net/url: disallow raw IPv6 addresses in host

RFC 3986 requires square brackets around IPv6 addresses.
Parse's acceptance of raw IPv6 addresses is non compliant,
and complicates splitting out a port.

This is a resubmission of CL 710176 after the revert in CL 711800,
this time with a new urlstrictipv6 godebug to control the behavior.

Fixes #31024
Fixes #75223

Change-Id: I4cbe5bb84266b3efe9c98cf4300421ddf1df7291
Reviewed-on: https://go-review.googlesource.com/c/go/+/712840
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

doc/godebug.md

@@ -163,6 +163,11 @@ will fail early. The default value is `httpcookiemaxnum=3000`. Setting
 number of cookies. To avoid denial of service attacks, this setting and default
 was backported to Go 1.25.2 and Go 1.24.8.
 
+Go 1.26 added a new `urlstrictcolons` setting that controls whether `net/url.Parse`
+allows malformed hostnames containing colons outside of a bracketed IPv6 address.
+The default `urlstrictcolons=1` rejects URLs such as `http://localhost:1:2` or `http://::1/`.
+Colons are permitted as part of a bracketed IPv6 address, such as `http://[::1]/`.
+
 ### Go 1.25
 
 Go 1.25 added a new `decoratemappings` setting that controls whether the Go

doc/next/6-stdlib/99-minor/net/url/31024.md

@@ -0,0 +1,4 @@
+[Parse] now rejects malformed URLs containing colons in the host subcomponent,
+such as `http://::1/` or `http://localhost:80:80/`.
+URLs containing bracketed IPv6 addresses, such as `http://[::1]/` are still accepted.
+The new GODEBUG=urlstrictcolons=0 setting restores the old behavior.

src/internal/godebugs/table.go

@@ -67,6 +67,7 @@ var All = []Info{
 	{Name: "tlssha1", Package: "crypto/tls", Changed: 25, Old: "1"},
 	{Name: "tlsunsafeekm", Package: "crypto/tls", Changed: 22, Old: "1"},
 	{Name: "updatemaxprocs", Package: "runtime", Changed: 25, Old: "0"},
+	{Name: "urlstrictcolons", Package: "net/url", Changed: 26, Old: "0"},
 	{Name: "winreadlinkvolume", Package: "os", Changed: 23, Old: "0"},
 	{Name: "winsymlink", Package: "os", Changed: 23, Old: "0"},
 	{Name: "x509keypairleaf", Package: "crypto/tls", Changed: 23, Old: "0"},

src/net/url/url.go

@@ -18,6 +18,7 @@ package url
 import (
 	"errors"
 	"fmt"
+	"internal/godebug"
 	"net/netip"
 	"path"
 	"slices"
@@ -26,6 +27,8 @@ import (
 	_ "unsafe" // for linkname
 )
 
+var urlstrictcolons = godebug.New("urlstrictcolons")
+
 // Error reports an error and the operation and URL that caused it.
 type Error struct {
 	Op  string
@@ -599,7 +602,11 @@ func parseHost(host string) (string, error) {
 			return "", errors.New("invalid IP-literal")
 		}
 		return "[" + unescapedHostname + "]" + unescapedColonPort, nil
-	} else if i := strings.LastIndex(host, ":"); i != -1 {
+	} else if i := strings.Index(host, ":"); i != -1 {
+		if j := strings.LastIndex(host, ":"); urlstrictcolons.Value() == "0" && j != i {
+			urlstrictcolons.IncNonDefault()
+			i = j
+		}
 		colonPort := host[i:]
 		if !validOptionalPort(colonPort) {
 			return "", fmt.Errorf("invalid port %q after host", colonPort)

src/net/url/url_test.go

@@ -13,6 +13,7 @@ import (
 	"io"
 	"net"
 	"reflect"
+	"strconv"
 	"strings"
 	"testing"
 )
@@ -506,26 +507,6 @@ var urltests = []URLTest{
 		},
 		"",
 	},
-	{
-		// Malformed IPv6 but still accepted.
-		"http://2b01:e34:ef40:7730:8e70:5aff:fefe:edac:8080/foo",
-		&URL{
-			Scheme: "http",
-			Host:   "2b01:e34:ef40:7730:8e70:5aff:fefe:edac:8080",
-			Path:   "/foo",
-		},
-		"",
-	},
-	{
-		// Malformed IPv6 but still accepted.
-		"http://2b01:e34:ef40:7730:8e70:5aff:fefe:edac:/foo",
-		&URL{
-			Scheme: "http",
-			Host:   "2b01:e34:ef40:7730:8e70:5aff:fefe:edac:",
-			Path:   "/foo",
-		},
-		"",
-	},
 	{
 		"http://[2b01:e34:ef40:7730:8e70:5aff:fefe:edac]:8080/foo",
 		&URL{
@@ -735,6 +716,9 @@ var parseRequestURLTests = []struct {
 	{"https://[0:0::test.com]:80", false},
 	{"https://[2001:db8::test.com]", false},
 	{"https://[test.com]", false},
+	{"https://1:2:3:4:5:6:7:8", false},
+	{"https://1:2:3:4:5:6:7:8:80", false},
+	{"https://example.com:80:", false},
 }
 
 func TestParseRequestURI(t *testing.T) {
@@ -2280,3 +2264,25 @@ func TestJoinPath(t *testing.T) {
 		}
 	}
 }
+
+func TestParseStrictIpv6(t *testing.T) {
+	t.Setenv("GODEBUG", "urlstrictcolons=0")
+
+	tests := []struct {
+		url string
+	}{
+		// Malformed URLs that used to parse.
+		{"https://1:2:3:4:5:6:7:8"},
+		{"https://1:2:3:4:5:6:7:8:80"},
+		{"https://example.com:80:"},
+	}
+	for i, tc := range tests {
+		t.Run(strconv.Itoa(i), func(t *testing.T) {
+			_, err := Parse(tc.url)
+			if err != nil {
+				t.Errorf("Parse(%q) error = %v, want nil", tc.url, err)
+			}
+		})
+	}
+
+}

src/runtime/metrics/doc.go

@@ -399,6 +399,11 @@ Below is the full list of supported metrics, ordered lexicographically.
 		The number of non-default behaviors executed by the runtime
 		package due to a non-default GODEBUG=updatemaxprocs=... setting.
 
+	/godebug/non-default-behavior/urlstrictcolons:events
+		The number of non-default behaviors executed by the net/url
+		package due to a non-default GODEBUG=urlstrictcolons=...
+		setting.
+
 	/godebug/non-default-behavior/winreadlinkvolume:events
 		The number of non-default behaviors executed by the os package
 		due to a non-default GODEBUG=winreadlinkvolume=... setting.