Replace CSRF cookie with CrossOriginProtection (#36183)

Removes the CSRF cookie in favor of
[`CrossOriginProtection`](https://pkg.go.dev/net/http#CrossOriginProtection)
which relies purely on HTTP headers.

Fixes: https://github.com/go-gitea/gitea/issues/11188
Fixes: https://github.com/go-gitea/gitea/issues/30333
Helps: https://github.com/go-gitea/gitea/issues/35107

TODOs:

- [x] Fix tests
- [ ] Ideally add tests to validates the protection

---------

Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/setting/markup.go

@@ -255,7 +255,7 @@ func newMarkupRenderer(name string, sec ConfigSection) {
 	}
 
 	// ATTENTION! at the moment, only a safe set like "allow-scripts" are allowed for sandbox mode.
-	// "allow-same-origin" should never be used, it leads to XSS attack, and it makes the JS in iframe can access parent window's config and CSRF token
+	// "allow-same-origin" should NEVER be used, it leads to XSS attack: makes the JS in iframe can access parent window's config and send requests with user's credentials.
 	renderContentSandbox := sec.Key("RENDER_CONTENT_SANDBOX").MustString("allow-scripts allow-popups")
 	if renderContentSandbox == "disabled" {
 		renderContentSandbox = ""

modules/setting/oauth2.go

@@ -133,7 +133,7 @@ func loadOAuth2From(rootCfg ConfigProvider) {
 
 	// FIXME: at the moment, no matter oauth2 is enabled or not, it must generate a "oauth2 JWT_SECRET"
 	// Because this secret is also used as GeneralTokenSigningSecret (as a quick not-that-breaking fix for some legacy problems).
-	// Including: CSRF token, account validation token, etc ...
+	// Including: account validation token, etc ...
 	// In main branch, the signing token should be refactored (eg: one unique for LFS/OAuth2/etc ...)
 	jwtSecretBase64 := loadSecret(sec, "JWT_SECRET_URI", "JWT_SECRET")
 	if InstallLock {

modules/setting/security.go

@@ -36,8 +36,6 @@ var (
 	PasswordCheckPwn                   bool
 	SuccessfulTokensCacheSize          int
 	DisableQueryAuthToken              bool
-	CSRFCookieName                     = "_csrf"
-	CSRFCookieHTTPOnly                 = true
 	RecordUserSignupMetadata           = false
 	TwoFactorAuthEnforced              = false
 )
@@ -139,7 +137,6 @@ func loadSecurityFrom(rootCfg ConfigProvider) {
 		log.Fatal("The provided password hash algorithm was invalid: %s", sec.Key("PASSWORD_HASH_ALGO").MustString(""))
 	}
 
-	CSRFCookieHTTPOnly = sec.Key("CSRF_COOKIE_HTTP_ONLY").MustBool(true)
 	PasswordCheckPwn = sec.Key("PASSWORD_CHECK_PWN").MustBool(false)
 	SuccessfulTokensCacheSize = sec.Key("SUCCESSFUL_TOKENS_CACHE_SIZE").MustInt(20)