html/template: properly escape URLs in meta content attributes

The meta tag can include a content attribute that contains URLs, which
we currently don't escape if they are inserted via a template action.
This can plausibly lead to XSS vulnerabilities if untrusted data is
inserted there, the http-equiv attribute is set to "refresh", and the
content attribute contains an action like `url={{.}}`.

Track whether we are inside of a meta element, if we are inside of a
content attribute, _and_ if the content attribute contains "url=". If
all of those are true, then we will apply the same URL escaping that we
use elsewhere.

Also add a new GODEBUG, htmlmetacontenturlescape, to allow disabling this
escaping for cases where this behavior is considered safe. The behavior
can be disabled by setting htmlmetacontenturlescape=0.

Fixes CVE-2026-27142
Fixes #77954

Change-Id: I9bbca263be9894688e6ef1e9a8f8d2f4304f5873
Reviewed-on: https://go-internal-review.googlesource.com/c/go/+/3360
Reviewed-by: Neal Patel <nealpatel@google.com>
Reviewed-by: Nicholas Husin <husin@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/752181
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

doc/godebug.md

@@ -160,6 +160,13 @@ and the [go command documentation](/cmd/go#hdr-Build_and_test_caching).
 
 Go 1.27 removed the `gotypesalias` setting, as noted in the [Go 1.22][#go-122] section.
 
+Go 1.27 added a new `htmlmetacontenturlescape` setting that controls whether
+html/template will escape URLs in the `url=` portion of the content attribute of
+HTML meta tags. The default `htmlmetacontentescape=1` will cause URLs to be
+escaped. Setting `htmlmetacontentescape=0` disables this behavior. To avoid
+content injection attacks, this setting and default was backported to Go 1.25.8
+and Go 1.26.1.
+
 ### Go 1.26
 
 Go 1.26 added a new `httpcookiemaxnum` setting that controls the maximum number

src/html/template/attr_string.go

@@ -14,11 +14,12 @@ func _() {
 	_ = x[attrStyle-3]
 	_ = x[attrURL-4]
 	_ = x[attrSrcset-5]
+	_ = x[attrMetaContent-6]
 }
 
-const _attr_name = "attrNoneattrScriptattrScriptTypeattrStyleattrURLattrSrcset"
+const _attr_name = "attrNoneattrScriptattrScriptTypeattrStyleattrURLattrSrcsetattrMetaContent"
 
-var _attr_index = [...]uint8{0, 8, 18, 32, 41, 48, 58}
+var _attr_index = [...]uint8{0, 8, 18, 32, 41, 48, 58, 73}
 
 func (i attr) String() string {
 	if i >= attr(len(_attr_index)-1) {

src/html/template/context.go

@@ -156,6 +156,10 @@ const (
 	// stateError is an infectious error state outside any valid
 	// HTML/CSS/JS construct.
 	stateError
+	// stateMetaContent occurs inside a HTML meta element content attribute.
+	stateMetaContent
+	// stateMetaContentURL occurs inside a "url=" tag in a HTML meta element content attribute.
+	stateMetaContentURL
 	// stateDead marks unreachable code after a {{break}} or {{continue}}.
 	stateDead
 )
@@ -267,6 +271,8 @@ const (
 	elementTextarea
 	// elementTitle corresponds to the RCDATA <title> element.
 	elementTitle
+	// elementMeta corresponds to the HTML <meta> element.
+	elementMeta
 )
 
 //go:generate stringer -type attr
@@ -288,4 +294,6 @@ const (
 	attrURL
 	// attrSrcset corresponds to a srcset attribute.
 	attrSrcset
+	// attrMetaContent corresponds to the content attribute in meta HTML element.
+	attrMetaContent
 )

src/html/template/element_string.go

@@ -13,11 +13,12 @@ func _() {
 	_ = x[elementStyle-2]
 	_ = x[elementTextarea-3]
 	_ = x[elementTitle-4]
+	_ = x[elementMeta-5]
 }
 
