Disallow dangerous url schemes (#25960)

Regression: https://github.com/go-gitea/gitea/pull/24805
Closes: #25945

- Disallow `javascript`, `vbscript` and `data` (data uri images still
work) url schemes even if all other schemes are allowed
- Fixed older `cbthunderlink` tests

---------

Co-authored-by: delvh <dev.lh@web.de>

modules/markup/sanitizer.go

@@ -6,6 +6,7 @@ package markup
 
 import (
 	"io"
+	"net/url"
 	"regexp"
 	"sync"
 
@@ -79,6 +80,14 @@ func createDefaultPolicy() *bluemonday.Policy {
 		policy.AllowURLSchemes(setting.Markdown.CustomURLSchemes...)
 	} else {
 		policy.AllowURLSchemesMatching(allowAllRegex)
+
+		// Even if every scheme is allowed, these three are blocked for security reasons
+		disallowScheme := func(*url.URL) bool {
+			return false
+		}
+		policy.AllowURLSchemeWithCustomPolicy("javascript", disallowScheme)
+		policy.AllowURLSchemeWithCustomPolicy("vbscript", disallowScheme)
+		policy.AllowURLSchemeWithCustomPolicy("data", disallowScheme)
 	}
 
 	// Allow classes for anchors

modules/markup/sanitizer_test.go

@@ -54,8 +54,13 @@ func Test_Sanitizer(t *testing.T) {
 		`<code style="bad-color: red">Hello World</code>`, `<code>Hello World</code>`,
 
 		// URLs
-		`[my custom URL scheme](cbthunderlink://somebase64string)`, `[my custom URL scheme](cbthunderlink://somebase64string)`,
-		`[my custom URL scheme](matrix:roomid/psumPMeAfzgAeQpXMG:feneas.org?action=join)`, `[my custom URL scheme](matrix:roomid/psumPMeAfzgAeQpXMG:feneas.org?action=join)`,
+		`<a href="cbthunderlink://somebase64string)">my custom URL scheme</a>`, `<a href="cbthunderlink://somebase64string)" rel="nofollow">my custom URL scheme</a>`,
+		`<a href="matrix:roomid/psumPMeAfzgAeQpXMG:feneas.org?action=join">my custom URL scheme</a>`, `<a href="matrix:roomid/psumPMeAfzgAeQpXMG:feneas.org?action=join" rel="nofollow">my custom URL scheme</a>`,
+
+		// Disallow dangerous url schemes
+		`<a href="javascript:alert('xss')">bad</a>`, `bad`,
+		`<a href="vbscript:no">bad</a>`, `bad`,
+		`<a href="data:1234">bad</a>`, `bad`,
 	}
 
 	for i := 0; i < len(testCases); i += 2 {