html/template: escape script tags in JS errors case insensitively

Thanks to Juho Forsén of Mattermost for reporting this issue.

Fixes #70740

Change-Id: I1a49b199dee91cd2bb4df5b174aaa958dc040c18
Reviewed-on: https://go-review.googlesource.com/c/go/+/634696


LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>

src/html/template/js.go

@@ -9,6 +9,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"reflect"
+	"regexp"
 	"strings"
 	"unicode/utf8"
 )
@@ -144,6 +145,8 @@ func indirectToJSONMarshaler(a any) any {
 	return v.Interface()
 }
 
+var scriptTagRe = regexp.MustCompile("(?i)<(/?)script")
+
 // jsValEscaper escapes its inputs to a JS Expression (section 11.14) that has
 // neither side-effects nor free variables outside (NaN, Infinity).
 func jsValEscaper(args ...any) string {
@@ -181,9 +184,9 @@ func jsValEscaper(args ...any) string {
 		// In particular we:
 		//   * replace "*/" comment end tokens with "* /", which does not
 		//     terminate the comment
-		//   * replace "</script" with "\x3C/script", and "<!--" with
-		//     "\x3C!--", which prevents confusing script block termination
-		//     semantics
+		//   * replace "<script" and "</script" with "\x3Cscript" and "\x3C/script"
+		//     (case insensitively), and "<!--" with "\x3C!--", which prevents
+		//     confusing script block termination semantics
 		//
 		// We also put a space before the comment so that if it is flush against
 		// a division operator it is not turned into a line comment:
@@ -192,8 +195,8 @@ func jsValEscaper(args ...any) string {
 		//     x//* error marshaling y:
 		//          second line of error message */null
 		errStr := err.Error()
+		errStr = string(scriptTagRe.ReplaceAll([]byte(errStr), []byte(`\x3C${1}script`)))
 		errStr = strings.ReplaceAll(errStr, "*/", "* /")
-		errStr = strings.ReplaceAll(errStr, "</script", `\x3C/script`)
 		errStr = strings.ReplaceAll(errStr, "<!--", `\x3C!--`)
 		return fmt.Sprintf(" /* %s */null ", errStr)
 	}

src/html/template/js_test.go

@@ -107,7 +107,7 @@ func TestNextJsCtx(t *testing.T) {
 type jsonErrType struct{}
 
 func (e *jsonErrType) MarshalJSON() ([]byte, error) {
-	return nil, errors.New("beep */ boop </script blip <!--")
+	return nil, errors.New("a */ b <script c </script d <!-- e <sCrIpT f </sCrIpT")
 }
 
 func TestJSValEscaper(t *testing.T) {
@@ -160,7 +160,7 @@ func TestJSValEscaper(t *testing.T) {
 		{"</script", `"\u003c/script"`, false},
 		{"\U0001D11E", "\"\U0001D11E\"", false}, // or "\uD834\uDD1E"
 		{nil, " null ", false},
-		{&jsonErrType{}, " /* json: error calling MarshalJSON for type *template.jsonErrType: beep * / boop \\x3C/script blip \\x3C!-- */null ", true},
+		{&jsonErrType{}, " /* json: error calling MarshalJSON for type *template.jsonErrType: a * / b \\x3Cscript c \\x3C/script d \\x3C!-- e \\x3Cscript f \\x3C/script */null ", true},
 	}
 
 	for _, test := range tests {