fix(mailer): reject incoming email tokens of users who cannot sign in (#39833) (#39841)

Backport #39833 by @basavaraj-sm05

Incoming email reply tokens stay valid for a year and are verified with
`user.Rands`, which is not rotated when an account is deactivated or
prohibited from signing in. A suspended user could therefore keep
commenting on public repos by replying to old notifications.

`DecodeToken` now rejects tokens of organizations, inactive users and
users prohibited from signing in, which covers every incoming mail
handler.

Co-authored-by: Basavaraj S m <basavaraj@digiscrypt.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

models/user/user.go

@@ -1021,7 +1021,8 @@ func GetInactiveUsers(ctx context.Context, olderThan time.Duration) ([]*User, er
 		Find(&users)
 }
 
-// GetUserByID returns the user object by given ID if exists.
+// GetUserByID returns the user object by given ID if exists. TODO: for most cases, this function should be avoided.
+// Use correct GetXxxUser functions instead to handle user types & states correctly.
 func GetUserByID(ctx context.Context, id int64) (*User, error) {
 	u := new(User)
 	has, err := db.GetEngine(ctx).ID(id).Get(u)
@@ -1033,7 +1034,8 @@ func GetUserByID(ctx context.Context, id int64) (*User, error) {
 	return u, nil
 }
 
-// GetUserByIDs returns the user objects by given IDs if exists.
+// GetUserByIDs returns the user objects by given IDs if exists. TODO: for most cases, this function should be avoided.
+// Use correct GetXxxUser functions instead to handle user types & states correctly.
 func GetUserByIDs(ctx context.Context, ids []int64) ([]*User, error) {
 	if len(ids) == 0 {
 		return nil, nil
@@ -1046,6 +1048,13 @@ func GetUserByIDs(ctx context.Context, ids []int64) ([]*User, error) {
 	return users, err
 }
 
+// Glossaries for XxxUser:
+// * Possible: The user may or may not exist in the database, but we can still return a valid user object (e.g. Ghost user) for the given ID.
+// * Individual: The user is a real person, not an organization or a bot.
+// * Doer: The user is the one performing an action
+// * Valid: The user is an active user and not prohibited from signing in
+// * System: Negative ID (e.g. Ghost, Actions, DeployKey, etc.) and not a real person, Ghost is not included if there is no "Possible" keyword
+
 // GetPossibleUserByID returns the possible user and its ID. If the user doesn't exist, it returns Ghost user
 func GetPossibleUserByID(ctx context.Context, id int64) (_ int64, u *User, err error) {
 	if id < 0 {
@@ -1082,6 +1091,23 @@ func GetPossibleUserByIDs(ctx context.Context, ids []int64) ([]*User, error) {
 	return users, nil
 }
 
+func GetValidDoerUserByID(ctx context.Context, id int64) (*User, error) {
+	u, err := GetUserByID(ctx, id)
+	if err != nil {
+		return nil, err
+	}
+	if !u.IsIndividual() && !u.IsTypeBot() {
+		return nil, util.NewNotExistErrorf("user %d is not a doer", id)
+	}
+	if !u.IsActive {
+		return nil, util.NewPermissionDeniedErrorf("user %d is inactive", id)
+	}
+	if u.ProhibitLogin {
+		return nil, util.NewPermissionDeniedErrorf("user %d is prohibited from login", id)
+	}
+	return u, nil
+}
+
 func getUserByNameWithTypes(ctx context.Context, name string, types ...UserType) (*User, error) {
 	u := &User{}
 	sess := db.GetEngine(ctx).Where(builder.Eq{"lower_name": strings.ToLower(name)})

services/mailer/token/token.go

@@ -98,7 +98,7 @@ func DecodeToken(ctx context.Context, token string) (HandlerType, *user_model.Us
 		return UnknownHandlerType, nil, nil, err
 	}
 
-	user, err := user_model.GetUserByID(ctx, userID)
+	user, err := user_model.GetValidDoerUserByID(ctx, userID)
 	if err != nil {
 		return UnknownHandlerType, nil, nil, err
 	}

services/mailer/token/token_test.go

@@ -0,0 +1,34 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package token
+
+import (
+	"testing"
+
+	"gitea.dev/models/unittest"
+	user_model "gitea.dev/models/user"
+
+	_ "gitea.dev/models"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestMain(m *testing.M) {
+	unittest.MainTest(m)
+}
+
+func TestDecodeTokenRejectsUserWhoCannotSignIn(t *testing.T) {
+	require.NoError(t, unittest.PrepareTestDatabase())
+	for id, expectedErr := range map[int64]string{2: "", 3: "not a doer", 9: "inactive", 37: "prohibited"} {
+		token, err := CreateToken(ReplyHandlerType, unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: id}), []byte{1})
+		require.NoError(t, err)
+		_, _, _, err = DecodeToken(t.Context(), token)
+		if expectedErr == "" {
+			assert.NoError(t, err)
+		} else {
+			assert.ErrorContains(t, err, expectedErr)
+		}
+	}
+}