Enhanced auth token / remember me (#27606)

Closes #27455

> The mechanism responsible for long-term authentication (the 'remember
me' cookie) uses a weak construction technique. It will hash the user's
hashed password and the rands value; it will then call the secure cookie
code, which will encrypt the user's name with the computed hash. If one
were able to dump the database, they could extract those two values to
rebuild that cookie and impersonate a user. That vulnerability exists
from the date the dump was obtained until a user changed their password.
> 
> To fix this security issue, the cookie could be created and verified
using a different technique such as the one explained at
https://paragonie.com/blog/2015/04/secure-authentication-php-with-long-term-persistence#secure-remember-me-cookies.

The PR removes the now obsolete setting `COOKIE_USERNAME`.

3 files changed, 244 insertions, 0 deletions

diff --git a/services/auth/auth_token.go b/services/auth/auth_token.go
new file mode 100644
index 0000000000..6b59238c98
--- /dev/null
+++ b/services/auth/auth_token.go
@@ -0,0 +1,123 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package auth
+
+import (
+	"context"
+	"crypto/sha256"
+	"crypto/subtle"
+	"encoding/hex"
+	"errors"
+	"strings"
+	"time"
+
+	auth_model "code.gitea.io/gitea/models/auth"
+	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/timeutil"
+	"code.gitea.io/gitea/modules/util"
+)
+
+// Based on https://paragonie.com/blog/2015/04/secure-authentication-php-with-long-term-persistence#secure-remember-me-cookies
+
+// The auth token consists of two parts: ID and token hash
+// Every device login creates a new auth token with an individual id and hash.
+// If a device uses the token to login into the instance, a fresh token gets generated which has the same id but a new hash.
+
+var (
+	ErrAuthTokenInvalidFormat = util.NewInvalidArgumentErrorf("auth token has an invalid format")
+	ErrAuthTokenExpired       = util.NewInvalidArgumentErrorf("auth token has expired")
+	ErrAuthTokenInvalidHash   = util.NewInvalidArgumentErrorf("auth token is invalid")
+)
+
+func CheckAuthToken(ctx context.Context, value string) (*auth_model.AuthToken, error) {
+	if len(value) == 0 {
+		return nil, nil
+	}
+
+	parts := strings.SplitN(value, ":", 2)
+	if len(parts) != 2 {
+		return nil, ErrAuthTokenInvalidFormat
+	}
+
+	t, err := auth_model.GetAuthTokenByID(ctx, parts[0])
+	if err != nil {
+		if errors.Is(err, util.ErrNotExist) {
+			return nil, ErrAuthTokenExpired
+		}
+		return nil, err
+	}
+
+	if t.ExpiresUnix < timeutil.TimeStampNow() {
+		return nil, ErrAuthTokenExpired
+	}
+
+	hashedToken := sha256.Sum256([]byte(parts[1]))
+
+	if subtle.ConstantTimeCompare([]byte(t.TokenHash), []byte(hex.EncodeToString(hashedToken[:]))) == 0 {
+		// If an attacker steals a token and uses the token to create a new session the hash gets updated.
+		// When the victim uses the old token the hashes don't match anymore and the victim should be notified about the compromised token.
+		return nil, ErrAuthTokenInvalidHash
+	}
+
+	return t, nil
+}
+
+func RegenerateAuthToken(ctx context.Context, t *auth_model.AuthToken) (*auth_model.AuthToken, string, error) {
+	token, hash, err := generateTokenAndHash()
+	if err != nil {
+		return nil, "", err
+	}
+
+	newToken := &auth_model.AuthToken{
+		ID:          t.ID,
+		TokenHash:   hash,
+		UserID:      t.UserID,
+		ExpiresUnix: timeutil.TimeStampNow().AddDuration(time.Duration(setting.LogInRememberDays*24) * time.Hour),
+	}
+
+	if err := auth_model.UpdateAuthTokenByID(ctx, newToken); err != nil {
+		return nil, "", err
+	}
+
+	return newToken, token, nil
+}
+
+func CreateAuthTokenForUserID(ctx context.Context, userID int64) (*auth_model.AuthToken, string, error) {
+	t := &auth_model.AuthToken{
+		UserID:      userID,
+		ExpiresUnix: timeutil.TimeStampNow().AddDuration(time.Duration(setting.LogInRememberDays*24) * time.Hour),
+	}
+
+	var err error
+	t.ID, err = util.CryptoRandomString(10)
+	if err != nil {
+		return nil, "", err
+	}
+
+	token, hash, err := generateTokenAndHash()