-const _element_name = "elementNoneelementScriptelementStyleelementTextareaelementTitle"
+const _element_name = "elementNoneelementScriptelementStyleelementTextareaelementTitleelementMeta"
 
-var _element_index = [...]uint8{0, 11, 24, 36, 51, 63}
+var _element_index = [...]uint8{0, 11, 24, 36, 51, 63, 74}
 
 func (i element) String() string {
 	if i >= element(len(_element_index)-1) {

src/html/template/escape.go

@@ -166,6 +166,8 @@ func (e *escaper) escape(c context, n parse.Node) context {
 
 var debugAllowActionJSTmpl = godebug.New("jstmpllitinterp")
 
+var htmlmetacontenturlescape = godebug.New("htmlmetacontenturlescape")
+
 // escapeAction escapes an action template node.
 func (e *escaper) escapeAction(c context, n *parse.ActionNode) context {
 	if len(n.Pipe.Decl) != 0 {
@@ -223,6 +225,18 @@ func (e *escaper) escapeAction(c context, n *parse.ActionNode) context {
 		default:
 			panic(c.urlPart.String())
 		}
+	case stateMetaContent:
+		// Handled below in delim check.
+	case stateMetaContentURL:
+		if htmlmetacontenturlescape.Value() != "0" {
+			s = append(s, "_html_template_urlfilter")
+		} else {
+			// We don't have a great place to increment this, since it's hard to
+			// know if we actually escape any urls in _html_template_urlfilter,
+			// since it has no information about what context it is being
+			// executed in etc. This is probably the best we can do.
+			htmlmetacontenturlescape.IncNonDefault()
+		}
 	case stateJS:
 		s = append(s, "_html_template_jsvalescaper")
 		// A slash after a value starts a div operator.

src/html/template/escape_test.go

@@ -8,6 +8,7 @@ import (
 	"bytes"
 	"encoding/json"
 	"fmt"
+	"internal/testenv"
 	"os"
 	"strings"
 	"testing"
@@ -734,6 +735,16 @@ func TestEscape(t *testing.T) {
 			"<script>var a = `${ var a = \"{{\"a \\\" d\"}}\" }`</script>",
 			"<script>var a = `${ var a = \"a \\u0022 d\" }`</script>",
 		},
+		{
+			"meta content attribute url",
+			`<meta http-equiv="refresh" content="asd; url={{"javascript:alert(1)"}}; asd; url={{"vbscript:alert(1)"}}; asd">`,
+			`<meta http-equiv="refresh" content="asd; url=#ZgotmplZ; asd; url=#ZgotmplZ; asd">`,
+		},
+		{
+			"meta content string",
+			`<meta http-equiv="refresh" content="{{"asd: 123"}}">`,
+			`<meta http-equiv="refresh" content="asd: 123">`,
+		},
 	}
 
 	for _, test := range tests {
@@ -1016,6 +1027,14 @@ func TestErrors(t *testing.T) {
 			"<script>var tmpl = `asd ${return \"{\"}`;</script>",
 			``,
 		},
+		{
+			`{{if eq "" ""}}<meta>{{end}}`,
+			``,
+		},
+		{
+			`{{if eq "" ""}}<meta content="url={{"asd"}}">{{end}}`,
+			``,
+		},
 
 		// Error cases.
 		{
@@ -2198,3 +2217,16 @@ func TestAliasedParseTreeDoesNotOverescape(t *testing.T) {
 		t.Fatalf(`Template "foo" and "bar" rendered %q and %q respectively, expected equal values`, got1, got2)
 	}
 }
+
+func TestMetaContentEscapeGODEBUG(t *testing.T) {
+	testenv.SetGODEBUG(t, "htmlmetacontenturlescape=0")
+	tmpl := Must(New("").Parse(`<meta http-equiv="refresh" content="asd; url={{"javascript:alert(1)"}}; asd; url={{"vbscript:alert(1)"}}; asd">`))
+	var b strings.Builder
+	if err := tmpl.Execute(&b, nil); err != nil {
+		t.Fatalf("unexpected error: %s", err)
+	}
+	want := `<meta http-equiv="refresh" content="asd; url=javascript:alert(1); asd; url=vbscript:alert(1); asd">`
+	if got := b.String(); got != want {
+		t.Fatalf("got %q, want %q", got, want)
+	}
+}

