Merge main into embedded-vnc

# Conflicts:
#	client/configs/configs.go

client/internal/debug/debug.go

@@ -229,7 +229,6 @@ scutil_dns.txt (macOS only):
 
 const (
 	clientLogFile = "client.log"
-	uiLogFile     = "gui-client.log"
 	errorLogFile  = "netbird.err"
 	stdoutLogFile = "netbird.out"
 
@@ -248,6 +247,20 @@ type MetricsExporter interface {
 	Export(w io.Writer) error
 }
 
+// LogOpener opens a log file for inclusion in the bundle. It exists so that log
+// files whose path was supplied by an IPC caller can be opened under a check
+// the daemon defines, instead of being opened with the daemon's privileges
+// unconditionally.
+type LogOpener func(path string) (*os.File, error)
+
+func openLogFile(path string) (*os.File, error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, fmt.Errorf("open %s: %w", path, err)
+	}
+	return f, nil
+}
+
 type BundleGenerator struct {
 	anonymizer *anonymize.Anonymizer
 
@@ -257,6 +270,7 @@ type BundleGenerator struct {
 	syncResponse   *mgmProto.SyncResponse
 	logPath        string
 	uiLogPath      string
+	uiLogOpener    LogOpener
 	tempDir        string
 	statePath      string
 	cpuProfile     []byte
@@ -285,14 +299,20 @@ type GeneratorDependencies struct {
 	SyncResponse   *mgmProto.SyncResponse
 	LogPath        string
 	UILogPath      string // Absolute path to the desktop UI's gui-client.log, reported via RegisterUILog. Empty if no UI registered one.
-	TempDir        string // Directory for temporary bundle zip files. If empty, os.TempDir() is used.
-	StatePath      string // Path to the state file. If empty, the ServiceManager default path is used.
-	CPUProfile     []byte
-	CapturePath    string
-	RefreshStatus  func()
-	ClientMetrics  MetricsExporter
-	DaemonVersion  string
-	CliVersion     string
+	// UILogOpener opens the UI log and its rotated siblings. The path comes from
+	// a local IPC caller, so the daemon must not open it with plain os.Open: the
+	// opener is where the caller's right to that file is enforced. Defaults to
+	// os.Open, which is only correct where the path is not caller-supplied
+	// (mobile).
+	UILogOpener   LogOpener
+	TempDir       string // Directory for temporary bundle zip files. If empty, os.TempDir() is used.
+	StatePath     string // Path to the state file. If empty, the ServiceManager default path is used.
+	CPUProfile    []byte
+	CapturePath   string
+	RefreshStatus func()
+	ClientMetrics MetricsExporter
+	DaemonVersion string
+	CliVersion    string
 }
 
 func NewBundleGenerator(deps GeneratorDependencies, cfg BundleConfig) *BundleGenerator {
@@ -302,6 +322,11 @@ func NewBundleGenerator(deps GeneratorDependencies, cfg BundleConfig) *BundleGen
 		logFileCount = 1
 	}
 
+	uiLogOpener := deps.UILogOpener
+	if uiLogOpener == nil {
+		uiLogOpener = openLogFile
+	}
+
 	return &BundleGenerator{
 		anonymizer: anonymize.NewAnonymizer(anonymize.DefaultAddresses()),
 
@@ -310,6 +335,7 @@ func NewBundleGenerator(deps GeneratorDependencies, cfg BundleConfig) *BundleGen
 		syncResponse:   deps.SyncResponse,
 		logPath:        deps.LogPath,
 		uiLogPath:      deps.UILogPath,
+		uiLogOpener:    uiLogOpener,
 		tempDir:        deps.TempDir,
 		statePath:      deps.StatePath,
 		cpuProfile:     deps.CPUProfile,
@@ -1002,11 +1028,11 @@ func (g *BundleGenerator) addLogfile() error {
 
 	logDir := filepath.Dir(g.logPath)
 
