cmd/go: restore netrc preferences for GOAUTH and fix domain lookup

Store netrc lines into the credential map backward so that earlier lines
take priority over later lines. This matches Go 1.23 netrc lookup which
stopped at the first match it found.
Additionally, this fixes a security issue related to domain parsing
which could have allowed servers to read credentials belonging to other
servers. The fix was to switch from using path.Dir(currentPrefix) to
strings.Cut(currentPrefix, "/")

Thanks to Juho Forsén of Mattermost for reporting this issue.

Fixes #71249
Fixes CVE-2024-45340

Change-Id: I175a00d6d7f4d31c9e4d79b7cf1c2a0ad35b2781
Reviewed-on: https://go-internal-review.googlesource.com/c/go/+/1781
Reviewed-by: Tatiana Bradley <tatianabradley@google.com>
Commit-Queue: Roland Shoemaker <bracewell@google.com>
Reviewed-by: Roland Shoemaker <bracewell@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/643097
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>

Author: samthanawalla

src/cmd/go/internal/auth/auth.go

@@ -12,7 +12,6 @@ import (
 	"log"
 	"net/http"
 	"os"
-	"path"
 	"path/filepath"
 	"slices"
 	"strings"
@@ -73,7 +72,12 @@ func runGoAuth(client *http.Client, res *http.Response, url string) {
 			if err != nil {
 				base.Fatalf("go: could not parse netrc (GOAUTH=%s): %v", cfg.GOAUTH, err)
 			}
-			for _, l := range lines {
+			// Process lines in reverse so that if the same machine is listed
+			// multiple times, we end up saving the earlier one
+			// (overwriting later ones). This matches the way the go command
+			// worked before GOAUTH.
+			for i := len(lines) - 1; i >= 0; i-- {
+				l := lines[i]
 				r := http.Request{Header: make(http.Header)}
 				r.SetBasicAuth(l.login, l.password)
 				storeCredential(l.machine, r.Header)
@@ -137,11 +141,13 @@ func runGoAuth(client *http.Client, res *http.Response, url string) {
 func loadCredential(req *http.Request, url string) bool {
 	currentPrefix := strings.TrimPrefix(url, "https://")
 	// Iteratively try prefixes, moving up the path hierarchy.
-	for currentPrefix != "/" && currentPrefix != "." && currentPrefix != "" {
+	for {
 		headers, ok := credentialCache.Load(currentPrefix)
 		if !ok {
-			// Move to the parent directory.
-			currentPrefix = path.Dir(currentPrefix)
+			currentPrefix, _, ok = strings.Cut(currentPrefix, "/")
+			if !ok {
+				return false
+			}
 			continue
 		}
 		for key, values := range headers.(http.Header) {
@@ -151,7 +157,6 @@ func loadCredential(req *http.Request, url string) bool {
 		}
 		return true
 	}
-	return false
 }
 
 // storeCredential caches or removes credentials (represented by HTTP headers)

src/cmd/go/internal/auth/auth_test.go

@@ -25,7 +25,29 @@ func TestCredentialCache(t *testing.T) {
 		got := &http.Request{Header: make(http.Header)}
 		ok := loadCredential(got, tc.machine)
 		if !ok || !reflect.DeepEqual(got.Header, want.Header) {
-			t.Errorf("loadCredential:\nhave %q\nwant %q", got.Header, want.Header)
+			t.Errorf("loadCredential(%q):\nhave %q\nwant %q", tc.machine, got.Header, want.Header)
+		}
+	}
+
+	// Having stored those credentials, we should be able to look up longer URLs too.
+	extraCases := []netrcLine{
+		{"https://api.github.com/foo", "user", "pwd"},
+		{"https://api.github.com/foo/bar/baz", "user", "pwd"},
+		{"https://example.com/abc", "", ""},
+		{"https://example.com/?/../api.github.com/", "", ""},
+		{"https://example.com/?/../api.github.com", "", ""},
+		{"https://example.com/../api.github.com/", "", ""},
+		{"https://example.com/../api.github.com", "", ""},
+	}
+	for _, tc := range extraCases {
+		want := http.Request{Header: make(http.Header)}
+		if tc.login != "" {
+			want.SetBasicAuth(tc.login, tc.password)
+		}
+		got := &http.Request{Header: make(http.Header)}
+		loadCredential(got, tc.machine)
+		if !reflect.DeepEqual(got.Header, want.Header) {
+			t.Errorf("loadCredential(%q):\nhave %q\nwant %q", tc.machine, got.Header, want.Header)
 		}
 	}
 }

src/cmd/go/testdata/script/goauth_netrc.txt

@@ -2,8 +2,6 @@
 # credentials passed in HTTPS requests to VCS servers.
 # See golang.org/issue/26232
 
-[short] skip
-
 env GOPROXY=direct
 env GOSUMDB=off
 
@@ -55,11 +53,14 @@ go get vcs-test.golang.org/auth/or401
 env NETRC=$WORK/missing
 ! go get vcs-test.golang.org/auth/or401
 stderr '^\tserver response: ACCESS DENIED, buddy$'
-
 -- go.mod --
 module private.example.com
 -- $WORK/empty --
 -- $WORK/netrc --
 machine vcs-test.golang.org
 	login aladdin
 	password opensesame
+# first one should override this one
+machine vcs-test.golang.org
+	login aladdin
+	password ignored