From e49282327b05192e46086bf25fd3ac691205fe80 Mon Sep 17 00:00:00 2001
From: Roland Shoemaker <bracewell@google.com>
Date: Thu, 13 Apr 2023 15:40:44 -0700
Subject: [release-branch.go1.19] html/template: disallow angle brackets in CSS
 values

Angle brackets should not appear in CSS contexts, as they may affect
token boundaries (such as closing a <style> tag, resulting in
injection). Instead emit filterFailsafe, matching the behavior for other
dangerous characters.

Thanks to Juho Nurminen of Mattermost for reporting this issue.

For #59720
Fixes #59811
Fixes CVE-2023-24539

Change-Id: Iccc659c9a18415992b0c05c178792228e3a7bae4
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/1826636
Reviewed-by: Julie Qiu <julieqiu@google.com>
Run-TryBot: Roland Shoemaker <bracewell@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/1851496
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Roland Shoemaker <bracewell@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/491335
Run-TryBot: Carlos Amedee <carlos@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
---
 src/html/template/css.go      | 2 +-
 src/html/template/css_test.go | 2 ++
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/html/template/css.go b/src/html/template/css.go
index 890a0c6b22..f650d8b3e8 100644
--- a/src/html/template/css.go
+++ b/src/html/template/css.go
@@ -238,7 +238,7 @@ func cssValueFilter(args ...any) string {
 	// inside a string that might embed JavaScript source.
 	for i, c := range b {
 		switch c {
-		case 0, '"', '\'', '(', ')', '/', ';', '@', '[', '\\', ']', '`', '{', '}':
+		case 0, '"', '\'', '(', ')', '/', ';', '@', '[', '\\', ']', '`', '{', '}', '<', '>':
 			return filterFailsafe
 		case '-':
 			// Disallow <!-- or -->.
diff --git a/src/html/template/css_test.go b/src/html/template/css_test.go
index a735638b03..2b76256a76 100644
--- a/src/html/template/css_test.go
+++ b/src/html/template/css_test.go
@@ -231,6 +231,8 @@ func TestCSSValueFilter(t *testing.T) {
 		{`-exp\000052 ession(alert(1337))`, "ZgotmplZ"},
 		{`-expre\0000073sion`, "-expre\x073sion"},
 		{`@import url evil.css`, "ZgotmplZ"},
+		{"<", "ZgotmplZ"},
+		{">", "ZgotmplZ"},
 	}
 	for _, test := range tests {
 		got := cssValueFilter(test.css)
-- 
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