-	if err := g.addSingleLogfile(g.logPath, clientLogFile); err != nil {
+	if err := g.addSingleLogfile(openLogFile, g.logPath, clientLogFile); err != nil {
 		return fmt.Errorf("add client log file to zip: %w", err)
 	}
 
-	g.addRotatedLogFiles(logDir, clientLogPrefix)
+	g.addRotatedLogFiles(openLogFile, logDir, clientLogPrefix)
 
 	stdErrLogPath := filepath.Join(logDir, errorLogFile)
 	stdoutLogPath := filepath.Join(logDir, stdoutLogFile)
@@ -1015,11 +1041,11 @@ func (g *BundleGenerator) addLogfile() error {
 		stdoutLogPath = darwinStdoutLogPath
 	}
 
-	if err := g.addSingleLogfile(stdErrLogPath, errorLogFile); err != nil {
+	if err := g.addSingleLogfile(openLogFile, stdErrLogPath, errorLogFile); err != nil {
 		log.Warnf("Failed to add %s to zip: %v", errorLogFile, err)
 	}
 
-	if err := g.addSingleLogfile(stdoutLogPath, stdoutLogFile); err != nil {
+	if err := g.addSingleLogfile(openLogFile, stdoutLogPath, stdoutLogFile); err != nil {
 		log.Warnf("Failed to add %s to zip: %v", stdoutLogFile, err)
 	}
 
@@ -1036,18 +1062,18 @@ func (g *BundleGenerator) addUILog() error {
 		return nil
 	}
 
-	if err := g.addSingleLogfile(g.uiLogPath, uiLogFile); err != nil {
+	if err := g.addSingleLogfile(g.uiLogOpener, g.uiLogPath, configs.UILogFile); err != nil {
 		return fmt.Errorf("add UI log file to zip: %w", err)
 	}
 
-	g.addRotatedLogFiles(filepath.Dir(g.uiLogPath), uiLogPrefix)
+	g.addRotatedLogFiles(g.uiLogOpener, filepath.Dir(g.uiLogPath), uiLogPrefix)
 
 	return nil
 }
 
 // addSingleLogfile adds a single log file to the archive
-func (g *BundleGenerator) addSingleLogfile(logPath, targetName string) error {
-	logFile, err := os.Open(logPath)
+func (g *BundleGenerator) addSingleLogfile(open LogOpener, logPath, targetName string) error {
+	logFile, err := open(logPath)
 	if err != nil {
 		return fmt.Errorf("open log file %s: %w", targetName, err)
 	}
@@ -1072,8 +1098,8 @@ func (g *BundleGenerator) addSingleLogfile(logPath, targetName string) error {
 }
 
 // addSingleLogFileGz adds a single gzipped log file to the archive
-func (g *BundleGenerator) addSingleLogFileGz(logPath, targetName string) error {
-	f, err := os.Open(logPath)
+func (g *BundleGenerator) addSingleLogFileGz(open LogOpener, logPath, targetName string) error {
+	f, err := open(logPath)
 	if err != nil {
 		return fmt.Errorf("open gz log file %s: %w", targetName, err)
 	}
@@ -1120,7 +1146,7 @@ func (g *BundleGenerator) addSingleLogFileGz(logPath, targetName string) error {
 // addRotatedLogFiles adds rotated log files to the bundle based on logFileCount.
 // prefix is the base log name without extension (e.g. "client", "gui-client");
 // the glob matches both files rotated by us and by logrotate on linux.
-func (g *BundleGenerator) addRotatedLogFiles(logDir, prefix string) {
+func (g *BundleGenerator) addRotatedLogFiles(open LogOpener, logDir, prefix string) {
 	if g.logFileCount == 0 {
 		return
 	}
@@ -1160,9 +1186,9 @@ func (g *BundleGenerator) addRotatedLogFiles(logDir, prefix string) {
 	for i := 0; i < maxFiles; i++ {
 		name := filepath.Base(files[i])
 		if strings.HasSuffix(name, ".gz") {
-			err = g.addSingleLogFileGz(files[i], name)
+			err = g.addSingleLogFileGz(open, files[i], name)
 		} else {
-			err = g.addSingleLogfile(files[i], name)
+			err = g.addSingleLogfile(open, files[i], name)
 		}
 		if err != nil {
 			log.Warnf("failed to add rotated log %s: %v", name, err)

