Allow all URL schemes in Markdown links by default (#24805)

- Closes #21146
- Closes #16721

## :warning: BREAKING :warning:
This changes the default behavior to now create links for any URL scheme
when the user uses the markdown form for links (`[label](URL)`), this
doesn't affect the rendering of inline links. To opt-out set the
`markdown.CUSTOM_URL_SCHEMES` setting to a list of allowed schemes, all
other schemes (except `http` and `https`) won't be allowed.

# Before

![image](https://github.com/go-gitea/gitea/assets/20454870/35fa18ce-7dda-4995-b5b3-3f360f38296d)

# After

![image](https://github.com/go-gitea/gitea/assets/20454870/0922216b-0b35-4b77-9919-21a5c21dd5d0)

---------

Signed-off-by: Yarden Shoham <git@yardenshoham.com>
Co-authored-by: Giteabot <teabot@gitea.io>

6 files changed, 18 insertions, 9 deletions

diff --git a/custom/conf/app.example.ini b/custom/conf/app.example.ini
index 9046f54734..91d829c0ca 100644
--- a/custom/conf/app.example.ini
+++ b/custom/conf/app.example.ini
@@ -1327,6 +1327,7 @@ ROUTER = console
 ;; Comma separated list of custom URL-Schemes that are allowed as links when rendering Markdown
 ;; for example git,magnet,ftp (more at https://en.wikipedia.org/wiki/List_of_URI_schemes)
 ;; URLs starting with http and https are always displayed, whatever is put in this entry.
+;; If this entry is empty, all URL schemes are allowed.
 ;CUSTOM_URL_SCHEMES =
 ;;
 ;; List of file extensions that should be rendered/edited as Markdown
diff --git a/docs/content/doc/administration/config-cheat-sheet.en-us.md b/docs/content/doc/administration/config-cheat-sheet.en-us.md
index 5bf5d6fbf9..fdd1235f89 100644
--- a/docs/content/doc/administration/config-cheat-sheet.en-us.md
+++ b/docs/content/doc/administration/config-cheat-sheet.en-us.md
@@ -276,7 +276,7 @@ The following configuration set `Content-Type: application/vnd.android.package-a
   trailing whitespace to paragraphs is not necessary to force a line break.
 - `CUSTOM_URL_SCHEMES`: Use a comma separated list (ftp,git,svn) to indicate additional
   URL hyperlinks to be rendered in Markdown. URLs beginning in http and https are
-  always displayed
+  always displayed. If this entry is empty, all URL schemes are allowed
 - `FILE_EXTENSIONS`: **.md,.markdown,.mdown,.mkd,.livemd**: List of file extensions that should be rendered/edited as Markdown. Separate the extensions with a comma. To render files without any extension as markdown, just put a comma.
 - `ENABLE_MATH`: **true**: Enables detection of `\(...\)`, `\[...\]`, `$...$` and `$$...$$` blocks as math blocks.
 
