a length prefix before appending their bytes
- encode integer fields as fixed-width binary values instead of decimal
text
- apply the same hardening to both:
  - Actions Artifact V4 signing in `routers/api/actions/artifactsv4.go`
  - artifact download signing in `routers/api/v1/repo/action.go`
- add regression tests that verify distinct field combinations produce
distinct payloads and signatures

Why:

The previous signing logic built HMAC inputs by appending multiple
fields without a strongly structured representation. That kind of
construction can create ambiguity at field boundaries, where different
parameter combinations may serialize into the same byte stream for
signing.

This change removes that ambiguity by constructing a deterministic
payload format with explicit boundaries between fields.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Nicolas <bircni@icloud.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>

modules/actions/artifacts.go

@@ -0,0 +1,29 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package actions
+
+import (
+	"crypto/hmac"
+	"crypto/sha256"
+	"encoding/binary"
+	"io"
+
+	"code.gitea.io/gitea/modules/setting"
+)
+
+type tagType string
+
+// BuildSignature builds a hmac signature for the input values.
+// "tag" is an internal pre-defined static string to distinguish the signatures for different purpose.
+func BuildSignature(tag tagType, vals ...string) []byte {
+	m := hmac.New(sha256.New, setting.GetGeneralTokenSigningSecret())
+	_, _ = io.WriteString(m, string(tag))
+	var buf8 [8]byte
+	for _, v := range vals {
+		binary.LittleEndian.PutUint64(buf8[:], uint64(len(v)))
+		_, _ = m.Write(buf8[:])
+		_, _ = io.WriteString(m, v)
+	}
+	return m.Sum(nil)
+}

modules/actions/artifacts_test.go

@@ -0,0 +1,36 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package actions
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestBuildSignature(t *testing.T) {
+	a := BuildSignature("v0", "x")
+	b := BuildSignature("v0", "x")
+	assert.Equal(t, a, b)
+
+	a = BuildSignature("v0", "x", "yz")
+	b = BuildSignature("v0", "xy", "z")
+	assert.NotEqual(t, a, b)
+
+	a = BuildSignature("v1", "x")
+	b = BuildSignature("v2", "x")
+	assert.NotEqual(t, a, b)
+
+	a = BuildSignature("v0", "x")
+	b = BuildSignature("v0x")
+	assert.NotEqual(t, a, b)
+
+	a = BuildSignature("v0", "", "x")
+	b = BuildSignature("v0", "x", "")
+	assert.NotEqual(t, a, b)
+
+	a = BuildSignature("v0")
+	b = BuildSignature("v0")
+	assert.Equal(t, a, b)
+}

routers/api/actions/artifactsv4.go

@@ -104,6 +104,7 @@ import (
 
 	actions_model "code.gitea.io/gitea/models/actions"
 	"code.gitea.io/gitea/models/db"
+	actions_module "code.gitea.io/gitea/modules/actions"
 	"code.gitea.io/gitea/modules/httplib"
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/optional"
@@ -162,13 +163,7 @@ func ArtifactsV4Routes(prefix string) *web.Router {
 }
 
 func (r *artifactV4Routes) buildSignature(endpoint, expires, artifactName string, taskID, artifactID int64) []byte {
-	mac := hmac.New(sha256.New, setting.GetGeneralTokenSigningSecret())
-	mac.Write([]byte(endpoint))
-	mac.Write([]byte(expires))
-	mac.Write([]byte(artifactName))
-	_, _ = fmt.Fprint(mac, taskID)
-	_, _ = fmt.Fprint(mac, artifactID)
-	return mac.Sum(nil)
+	return actions_module.BuildSignature("v4", endpoint, expires, artifactName, strconv.FormatInt(taskID, 10), strconv.FormatInt(artifactID, 10))
 }
 
 func (r *artifactV4Routes) buildArtifactURL(ctx *ArtifactContext, endpoint, artifactName string, taskID, artifactID int64) string {

routers/api/v1/repo/action.go

@@ -6,7 +6,6 @@ package repo
 import (
 	go_context "context"
 	"crypto/hmac"
-	"crypto/sha256"
 	"encoding/base64"
 	"errors"
 	"fmt"
@@ -21,9 +20,9 @@ import (
 	"code.gitea.io/gitea/models/db"
 	repo_model "code.gitea.io/gitea/models/repo"
 	secret_model "code.gitea.io/gitea/models/secret"
+	"code.gitea.io/gitea/modules/actions"
 	"code.gitea.io/gitea/modules/httplib"
 	"code.gitea.io/gitea/modules/optional"
-	"code.gitea.io/gitea/modules/setting"
 	api "code.gitea.io/gitea/modules/structs"
 	"code.gitea.io/gitea/modules/util"
 	"code.gitea.io/gitea/modules/web"
@@ -1959,11 +1958,7 @@ func DeleteArtifact(ctx *context.APIContext) {
 }
 
 func buildSignature(endp string, expires, artifactID int64) []byte {
-	mac := hmac.New(sha256.New, setting.GetGeneralTokenSigningSecret())
-	mac.Write([]byte(endp))
-	fmt.Fprint(mac, expires)
-	fmt.Fprint(mac, artifactID)
-	return mac.Sum(nil)
+	return actions.BuildSignature("api", endp, strconv.FormatInt(expires, 10), strconv.FormatInt(artifactID, 10))
 }
 
 func buildDownloadRawEndpoint(repo *repo_model.Repository, artifactID int64) string {