client/internal/debug/debug_ios.go

@@ -27,7 +27,7 @@ func (g *BundleGenerator) addPlatformLog() error {
 	}
 
 	swiftLogPath := filepath.Join(filepath.Dir(g.logPath), swiftLogFile)
-	if err := g.addSingleLogfile(swiftLogPath, swiftLogFile); err != nil {
+	if err := g.addSingleLogfile(openLogFile, swiftLogPath, swiftLogFile); err != nil {
 		// The Swift log is best-effort: the app may not have written it yet.
 		log.Warnf("failed to add %s to debug bundle: %v", swiftLogFile, err)
 	}

client/internal/debug/debug_logfiles_test.go

@@ -97,7 +97,7 @@ func runAddRotatedLogFilesPrefix(t *testing.T, dir, prefix string, logFileCount
 		archive:      zip.NewWriter(&buf),
 		logFileCount: logFileCount,
 	}
-	g.addRotatedLogFiles(dir, prefix)
+	g.addRotatedLogFiles(openLogFile, dir, prefix)
 	require.NoError(t, g.archive.Close())
 
 	zr, err := zip.NewReader(bytes.NewReader(buf.Bytes()), int64(buf.Len()))

client/internal/debug/uilog_test.go

@@ -0,0 +1,64 @@
+package debug
+
+import (
+	"archive/zip"
+	"bytes"
+	"fmt"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/netbirdio/netbird/client/configs"
+)
+
+// bundleEntries generates a bundle with the given generator and returns the
+// set of entry names in the resulting archive.
+func bundleEntries(t *testing.T, g *BundleGenerator) map[string]struct{} {
+	t.Helper()
+
+	path, err := g.Generate()
+	require.NoError(t, err)
+	t.Cleanup(func() { _ = os.Remove(path) })
+
+	data, err := os.ReadFile(path)
+	require.NoError(t, err)
+
+	zr, err := zip.NewReader(bytes.NewReader(data), int64(len(data)))
+	require.NoError(t, err)
+
+	names := make(map[string]struct{}, len(zr.File))
+	for _, f := range zr.File {
+		names[f.Name] = struct{}{}
+	}
+	return names
+}
+
+func TestBundleIncludesUILogWhenOpenerAllows(t *testing.T) {
+	path := filepath.Join(t.TempDir(), configs.UILogFile)
+	require.NoError(t, os.WriteFile(path, []byte("gui log"), 0600))
+
+	g := NewBundleGenerator(GeneratorDependencies{
+		UILogPath:   path,
+		UILogOpener: openLogFile,
+	}, BundleConfig{})
+
+	require.Contains(t, bundleEntries(t, g), configs.UILogFile)
+}
+
+// A UILogOpener that refuses (as the ownership check does for a foreign file)
+// keeps the UI log out of the bundle without failing bundle generation.
+func TestBundleExcludesUILogWhenOpenerRefuses(t *testing.T) {
+	path := filepath.Join(t.TempDir(), configs.UILogFile)
+	require.NoError(t, os.WriteFile(path, []byte("secret"), 0600))
+
+	g := NewBundleGenerator(GeneratorDependencies{
+		UILogPath: path,
+		UILogOpener: func(string) (*os.File, error) {
+			return nil, fmt.Errorf("not owned by the caller")
+		},
+	}, BundleConfig{})
+
+	require.NotContains(t, bundleEntries(t, g), configs.UILogFile)
+}

client/internal/debug/upload.go

@@ -3,10 +3,12 @@ package debug
 import (
 	"context"
 	"crypto/sha256"
+	"crypto/tls"
 	"encoding/json"
 	"fmt"
 	"io"
 	"net/http"
+	neturl "net/url"
 	"os"
 
