[release-branch.go1.19] html/template: disallow actions in JS template literals

ECMAScript 6 introduced template literals[0][1] which are delimited with
backticks. These need to be escaped in a similar fashion to the
delimiters for other string literals. Additionally template literals can
contain special syntax for string interpolation.

There is no clear way to allow safe insertion of actions within JS
template literals, as handling (JS) string interpolation inside of these
literals is rather complex. As such we've chosen to simply disallow
template actions within these template literals.

A new error code is added for this parsing failure case, errJsTmplLit,
but it is unexported as it is not backwards compatible with other minor
release versions to introduce an API change in a minor release. We will
export this code in the next major release.

The previous behavior (with the cavet that backticks are now escaped
properly) can be re-enabled with GODEBUG=jstmpllitinterp=1.

This change subsumes CL471455.

Thanks to Sohom Datta, Manipal Institute of Technology, for reporting
this issue.

Fixes CVE-2023-24538
For #59234
Fixes #59271

[0] https://tc39.es/ecma262/multipage/ecmascript-language-expressions.html#sec-template-literals
[1] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Template_literals

Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/1802457
Reviewed-by: Damien Neil <dneil@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Julie Qiu <julieqiu@google.com>
Reviewed-by: Roland Shoemaker <bracewell@google.com>
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/1802612
Run-TryBot: Roland Shoemaker <bracewell@google.com>
Change-Id: Ic7f10595615f2b2740d9c85ad7ef40dc0e78c04c
Reviewed-on: https://go-review.googlesource.com/c/go/+/481987
Auto-Submit: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

9 files changed, 116 insertions, 33 deletions

diff --git a/src/html/template/context.go b/src/html/template/context.go
index a97c8be56f9..c28fb0c5ea8 100644
--- a/src/html/template/context.go
+++ b/src/html/template/context.go
@@ -120,6 +120,8 @@ const (
 	stateJSDqStr
 	// stateJSSqStr occurs inside a JavaScript single quoted string.
 	stateJSSqStr
+	// stateJSBqStr occurs inside a JavaScript back quoted string.
+	stateJSBqStr
 	// stateJSRegexp occurs inside a JavaScript regexp literal.
 	stateJSRegexp
 	// stateJSBlockCmt occurs inside a JavaScript /* block comment */.
diff --git a/src/html/template/error.go b/src/html/template/error.go
index 5c51f772cbd..d7d6f5b3ab5 100644
--- a/src/html/template/error.go
+++ b/src/html/template/error.go
@@ -214,6 +214,19 @@ const (
 	//   pipeline occurs in an unquoted attribute value context, "html" is
 	//   disallowed. Avoid using "html" and "urlquery" entirely in new templates.
 	ErrPredefinedEscaper
+
+	// errJSTmplLit: "... appears in a JS template literal"
+	// Example:
+	//     <script>var tmpl = `{{.Interp}`</script>
+	// Discussion:
+	//   Package html/template does not support actions inside of JS template
+	//   literals.
+	//
+	// TODO(rolandshoemaker): we cannot add this as an exported error in a minor
+	// release, since it is backwards incompatible with the other minor
+	// releases. As such we need to leave it unexported, and then we'll add it
+	// in the next major release.
+	errJSTmplLit
 )
 
 func (e *Error) Error() string {
diff --git a/src/html/template/escape.go b/src/html/template/escape.go
index 54fbcdca333..3d4cc19b5da 100644
--- a/src/html/template/escape.go
+++ b/src/html/template/escape.go
@@ -8,6 +8,7 @@ import (
 	"bytes"
 	"fmt"
 	"html"
+	"internal/godebug"
 	"io"
 	"text/template"
 	"text/template/parse"
@@ -223,6 +224,16 @@ func (e *escaper) escapeAction(c context, n *parse.ActionNode) context {
 		c.jsCtx = jsCtxDivOp
 	case stateJSDqStr, stateJSSqStr:
 		s = append(s, "_html_template_jsstrescaper")
+	case stateJSBqStr:
+		debugAllowActionJSTmpl := godebug.Get("jstmpllitinterp")
+		if debugAllowActionJSTmpl == "1" {
+			s = append(s, "_html_template_jsstrescaper")
+		} else {
+			return context{
+				state: stateError,
+				err:   errorf(errJSTmplLit, n, n.Line, "%s appears in a JS template literal", n),
+			}
+		}
 	case stateJSRegexp:
 		s = append(s, "_html_template_jsregexpescaper")
 	case stateCSS:
diff --git a/src/html/template/escape_test.go b/src/html/template/escape_test.go
index 58f3f271b7a..972b00b921d 100644
--- a/src/html/template/escape_test.go
+++ b/src/html/template/escape_test.go
@@ -681,35 +681,31 @@ func TestEscape(t *testing.T) {
 	}
 
