net/http/cookiejar: treat localhost as secure origin

For development purposes, browsers treat localhost
as a secure origin regardless of protocol.

Fixes #60997

https://developer.mozilla.org/en-US/docs/Web/HTTP/Guides/Cookies#restrict_access_to_cookies
https://bugzilla.mozilla.org/show_bug.cgi?id=1618113
https://issues.chromium.org/issues/40120372

Change-Id: I6d31df4e055f2872c4b93571c53ae5160923852b
Reviewed-on: https://go-review.googlesource.com/c/go/+/717860
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>
Reviewed-by: Damien Neil <dneil@google.com>

src/net/http/cookiejar/jar.go

@@ -12,6 +12,7 @@ import (
 	"net"
 	"net/http"
 	"net/http/internal/ascii"
+	"net/netip"
 	"net/url"
 	"slices"
 	"strings"
@@ -120,7 +121,7 @@ func (e *entry) id() string {
 // request to host/path. It is the caller's responsibility to check if the
 // cookie is expired.
 func (e *entry) shouldSend(https bool, host, path string) bool {
-	return e.domainMatch(host) && e.pathMatch(path) && (https || !e.Secure)
+	return e.domainMatch(host) && e.pathMatch(path) && e.secureMatch(https)
 }
 
 // domainMatch checks whether e's Domain allows sending e back to host.
@@ -148,6 +149,38 @@ func (e *entry) pathMatch(requestPath string) bool {
 	return false
 }
 
+// secureMatch checks whether a cookie should be sent based on the protocol
+// and the Secure flag. Localhost is considered a secure origin regardless
+// of protocol, matching browser behavior.
+func (e *entry) secureMatch(https bool) bool {
+	if !e.Secure {
+		// Cookies not marked secure are always sent.
+		return true
+	}
+	// Everything below is about cookies marked secure.
+	if https {
+		// HTTPS request matches secure cookies.
+		return true
+	}
+	// Consider localhost to be secure like browsers.
+	if isLocalhost(e.Domain) {
+		return true
+	}
+	ip, err := netip.ParseAddr(e.Domain)
+	if err == nil && ip.IsLoopback() {
+		return true
+	}
+	return false
+}
+
+func isLocalhost(host string) bool {
+	host = strings.TrimSuffix(host, ".")
+	if idx := strings.LastIndex(host, "."); idx >= 0 {
+		host = host[idx+1:]
+	}
+	return ascii.EqualFold(host, "localhost")
+}
+
 // hasDotSuffix reports whether s ends in "."+suffix.
 func hasDotSuffix(s, suffix string) bool {
 	return len(s) > len(suffix) && s[len(s)-len(suffix)-1] == '.' && s[len(s)-len(suffix):] == suffix

src/net/http/cookiejar/jar_test.go

@@ -471,6 +471,62 @@ var basicsTests = [...]jarTest{
 			{"https://www.host.test/some/path", "A=a"},
 		},
 	},
+	{
+		"Secure cookies are sent for localhost",
+		"http://localhost:8910/",
+		[]string{"A=a; secure"},
+		"A=a",
+		[]query{
+			{"http://localhost:8910", "A=a"},
+			{"http://localhost:8910/", "A=a"},
+			{"http://localhost:8910/some/path", "A=a"},
+			{"https://localhost:8910", "A=a"},
+			{"https://localhost:8910/", "A=a"},
+			{"https://localhost:8910/some/path", "A=a"},
+		},
+	},
+	{
+		"Secure cookies are sent for localhost (tld)",
+		"http://example.LOCALHOST:8910/",
+		[]string{"A=a; secure"},
+		"A=a",
+		[]query{
+			{"http://example.LOCALHOST:8910", "A=a"},
+			{"http://example.LOCALHOST:8910/", "A=a"},
+			{"http://example.LOCALHOST:8910/some/path", "A=a"},
+			{"https://example.LOCALHOST:8910", "A=a"},
+			{"https://example.LOCALHOST:8910/", "A=a"},
+			{"https://example.LOCALHOST:8910/some/path", "A=a"},
+		},
+	},
+	{
+		"Secure cookies are sent for localhost (ipv6)",
+		"http://[::1]:8910/",
+		[]string{"A=a; secure"},
+		"A=a",
+		[]query{
+			{"http://[::1]:8910", "A=a"},
+			{"http://[::1]:8910/", "A=a"},
+			{"http://[::1]:8910/some/path", "A=a"},
+			{"https://[::1]:8910", "A=a"},
+			{"https://[::1]:8910/", "A=a"},
+			{"https://[::1]:8910/some/path", "A=a"},
+		},
+	},
+	{
+		"Localhost only if it's a segment",
+		"http://notlocalhost/",
+		[]string{"A=a; secure"},
+		"A=a",
+		[]query{
+			{"http://notlocalhost", ""},
+			{"http://notlocalhost/", ""},
+			{"http://notlocalhost/some/path", ""},
+			{"https://notlocalhost", "A=a"},
+			{"https://notlocalhost/", "A=a"},
+			{"https://notlocalhost/some/path", "A=a"},
+		},
+	},
 	{
 		"Explicit path.",
 		"http://www.host.test/",