 	"github.com/netbirdio/netbird/upload-server/types"
@@ -14,20 +16,80 @@ import (
 
 const maxBundleUploadSize = 50 * 1024 * 1024
 
-func UploadDebugBundle(ctx context.Context, url, managementURL, filePath string) (key string, err error) {
-	response, err := getUploadURL(ctx, url, managementURL)
+// requireHTTPS refuses any URL the daemon would fetch or upload to that is not
+// https. The daemon runs as root and the bundle carries its logs and state, so a
+// plaintext hop is a place to intercept the bundle or the presigned redirect.
+// The server-side gate already enforces this for the desktop path; this also
+// covers the mobile and job-runner callers that reach this package directly.
+// Skipped when the caller opted into an insecure upload (self-hosted server).
+func requireHTTPS(what, rawURL string) error {
+	parsed, err := neturl.Parse(rawURL)
+	if err != nil {
+		return fmt.Errorf("parse %s: %w", what, err)
+	}
+	if parsed.Scheme != "https" {
+		return fmt.Errorf("%s must use https, got scheme %q", what, parsed.Scheme)
+	}
+	return nil
+}
+
+// uploadClient returns the HTTP client for the upload requests. The default
+// client verifies TLS and refuses a redirect that would downgrade to a non-https
+// hop, so a bundle can never leave over http after an https start. The insecure
+// variant accepts http and untrusted certificates, and is only reachable for a
+// privileged caller that passed --upload-bundle-insecure (see
+// requirePrivilegeForUploadURL).
+func uploadClient(insecure bool) *http.Client {
+	if !insecure {
+		return &http.Client{CheckRedirect: rejectInsecureRedirect}
+	}
+	return &http.Client{
+		Transport: &http.Transport{
+			//nolint:gosec // opt-in, privileged, self-hosted upload servers
+			TLSClientConfig: &tls.Config{InsecureSkipVerify: true, MinVersion: tls.VersionTLS12},
+		},
+	}
+}
+
+// rejectInsecureRedirect refuses a redirect to a non-https target and keeps the
+// standard library's 10-hop limit that a custom CheckRedirect would otherwise
+// disable.
+func rejectInsecureRedirect(req *http.Request, via []*http.Request) error {
+	if req.URL.Scheme != "https" {
+		return fmt.Errorf("refusing redirect to non-https URL %s", req.URL.Redacted())
+	}
+	if len(via) >= 10 {
+		return fmt.Errorf("stopped after 10 redirects")
+	}
+	return nil
+}
+
+func UploadDebugBundle(ctx context.Context, url, managementURL, filePath string, insecure bool) (key string, err error) {
+	if !insecure {
+		if err := requireHTTPS("upload service URL", url); err != nil {
+			return "", err
+		}
+	}
+
+	response, err := getUploadURL(ctx, url, managementURL, insecure)
 	if err != nil {
 		return "", err
 	}
 
-	err = upload(ctx, filePath, response)
+	if !insecure {
+		if err := requireHTTPS("upload URL from service", response.URL); err != nil {
+			return "", err
+		}
+	}
+
+	err = upload(ctx, filePath, response, insecure)
 	if err != nil {
 		return "", err
 	}
 	return response.Key, nil
 }
 
-func upload(ctx context.Context, filePath string, response *types.GetURLResponse) error {
+func upload(ctx context.Context, filePath string, response *types.GetURLResponse, insecure bool) error {
 	fileData, err := os.Open(filePath)
 	if err != nil {
 		return fmt.Errorf("open file: %w", err)
@@ -52,7 +114,7 @@ func upload(ctx context.Context, filePath string, response *types.GetURLResponse
 	req.ContentLength = stat.Size()
 	req.Header.Set("Content-Type", "application/octet-stream")
 
-	putResp, err := http.DefaultClient.Do(req)
+	putResp, err := uploadClient(insecure).Do(req)
 	if err != nil {
 		return fmt.Errorf("upload failed: %v", err)
 	}
@@ -65,16 +127,23 @@ func upload(ctx context.Context, filePath string, response *types.GetURLResponse
 	return nil
 }
 