 	for _, test := range tests {
-		tmpl := New(test.name)
-		tmpl = Must(tmpl.Parse(test.input))
-		// Check for bug 6459: Tree field was not set in Parse.
-		if tmpl.Tree != tmpl.text.Tree {
-			t.Errorf("%s: tree not set properly", test.name)
-			continue
-		}
-		b := new(bytes.Buffer)
-		if err := tmpl.Execute(b, data); err != nil {
-			t.Errorf("%s: template execution failed: %s", test.name, err)
-			continue
-		}
-		if w, g := test.output, b.String(); w != g {
-			t.Errorf("%s: escaped output: want\n\t%q\ngot\n\t%q", test.name, w, g)
-			continue
-		}
-		b.Reset()
-		if err := tmpl.Execute(b, pdata); err != nil {
-			t.Errorf("%s: template execution failed for pointer: %s", test.name, err)
-			continue
-		}
-		if w, g := test.output, b.String(); w != g {
-			t.Errorf("%s: escaped output for pointer: want\n\t%q\ngot\n\t%q", test.name, w, g)
-			continue
-		}
-		if tmpl.Tree != tmpl.text.Tree {
-			t.Errorf("%s: tree mismatch", test.name)
-			continue
-		}
+		t.Run(test.name, func(t *testing.T) {
+			tmpl := New(test.name)
+			tmpl = Must(tmpl.Parse(test.input))
+			// Check for bug 6459: Tree field was not set in Parse.
+			if tmpl.Tree != tmpl.text.Tree {
+				t.Fatalf("%s: tree not set properly", test.name)
+			}
+			b := new(strings.Builder)
+			if err := tmpl.Execute(b, data); err != nil {
+				t.Fatalf("%s: template execution failed: %s", test.name, err)
+			}
+			if w, g := test.output, b.String(); w != g {
+				t.Fatalf("%s: escaped output: want\n\t%q\ngot\n\t%q", test.name, w, g)
+			}
+			b.Reset()
+			if err := tmpl.Execute(b, pdata); err != nil {
+				t.Fatalf("%s: template execution failed for pointer: %s", test.name, err)
+			}
+			if w, g := test.output, b.String(); w != g {
+				t.Fatalf("%s: escaped output for pointer: want\n\t%q\ngot\n\t%q", test.name, w, g)
+			}
+			if tmpl.Tree != tmpl.text.Tree {
+				t.Fatalf("%s: tree mismatch", test.name)
+			}
+		})
 	}
 }
 