diff --git a/go.mod b/go.mod
index 24765df181..93b8052daf 100644
--- a/go.mod
+++ b/go.mod
@@ -76,7 +76,7 @@ require (
 	github.com/mattn/go-sqlite3 v1.14.16
 	github.com/meilisearch/meilisearch-go v0.24.0
 	github.com/mholt/archiver/v3 v3.5.1
-	github.com/microcosm-cc/bluemonday v1.0.23
+	github.com/microcosm-cc/bluemonday v1.0.24
 	github.com/minio/minio-go/v7 v7.0.52
 	github.com/minio/sha256-simd v1.0.0
 	github.com/msteinert/pam v1.1.0
@@ -109,7 +109,7 @@ require (
 	github.com/yuin/goldmark-meta v1.1.0
 	golang.org/x/crypto v0.8.0
 	golang.org/x/image v0.7.0
-	golang.org/x/net v0.9.0
+	golang.org/x/net v0.10.0
 	golang.org/x/oauth2 v0.7.0
 	golang.org/x/sys v0.8.0
 	golang.org/x/text v0.9.0
@@ -288,7 +288,6 @@ require (
 	go.uber.org/zap v1.24.0 // indirect
 	golang.org/x/mod v0.10.0 // indirect
 	golang.org/x/sync v0.2.0 // indirect
-	golang.org/x/term v0.8.0 // indirect
 	golang.org/x/time v0.3.0 // indirect
 	google.golang.org/appengine v1.6.7 // indirect
 	google.golang.org/genproto v0.0.0-20230306155012-7f2fa6fef1f4 // indirect
diff --git a/go.sum b/go.sum
index 62c01c690a..8e5b728aac 100644
--- a/go.sum
+++ b/go.sum
@@ -876,8 +876,8 @@ github.com/mholt/acmez v1.1.0 h1:IQ9CGHKOHokorxnffsqDvmmE30mDenO1lptYZ1AYkHY=
 github.com/mholt/acmez v1.1.0/go.mod h1:zwo5+fbLLTowAX8o8ETfQzbDtwGEXnPhkmGdKIP+bgs=
 github.com/mholt/archiver/v3 v3.5.1 h1:rDjOBX9JSF5BvoJGvjqK479aL70qh9DIpZCl+k7Clwo=
 github.com/mholt/archiver/v3 v3.5.1/go.mod h1:e3dqJ7H78uzsRSEACH1joayhuSyhnonssnDhppzS1L4=
-github.com/microcosm-cc/bluemonday v1.0.23 h1:SMZe2IGa0NuHvnVNAZ+6B38gsTbi5e4sViiWJyDDqFY=
-github.com/microcosm-cc/bluemonday v1.0.23/go.mod h1:mN70sk7UkkF8TUr2IGBpNN0jAgStuPzlK76QuruE/z4=
+github.com/microcosm-cc/bluemonday v1.0.24 h1:NGQoPtwGVcbGkKfvyYk1yRqknzBuoMiUrO6R7uFTPlw=
+github.com/microcosm-cc/bluemonday v1.0.24/go.mod h1:ArQySAMps0790cHSkdPEJ7bGkF2VePWH773hsJNSHf8=
 github.com/miekg/dns v1.0.14/go.mod h1:W1PPwlIAgtquWBMBEV9nkV9Cazfe8ScdGz/Lj7v3Nrg=
 github.com/miekg/dns v1.1.54 h1:5jon9mWcb0sFJGpnI99tOMhCPyJ+RPVz5b63MQG0VWI=
 github.com/miekg/dns v1.1.54/go.mod h1:uInx36IzPl7FYnDcMeVWxj9byh7DutNykX4G9Sj60FY=
@@ -1418,8 +1418,9 @@ golang.org/x/net v0.2.0/go.mod h1:KqCZLdyyvdV855qA2rE3GC2aiw5xGR5TEjj8smXukLY=
 golang.org/x/net v0.6.0/go.mod h1:2Tu9+aMcznHK/AK1HMvgo6xiTLG5rD5rZLDS+rp2Bjs=
 golang.org/x/net v0.7.0/go.mod h1:2Tu9+aMcznHK/AK1HMvgo6xiTLG5rD5rZLDS+rp2Bjs=
 golang.org/x/net v0.8.0/go.mod h1:QVkue5JL9kW//ek3r6jTKnTFis1tRmNAW2P1shuFdJc=
-golang.org/x/net v0.9.0 h1:aWJ/m6xSmxWBx+V0XRHTlrYrPG56jKsLdTFmsSsCzOM=
 golang.org/x/net v0.9.0/go.mod h1:d48xBJpPfHeWQsugry2m+kC02ZBRGRgulfHnEXEuWns=
+golang.org/x/net v0.10.0 h1:X2//UzNDwYmtCLn7To6G58Wr6f5ahEAQgKNzv9Y951M=
+golang.org/x/net v0.10.0/go.mod h1:0qNGK6F8kojg2nk9dLZ2mShWaEBan6FAoqfSigmmuDg=
 golang.org/x/oauth2 v0.0.0-20180821212333-d2e6202438be/go.mod h1:N/0e6XlmueqKjAGxoOufVs8QHGRruUQn6yWY3a++T0U=
 golang.org/x/oauth2 v0.0.0-20190226205417-e64efc72b421/go.mod h1:gOpvHmFTYa4IltrdGE7lF6nIHvwfUNPOp7c8zoXwtLw=
 golang.org/x/oauth2 v0.0.0-20190604053449-0f29369cfe45/go.mod h1:gOpvHmFTYa4IltrdGE7lF6nIHvwfUNPOp7c8zoXwtLw=
@@ -1543,7 +1544,6 @@ golang.org/x/term v0.5.0/go.mod h1:jMB1sMXY+tzblOD4FWmEbocvup2/aLOaQEp7JmGp78k=
 golang.org/x/term v0.6.0/go.mod h1:m6U89DPEgQRMq3DNkDClhWw02AUbt2daBVO4cn4Hv9U=
 golang.org/x/term v0.7.0/go.mod h1:P32HKFT3hSsZrRxla30E9HqToFYAQPCMs/zFMBUFqPY=
 golang.org/x/term v0.8.0 h1:n5xxQn2i3PC0yLAbjTpNT85q/Kgzcr2gIoX9OrJUols=
-golang.org/x/term v0.8.0/go.mod h1:xPskH00ivmX89bAKVGSKKtLOWNx2+17Eiy94tnKShWo=
 golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
 golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
 golang.org/x/text v0.3.1-0.20180807135948-17ff2d5776d2/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
diff --git a/modules/markup/sanitizer.go b/modules/markup/sanitizer.go
index 600ccbf3c6..a0c9ee171f 100644
--- a/modules/markup/sanitizer.go
+++ b/modules/markup/sanitizer.go
@@ -22,7 +22,10 @@ type Sanitizer struct {
 	init             sync.Once
 }
 
-var sanitizer = &Sanitizer{}
+var (
+	sanitizer     = &Sanitizer{}
+	allowAllRegex = regexp.MustCompile(".+")
+)
 
 // NewSanitizer initializes sanitizer with allowed attributes based on settings.
 // Multiple calls to this function will only create one instance of Sanitizer during
@@ -74,6 +77,8 @@ func createDefaultPolicy() *bluemonday.Policy {
 	// Custom URL-Schemes
 	if len(setting.Markdown.CustomURLSchemes) > 0 {
 		policy.AllowURLSchemes(setting.Markdown.CustomURLSchemes...)
+	} else {
+		policy.AllowURLSchemesMatching(allowAllRegex)
 	}
 
 	// Allow classes for anchors
diff --git a/modules/markup/sanitizer_test.go b/modules/markup/sanitizer_test.go
index c792ec2dc6..0c22ce3ba0 100644
--- a/modules/markup/sanitizer_test.go
+++ b/modules/markup/sanitizer_test.go
@@ -52,6 +52,10 @@ func Test_Sanitizer(t *testing.T) {
 		`<span style="bad-color: red">Hello World</span>`, `<span>Hello World</span>`,
 		`<p style="bad-color: red">Hello World</p>`, `<p>Hello World</p>`,
 		`<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)`,
 	}
 
 	for i := 0; i < len(testCases); i += 2 {