-func getUploadURL(ctx context.Context, url string, managementURL string) (*types.GetURLResponse, error) {
-	id := getURLHash(managementURL)
-	getReq, err := http.NewRequestWithContext(ctx, "GET", url+"?id="+id, nil)
+func getUploadURL(ctx context.Context, serviceURL string, managementURL string, insecure bool) (*types.GetURLResponse, error) {
+	parsed, err := neturl.Parse(serviceURL)
+	if err != nil {
+		return nil, fmt.Errorf("parse upload service URL: %w", err)
+	}
+	q := parsed.Query()
+	q.Set("id", getURLHash(managementURL))
+	parsed.RawQuery = q.Encode()
+
+	getReq, err := http.NewRequestWithContext(ctx, "GET", parsed.String(), nil)
 	if err != nil {
 		return nil, fmt.Errorf("create GET request: %w", err)
 	}
 
 	getReq.Header.Set(types.ClientHeader, types.ClientHeaderValue)
 
-	resp, err := http.DefaultClient.Do(getReq)
+	resp, err := uploadClient(insecure).Do(getReq)
 	if err != nil {
 		return nil, fmt.Errorf("get presigned URL: %w", err)
 	}

client/internal/debug/upload_test.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"net"
 	"net/http"
+	"net/http/httptest"
 	"os"
 	"path/filepath"
 	"testing"
@@ -43,7 +44,7 @@ func TestUpload(t *testing.T) {
 	fileContent := []byte("test file content")
 	err := os.WriteFile(file, fileContent, 0640)
 	require.NoError(t, err)
-	key, err := UploadDebugBundle(context.Background(), testURL+types.GetURLPath, testURL, file)
+	key, err := UploadDebugBundle(context.Background(), testURL+types.GetURLPath, testURL, file, true)
 	require.NoError(t, err)
 	id := getURLHash(testURL)
 	require.Contains(t, key, id+"/")
@@ -79,3 +80,47 @@ func waitForServer(t *testing.T, addr string) {
 	}
 	t.Fatalf("server did not start listening on %s in time", addr)
 }
+
+func TestRequireHTTPS(t *testing.T) {
+	require.NoError(t, requireHTTPS("upload URL", "https://upload.example/path"))
+	require.Error(t, requireHTTPS("upload URL", "http://upload.example/path"))
+	require.Error(t, requireHTTPS("upload URL", "ftp://upload.example/path"))
+	require.Error(t, requireHTTPS("upload URL", "://malformed"))
+}
+
+func TestRejectInsecureRedirect(t *testing.T) {
+	httpsReq, err := http.NewRequest(http.MethodGet, "https://a.example/", nil)
+	require.NoError(t, err)
+	require.NoError(t, rejectInsecureRedirect(httpsReq, nil), "https redirect target must be allowed")
+
+	httpReq, err := http.NewRequest(http.MethodGet, "http://a.example/", nil)
+	require.NoError(t, err)
+	require.Error(t, rejectInsecureRedirect(httpReq, nil), "http redirect target must be refused")
+
+	require.Error(t, rejectInsecureRedirect(httpsReq, make([]*http.Request, 10)), "the 10-redirect limit must be enforced")
+}
+
+// The secure client refuses to follow an https response that redirects to http,
+// so a bundle can't be downgraded onto plaintext mid-flight.
+func TestUploadClientRefusesHTTPSToHTTPRedirect(t *testing.T) {
+	plain := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	}))
+	t.Cleanup(plain.Close)
+
+	secure := httptest.NewTLSServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		http.Redirect(w, r, plain.URL, http.StatusFound)
+	}))
+	t.Cleanup(secure.Close)
+
+	client := uploadClient(false)
+	// Trust the test server's cert without disabling verification globally.
+	client.Transport = secure.Client().Transport
+
+	resp, err := client.Get(secure.URL)
+	if resp != nil {
+		_ = resp.Body.Close()
+	}
+	require.Error(t, err, "redirect from https to http must be refused")
+	require.Contains(t, err.Error(), "non-https")
+}