@@ -936,6 +932,10 @@ func TestErrors(t *testing.T) {
 			"{{range .Items}}<a{{if .X}}{{end}}>{{if .X}}{{break}}{{end}}{{end}}",
 			"",
 		},
+		{
+			"<script>var a = `${a+b}`</script>`",
+			"",
+		},
 		// Error cases.
 		{
 			"{{if .Cond}}<a{{end}}",
@@ -1082,6 +1082,10 @@ func TestErrors(t *testing.T) {
 			// html is allowed since it is the last command in the pipeline, but urlquery is not.
 			`predefined escaper "urlquery" disallowed in template`,
 		},
+		{
+			"<script>var tmpl = `asd {{.}}`;</script>",
+			`{{.}} appears in a JS template literal`,
+		},
 	}
 	for _, test := range tests {
 		buf := new(bytes.Buffer)
@@ -1304,6 +1308,10 @@ func TestEscapeText(t *testing.T) {
 			context{state: stateJSSqStr, delim: delimDoubleQuote, attr: attrScript},
 		},
 		{
+			"<a onclick=\"`foo",
+			context{state: stateJSBqStr, delim: delimDoubleQuote, attr: attrScript},
+		},
+		{
 			`<A ONCLICK="'`,
 			context{state: stateJSSqStr, delim: delimDoubleQuote, attr: attrScript},
 		},
diff --git a/src/html/template/js.go b/src/html/template/js.go
index 50523d00f16..fe7054efe5c 100644
--- a/src/html/template/js.go
+++ b/src/html/template/js.go
@@ -308,6 +308,7 @@ var jsStrReplacementTable = []string{
 	// Encode HTML specials as hex so the output can be embedded
 	// in HTML attributes without further encoding.
 	'"':  `\u0022`,
+	'`':  `\u0060`,
 	'&':  `\u0026`,
 	'\'': `\u0027`,
 	'+':  `\u002b`,
@@ -331,6 +332,7 @@ var jsStrNormReplacementTable = []string{
 	'"':  `\u0022`,
 	'&':  `\u0026`,
 	'\'': `\u0027`,
+	'`':  `\u0060`,
 	'+':  `\u002b`,
 	'/':  `\/`,
 	'<':  `\u003c`,
diff --git a/src/html/template/js_test.go b/src/html/template/js_test.go
index 56579d8d30b..e07c695f7a7 100644
--- a/src/html/template/js_test.go
+++ b/src/html/template/js_test.go
@@ -292,7 +292,7 @@ func TestEscapersOnLower7AndSelectHighCodepoints(t *testing.T) {
 				`0123456789:;\u003c=\u003e?` +
 				`@ABCDEFGHIJKLMNO` +
 				`PQRSTUVWXYZ[\\]^_` +
-				"`abcdefghijklmno" +
+				"\\u0060abcdefghijklmno" +
 				"pqrstuvwxyz{|}~\u007f" +
 				"\u00A0\u0100\\u2028\\u2029\ufeff\U0001D11E",
 		},
diff --git a/src/html/template/jsctx_string.go b/src/html/template/jsctx_string.go
index dd1d87ee454..23948934c95 100644
--- a/src/html/template/jsctx_string.go
+++ b/src/html/template/jsctx_string.go
@@ -4,6 +4,15 @@ package template
 
 import "strconv"
 
+func _() {
+	// An "invalid array index" compiler error signifies that the constant values have changed.
+	// Re-run the stringer command to generate them again.
+	var x [1]struct{}
+	_ = x[jsCtxRegexp-0]
+	_ = x[jsCtxDivOp-1]
+	_ = x[jsCtxUnknown-2]
+}
+
 const _jsCtx_name = "jsCtxRegexpjsCtxDivOpjsCtxUnknown"
 
 var _jsCtx_index = [...]uint8{0, 11, 21, 33}
diff --git a/src/html/template/state_string.go b/src/html/template/state_string.go
index 05104be89c2..6fb1a6eeb0e 100644
--- a/src/html/template/state_string.go
+++ b/src/html/template/state_string.go
@@ -4,9 +4,42 @@ package template
 
 import "strconv"
 