src/html/template/state_string.go

@@ -36,12 +36,14 @@ func _() {
 	_ = x[stateCSSBlockCmt-25]
 	_ = x[stateCSSLineCmt-26]
 	_ = x[stateError-27]
-	_ = x[stateDead-28]
+	_ = x[stateMetaContent-28]
+	_ = x[stateMetaContentURL-29]
+	_ = x[stateDead-30]
 }
 
-const _state_name = "stateTextstateTagstateAttrNamestateAfterNamestateBeforeValuestateHTMLCmtstateRCDATAstateAttrstateURLstateSrcsetstateJSstateJSDqStrstateJSSqStrstateJSTmplLitstateJSRegexpstateJSBlockCmtstateJSLineCmtstateJSHTMLOpenCmtstateJSHTMLCloseCmtstateCSSstateCSSDqStrstateCSSSqStrstateCSSDqURLstateCSSSqURLstateCSSURLstateCSSBlockCmtstateCSSLineCmtstateErrorstateDead"
+const _state_name = "stateTextstateTagstateAttrNamestateAfterNamestateBeforeValuestateHTMLCmtstateRCDATAstateAttrstateURLstateSrcsetstateJSstateJSDqStrstateJSSqStrstateJSTmplLitstateJSRegexpstateJSBlockCmtstateJSLineCmtstateJSHTMLOpenCmtstateJSHTMLCloseCmtstateCSSstateCSSDqStrstateCSSSqStrstateCSSDqURLstateCSSSqURLstateCSSURLstateCSSBlockCmtstateCSSLineCmtstateErrorstateMetaContentstateMetaContentURLstateDead"
 
-var _state_index = [...]uint16{0, 9, 17, 30, 44, 60, 72, 83, 92, 100, 111, 118, 130, 142, 156, 169, 184, 198, 216, 235, 243, 256, 269, 282, 295, 306, 322, 337, 347, 356}
+var _state_index = [...]uint16{0, 9, 17, 30, 44, 60, 72, 83, 92, 100, 111, 118, 130, 142, 156, 169, 184, 198, 216, 235, 243, 256, 269, 282, 295, 306, 322, 337, 347, 363, 382, 391}
 
 func (i state) String() string {
 	if i >= state(len(_state_index)-1) {

src/html/template/transition.go

@@ -23,6 +23,8 @@ var transitionFunc = [...]func(context, []byte) (context, int){
 	stateRCDATA:         tSpecialTagEnd,
 	stateAttr:           tAttr,
 	stateURL:            tURL,
+	stateMetaContent:    tMetaContent,
+	stateMetaContentURL: tMetaContentURL,
 	stateSrcset:         tURL,
 	stateJS:             tJS,
 	stateJSDqStr:        tJSDelimited,
@@ -83,6 +85,7 @@ var elementContentType = [...]state{
 	elementStyle:    stateCSS,
 	elementTextarea: stateRCDATA,
 	elementTitle:    stateRCDATA,
+	elementMeta:     stateText,
 }
 