client/internal/ipcauth/ownedfile.go

@@ -0,0 +1,63 @@
+package ipcauth
+
+import (
+	"fmt"
+	"os"
+)
+
+// OpenOwnedFile opens path for reading on behalf of the IPC caller identified by
+// id, and fails unless the opened file is a regular file that id owns.
+//
+// It exists for the paths a local caller hands to the daemon over the IPC. The
+// daemon runs as root, so opening such a path unchecked lets any local user read
+// any file through it. Ownership is the invariant that keeps the daemon from
+// reading, with its own privileges, a file the caller could not read itself: a
+// symlink or hard link planted at the path resolves to a file someone else owns
+// and is refused.
+//
+// The check is made against the open descriptor rather than the path, so
+// swapping the path between the check and the read cannot change the answer.
+//
+// A privileged caller is exempt: it can read the file directly, so refusing it
+// here would protect nothing. The regular-file requirement still applies to
+// everyone, since a fifo or device planted at the path is never a log file.
+func OpenOwnedFile(id Identity, path string) (*os.File, error) {
+	f, err := openForRead(path)
+	if err != nil {
+		return nil, err
+	}
+
+	if err := checkOwnership(id, f); err != nil {
+		if cerr := f.Close(); cerr != nil {
+			return nil, fmt.Errorf("%w (close: %v)", err, cerr)
+		}
+		return nil, err
+	}
+
+	return f, nil
+}
+
+func checkOwnership(id Identity, f *os.File) error {
+	info, err := f.Stat()
+	if err != nil {
+		return fmt.Errorf("stat %s: %w", f.Name(), err)
+	}
+
+	if !info.Mode().IsRegular() {
+		return fmt.Errorf("%s is not a regular file", f.Name())
+	}
+
+	if IsPrivilegedCaller(id) {
+		return nil
+	}
+
+	owned, err := fileOwnedBy(id, f)
+	if err != nil {
+		return fmt.Errorf("read owner of %s: %w", f.Name(), err)
+	}
+	if !owned {
+		return fmt.Errorf("%s is not owned by the caller (%s)", f.Name(), id)
+	}
+
+	return nil
+}

client/internal/ipcauth/ownedfile_test.go

@@ -0,0 +1,64 @@
+package ipcauth
+
+import (
+	"io"
+	"os"
+	"path/filepath"
+	"runtime"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+// otherIdentity is an unprivileged caller that owns nothing the test creates.
+func otherIdentity(t *testing.T) Identity {
+	t.Helper()
+	if runtime.GOOS == "windows" {
+		return Identity{SID: "S-1-5-21-1-2-3-1001"}
+	}
+	return Identity{UID: uint32(os.Geteuid() + 1), GID: uint32(os.Getegid() + 1)}
+}
+
+func TestOpenOwnedFileReadsFileOwnedByCaller(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "gui-client.log")
+	require.NoError(t, os.WriteFile(path, []byte("hello"), 0600))
+
+	id, err := CurrentProcessIdentity()
+	require.NoError(t, err)
+
+	f, err := OpenOwnedFile(id, path)
+	require.NoError(t, err)
+	t.Cleanup(func() { _ = f.Close() })
+
+	content, err := io.ReadAll(f)
+	require.NoError(t, err)
+	require.Equal(t, "hello", string(content))
+}
+
+func TestOpenOwnedFileRefusesFileOwnedByAnother(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "gui-client.log")
+	require.NoError(t, os.WriteFile(path, []byte("secret"), 0600))
+
+	_, err := OpenOwnedFile(otherIdentity(t), path)
+	require.ErrorContains(t, err, "not owned by the caller")
+}
+
+func TestOpenOwnedFileRefusesNonRegularFile(t *testing.T) {
+	dir := t.TempDir()
+
+	// The caller owns the directory, so this is the regular-file requirement
+	// talking, not the ownership check.
+	id, err := CurrentProcessIdentity()
+	require.NoError(t, err)
+
+	_, err = OpenOwnedFile(id, dir)
+	require.ErrorContains(t, err, "not a regular file")
+}
+
+func TestOpenOwnedFileRefusesMissingFile(t *testing.T) {
+	id, err := CurrentProcessIdentity()
+	require.NoError(t, err)
+
+	_, err = OpenOwnedFile(id, filepath.Join(t.TempDir(), "absent.log"))
+	require.Error(t, err)
+}