-const _state_name = "stateTextstateTagstateAttrNamestateAfterNamestateBeforeValuestateHTMLCmtstateRCDATAstateAttrstateURLstateSrcsetstateJSstateJSDqStrstateJSSqStrstateJSRegexpstateJSBlockCmtstateJSLineCmtstateCSSstateCSSDqStrstateCSSSqStrstateCSSDqURLstateCSSSqURLstateCSSURLstateCSSBlockCmtstateCSSLineCmtstateError"
+func _() {
+	// An "invalid array index" compiler error signifies that the constant values have changed.
+	// Re-run the stringer command to generate them again.
+	var x [1]struct{}
+	_ = x[stateText-0]
+	_ = x[stateTag-1]
+	_ = x[stateAttrName-2]
+	_ = x[stateAfterName-3]
+	_ = x[stateBeforeValue-4]
+	_ = x[stateHTMLCmt-5]
+	_ = x[stateRCDATA-6]
+	_ = x[stateAttr-7]
+	_ = x[stateURL-8]
+	_ = x[stateSrcset-9]
+	_ = x[stateJS-10]
+	_ = x[stateJSDqStr-11]
+	_ = x[stateJSSqStr-12]
+	_ = x[stateJSBqStr-13]
+	_ = x[stateJSRegexp-14]
+	_ = x[stateJSBlockCmt-15]
+	_ = x[stateJSLineCmt-16]
+	_ = x[stateCSS-17]
+	_ = x[stateCSSDqStr-18]
+	_ = x[stateCSSSqStr-19]
+	_ = x[stateCSSDqURL-20]
+	_ = x[stateCSSSqURL-21]
+	_ = x[stateCSSURL-22]
+	_ = x[stateCSSBlockCmt-23]
+	_ = x[stateCSSLineCmt-24]
+	_ = x[stateError-25]
+	_ = x[stateDead-26]
+}
+
+const _state_name = "stateTextstateTagstateAttrNamestateAfterNamestateBeforeValuestateHTMLCmtstateRCDATAstateAttrstateURLstateSrcsetstateJSstateJSDqStrstateJSSqStrstateJSBqStrstateJSRegexpstateJSBlockCmtstateJSLineCmtstateCSSstateCSSDqStrstateCSSSqStrstateCSSDqURLstateCSSSqURLstateCSSURLstateCSSBlockCmtstateCSSLineCmtstateErrorstateDead"
 
-var _state_index = [...]uint16{0, 9, 17, 30, 44, 60, 72, 83, 92, 100, 111, 118, 130, 142, 155, 170, 184, 192, 205, 218, 231, 244, 255, 271, 286, 296}
+var _state_index = [...]uint16{0, 9, 17, 30, 44, 60, 72, 83, 92, 100, 111, 118, 130, 142, 154, 167, 182, 196, 204, 217, 230, 243, 256, 267, 283, 298, 308, 317}
 
 func (i state) String() string {
 	if i >= state(len(_state_index)-1) {
diff --git a/src/html/template/transition.go b/src/html/template/transition.go
index 06df679330d..92eb3519063 100644
--- a/src/html/template/transition.go
+++ b/src/html/template/transition.go
@@ -27,6 +27,7 @@ var transitionFunc = [...]func(context, []byte) (context, int){
 	stateJS:          tJS,
 	stateJSDqStr:     tJSDelimited,
 	stateJSSqStr:     tJSDelimited,
+	stateJSBqStr:     tJSDelimited,
 	stateJSRegexp:    tJSDelimited,
 	stateJSBlockCmt:  tBlockCmt,
 	stateJSLineCmt:   tLineCmt,
@@ -262,7 +263,7 @@ func tURL(c context, s []byte) (context, int) {
 
 // tJS is the context transition function for the JS state.
 func tJS(c context, s []byte) (context, int) {
-	i := bytes.IndexAny(s, `"'/`)
+	i := bytes.IndexAny(s, "\"`'/")
 	if i == -1 {
 		// Entire input is non string, comment, regexp tokens.
 		c.jsCtx = nextJSCtx(s, c.jsCtx)
@@ -274,6 +275,8 @@ func tJS(c context, s []byte) (context, int) {
 		c.state, c.jsCtx = stateJSDqStr, jsCtxRegexp
 	case '\'':
 		c.state, c.jsCtx = stateJSSqStr, jsCtxRegexp
+	case '`':
+		c.state, c.jsCtx = stateJSBqStr, jsCtxRegexp
 	case '/':
 		switch {
 		case i+1 < len(s) && s[i+1] == '/':
@@ -303,6 +306,8 @@ func tJSDelimited(c context, s []byte) (context, int) {
 	switch c.state {
 	case stateJSSqStr:
 		specials = `\'`
+	case stateJSBqStr:
+		specials = "`\\"
 	case stateJSRegexp:
 		specials = `\/[]`
 	}