[release-branch.go1.26] 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.

Updates #77954
Fixes #77972
Fixes CVE-2026-27142

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-internal-review.googlesource.com/c/go/+/3643
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/752081
Auto-Submit: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Bypass: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/src/html/template/escape.go b/src/html/template/escape.go
index 1f963e61b4..d8e1b8cb54 100644
--- a/src/html/template/escape.go
+++ b/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.