+	if err != nil {
+		return nil, "", err
+	}
+
+	t.TokenHash = hash
+
+	if err := auth_model.InsertAuthToken(ctx, t); err != nil {
+		return nil, "", err
+	}
+
+	return t, token, nil
+}
+
+func generateTokenAndHash() (string, string, error) {
+	buf, err := util.CryptoRandomBytes(32)
+	if err != nil {
+		return "", "", err
+	}
+
+	token := hex.EncodeToString(buf)
+
+	hashedToken := sha256.Sum256([]byte(token))
+
+	return token, hex.EncodeToString(hashedToken[:]), nil
+}
diff --git a/services/auth/auth_token_test.go b/services/auth/auth_token_test.go
new file mode 100644
index 0000000000..654275df17
--- /dev/null
+++ b/services/auth/auth_token_test.go
@@ -0,0 +1,107 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package auth
+
+import (
+	"testing"
+	"time"
+
+	auth_model "code.gitea.io/gitea/models/auth"
+	"code.gitea.io/gitea/models/db"
+	"code.gitea.io/gitea/models/unittest"
+	"code.gitea.io/gitea/modules/timeutil"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestCheckAuthToken(t *testing.T) {
+	assert.NoError(t, unittest.PrepareTestDatabase())
+
+	t.Run("Empty", func(t *testing.T) {
+		token, err := CheckAuthToken(db.DefaultContext, "")
+		assert.NoError(t, err)
+		assert.Nil(t, token)
+	})
+
+	t.Run("InvalidFormat", func(t *testing.T) {
+		token, err := CheckAuthToken(db.DefaultContext, "dummy")
+		assert.ErrorIs(t, err, ErrAuthTokenInvalidFormat)
+		assert.Nil(t, token)
+	})
+
+	t.Run("NotFound", func(t *testing.T) {
+		token, err := CheckAuthToken(db.DefaultContext, "notexists:dummy")
+		assert.ErrorIs(t, err, ErrAuthTokenExpired)
+		assert.Nil(t, token)
+	})
+
+	t.Run("Expired", func(t *testing.T) {
+		timeutil.Set(time.Date(2023, 1, 1, 0, 0, 0, 0, time.UTC))
+
+		at, token, err := CreateAuthTokenForUserID(db.DefaultContext, 2)
+		assert.NoError(t, err)
+		assert.NotNil(t, at)
+		assert.NotEmpty(t, token)
+
+		timeutil.Unset()
+
+		at2, err := CheckAuthToken(db.DefaultContext, at.ID+":"+token)
+		assert.ErrorIs(t, err, ErrAuthTokenExpired)
+		assert.Nil(t, at2)
+
+		assert.NoError(t, auth_model.DeleteAuthTokenByID(db.DefaultContext, at.ID))
+	})
+
+	t.Run("InvalidHash", func(t *testing.T) {
+		at, token, err := CreateAuthTokenForUserID(db.DefaultContext, 2)
+		assert.NoError(t, err)
+		assert.NotNil(t, at)
+		assert.NotEmpty(t, token)
+
+		at2, err := CheckAuthToken(db.DefaultContext, at.ID+":"+token+"dummy")
+		assert.ErrorIs(t, err, ErrAuthTokenInvalidHash)
+		assert.Nil(t, at2)
+
+		assert.NoError(t, auth_model.DeleteAuthTokenByID(db.DefaultContext, at.ID))
+	})
+
+	t.Run("Valid", func(t *testing.T) {
+		at, token, err := CreateAuthTokenForUserID(db.DefaultContext, 2)
+		assert.NoError(t, err)
+		assert.NotNil(t, at)
+		assert.NotEmpty(t, token)
+
+		at2, err := CheckAuthToken(db.DefaultContext, at.ID+":"+token)
+		assert.NoError(t, err)
+		assert.NotNil(t, at2)
+
+		assert.NoError(t, auth_model.DeleteAuthTokenByID(db.DefaultContext, at.ID))
+	})
+}
+
+func TestRegenerateAuthToken(t *testing.T) {
+	assert.NoError(t, unittest.PrepareTestDatabase())
+
+	timeutil.Set(time.Date(2023, 1, 1, 0, 0, 0, 0, time.UTC))
+	defer timeutil.Unset()
+
+	at, token, err := CreateAuthTokenForUserID(db.DefaultContext, 2)
+	assert.NoError(t, err)
+	assert.NotNil(t, at)
+	assert.NotEmpty(t, token)
+
+	timeutil.Set(time.Date(2023, 1, 1, 0, 0, 1, 0, time.UTC))
+
+	at2, token2, err := RegenerateAuthToken(db.DefaultContext, at)
+	assert.NoError(t, err)
+	assert.NotNil(t, at2)
+	assert.NotEmpty(t, token2)
+
+	assert.Equal(t, at.ID, at2.ID)
+	assert.Equal(t, at.UserID, at2.UserID)
+	assert.NotEqual(t, token, token2)
+	assert.NotEqual(t, at.ExpiresUnix, at2.ExpiresUnix)
+
+	assert.NoError(t, auth_model.DeleteAuthTokenByID(db.DefaultContext, at.ID))
+}
diff --git a/services/auth/main_test.go b/services/auth/main_test.go
new file mode 100644
index 0000000000..b81c39a1f2
--- /dev/null
+++ b/services/auth/main_test.go
@@ -0,0 +1,14 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package auth
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/models/unittest"
+)
+
+func TestMain(m *testing.M) {
+	unittest.MainTest(m)
+}