 // tTag is the context transition function for the tag state.
@@ -93,6 +96,11 @@ func tTag(c context, s []byte) (context, int) {
 		return c, len(s)
 	}
 	if s[i] == '>' {
+		// Treat <meta> specially, because it doesn't have an end tag, and we
+		// want to transition into the correct state/element for it.
+		if c.element == elementMeta {
+			return context{state: stateText, element: elementNone}, i + 1
+		}
 		return context{
 			state:   elementContentType[c.element],
 			element: c.element,
@@ -113,6 +121,8 @@ func tTag(c context, s []byte) (context, int) {
 	attrName := strings.ToLower(string(s[i:j]))
 	if c.element == elementScript && attrName == "type" {
 		attr = attrScriptType
+	} else if c.element == elementMeta && attrName == "content" {
+		attr = attrMetaContent
 	} else {
 		switch attrType(attrName) {
 		case contentTypeURL:
@@ -162,12 +172,13 @@ func tAfterName(c context, s []byte) (context, int) {
 }
 
 var attrStartStates = [...]state{
-	attrNone:       stateAttr,
-	attrScript:     stateJS,
-	attrScriptType: stateAttr,
-	attrStyle:      stateCSS,
-	attrURL:        stateURL,
-	attrSrcset:     stateSrcset,
+	attrNone:        stateAttr,
+	attrScript:      stateJS,
+	attrScriptType:  stateAttr,
+	attrStyle:       stateCSS,
+	attrURL:         stateURL,
+	attrSrcset:      stateSrcset,
+	attrMetaContent: stateMetaContent,
 }
 
 // tBeforeValue is the context transition function for stateBeforeValue.
@@ -203,6 +214,7 @@ var specialTagEndMarkers = [...][]byte{
 	elementStyle:    []byte("style"),
 	elementTextarea: []byte("textarea"),
 	elementTitle:    []byte("title"),
+	elementMeta:     []byte(""),
 }
 
 var (
@@ -612,6 +624,28 @@ func tError(c context, s []byte) (context, int) {
 	return c, len(s)
 }
 
+// tMetaContent is the context transition function for the meta content attribute state.
+func tMetaContent(c context, s []byte) (context, int) {
+	for i := 0; i < len(s); i++ {
+		if i+3 <= len(s)-1 && bytes.Equal(bytes.ToLower(s[i:i+4]), []byte("url=")) {
+			c.state = stateMetaContentURL
+			return c, i + 4
+		}
+	}
+	return c, len(s)
+}
+
+// tMetaContentURL is the context transition function for the "url=" part of a meta content attribute state.
+func tMetaContentURL(c context, s []byte) (context, int) {
+	for i := 0; i < len(s); i++ {
+		if s[i] == ';' {
+			c.state = stateMetaContent
+			return c, i + 1
+		}
+	}
+	return c, len(s)
+}
+
 // eatAttrName returns the largest j such that s[i:j] is an attribute name.
 // It returns an error if s[i:] does not look like it begins with an
 // attribute name, such as encountering a quote mark without a preceding
@@ -638,6 +672,7 @@ var elementNameMap = map[string]element{
 	"style":    elementStyle,
 	"textarea": elementTextarea,
 	"title":    elementTitle,
+	"meta":     elementMeta,
 }
 
 // asciiAlpha reports whether c is an ASCII letter.

src/internal/godebugs/table.go

@@ -39,6 +39,7 @@ var All = []Info{
 	{Name: "gocachetest", Package: "cmd/go"},
 	{Name: "gocacheverify", Package: "cmd/go"},
 	{Name: "gotestjsonbuildtext", Package: "cmd/go", Changed: 24, Old: "1"},
+	{Name: "htmlmetacontenturlescape", Package: "html/template"},
 	{Name: "http2client", Package: "net/http"},
 	{Name: "http2debug", Package: "net/http", Opaque: true},
 	{Name: "http2server", Package: "net/http"},

src/runtime/metrics/doc.go

@@ -302,6 +302,11 @@ Below is the full list of supported metrics, ordered lexicographically.
 		package due to a non-default GODEBUG=gotestjsonbuildtext=...
 		setting.
 
+	/godebug/non-default-behavior/htmlmetacontenturlescape:events
+		The number of non-default behaviors executed by
+		the html/template package due to a non-default
+		GODEBUG=htmlmetacontenturlescape=... setting.
+
 	/godebug/non-default-behavior/http2client:events
 		The number of non-default behaviors executed by the net/http
 		package due to a non-default GODEBUG=http2client=... setting.