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Add a BlackFriday option for rel="noreferrer" on external links

Add a configuration option "noreferrerLinks". When set to "true" the "HTML_NOREFERRER_LINKS" flag is being passed to Blackfriday. Thereby all *absolute* links will get a "noreferrer" value for their "rel" attribute.

See #4722

--- a/docs/content/en/readfiles/bfconfig.md

+++ b/docs/content/en/readfiles/bfconfig.md

@@ -48,6 +48,11 @@

     Blackfriday flag: **`HTML_NOFOLLOW_LINKS`** <br>

     Purpose: `true` creates <s>external links</s> **absolute** links with `nofollow` being added to their `rel` attribute. Thereby crawlers are advised to not follow the link. While the `rel="nofollow"` attribute is typically used for external links, Blackfriday does that for _all_ absolute links. One needs to make note of this if they use absolute links throughout, for internal links too (for example, by setting `canonifyURLs` to `true` or via `absURL`).

+`noreferrerLinks`

+: default: **`false`** <br>

+    Blackfriday flag: **`HTML_NOREFERRER_LINKS`** <br>

+    Purpose: `true` creates <s>external links</s> **absolute** links with `noreferrer` being added to their `rel` attribute. Thus when following the link no referrer information will be leaked. While the `rel="noreferrer"` attribute is typically used for external links, Blackfriday does that for _all_ absolute links. One needs to make note of this if they use absolute links throughout, for internal links too (for example, by setting `canonifyURLs` to `true` or via `absURL`).

+

 `plainIDAnchors`

 : default **`true`** <br>

     Blackfriday flag: **`FootnoteAnchorPrefix` and `HeaderIDSuffix`** <br>

--- a/helpers/content.go

+++ b/helpers/content.go

@@ -109,6 +109,7 @@

 	Fractions             bool

 	HrefTargetBlank       bool

 	NofollowLinks         bool

+	NoreferrerLinks       bool

 	SmartDashes           bool

 	LatexDashes           bool

 	TaskLists             bool

@@ -126,6 +127,7 @@

 		"fractions":             true,

 		"hrefTargetBlank":       false,

 		"nofollowLinks":         false,

+		"noreferrerLinks":       false,

 		"smartDashes":           true,

 		"latexDashes":           true,

 		"plainIDAnchors":        true,

@@ -281,6 +283,10 @@

 	if ctx.Config.NofollowLinks {

 		htmlFlags |= blackfriday.HTML_NOFOLLOW_LINKS

+	}

+

+	if ctx.Config.NoreferrerLinks {

+		htmlFlags |= blackfriday.HTML_NOREFERRER_LINKS

 	}

 	if ctx.Config.SmartDashes {

--- a/helpers/content_test.go

+++ b/helpers/content_test.go

@@ -205,6 +205,7 @@

 		{blackfriday.HTML_SMARTYPANTS_FRACTIONS},

 		{blackfriday.HTML_HREF_TARGET_BLANK},

 		{blackfriday.HTML_NOFOLLOW_LINKS},

+		{blackfriday.HTML_NOREFERRER_LINKS},

 		{blackfriday.HTML_SMARTYPANTS_DASHES},

 		{blackfriday.HTML_SMARTYPANTS_LATEX_DASHES},

 	}

@@ -214,6 +215,7 @@

 	ctx.Config.Fractions = true

 	ctx.Config.HrefTargetBlank = true

 	ctx.Config.NofollowLinks = true

+	ctx.Config.NoreferrerLinks = true

 	ctx.Config.LatexDashes = true

 	ctx.Config.PlainIDAnchors = true

 	ctx.Config.SmartDashes = true