client/internal/ipcauth/ownedfile_unix.go

@@ -0,0 +1,35 @@
+//go:build !windows
+
+package ipcauth
+
+import (
+	"fmt"
+	"os"
+	"syscall"
+)
+
+// openForRead opens a caller-supplied path without following a symlink at its
+// final component and without blocking: a fifo planted at the path would
+// otherwise stall the open until a writer appears, and the daemon holds a lock
+// while it collects the file.
+func openForRead(path string) (*os.File, error) {
+	f, err := os.OpenFile(path, os.O_RDONLY|syscall.O_NOFOLLOW|syscall.O_NONBLOCK, 0)
+	if err != nil {
+		return nil, fmt.Errorf("open %s: %w", path, err)
+	}
+	return f, nil
+}
+
+func fileOwnedBy(id Identity, f *os.File) (bool, error) {
+	info, err := f.Stat()
+	if err != nil {
+		return false, err
+	}
+
+	stat, ok := info.Sys().(*syscall.Stat_t)
+	if !ok {
+		return false, fmt.Errorf("no owner information in %T", info.Sys())
+	}
+
+	return stat.Uid == id.UID, nil
+}

client/internal/ipcauth/ownedfile_unix_test.go

@@ -0,0 +1,57 @@
+//go:build !windows
+
+package ipcauth
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+	"syscall"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+)
+
+// A symlink is the shape the arbitrary-read attempt takes: the caller owns the
+// link, the file it points at belongs to someone else.
+func TestOpenOwnedFileRefusesSymlink(t *testing.T) {
+	dir := t.TempDir()
+	target := filepath.Join(dir, "target.log")
+	require.NoError(t, os.WriteFile(target, []byte("secret"), 0600))
+
+	link := filepath.Join(dir, "gui-client.log")
+	require.NoError(t, os.Symlink(target, link))
+
+	id, err := CurrentProcessIdentity()
+	require.NoError(t, err)
+
+	_, err = OpenOwnedFile(id, link)
+	// O_NOFOLLOW on a symlink reports ELOOP on Linux/Darwin and EMLINK on FreeBSD.
+	if !errors.Is(err, syscall.ELOOP) && !errors.Is(err, syscall.EMLINK) {
+		t.Fatalf("symlink open: got %v, want ELOOP or EMLINK", err)
+	}
+}
+
+// A fifo would block the open until a writer showed up, stalling the daemon
+// while it holds its lock.
+func TestOpenOwnedFileRefusesFifoWithoutBlocking(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "gui-client.log")
+	require.NoError(t, syscall.Mkfifo(path, 0600))
+
+	id, err := CurrentProcessIdentity()
+	require.NoError(t, err)
+
+	done := make(chan error, 1)
+	go func() {
+		_, err := OpenOwnedFile(id, path)
+		done <- err
+	}()
+
+	select {
+	case err := <-done:
+		require.ErrorContains(t, err, "not a regular file")
+	case <-time.After(5 * time.Second):
+		t.Fatal("opening a fifo blocked")
+	}
+}

client/internal/ipcauth/ownedfile_windows.go

@@ -0,0 +1,59 @@
+//go:build windows
+
+package ipcauth
+
+import (
+	"fmt"
+	"os"
+
+	"golang.org/x/sys/windows"
+)
+
+// openForRead opens a caller-supplied path without following a reparse point at
+// it. FILE_FLAG_OPEN_REPARSE_POINT is the Windows analogue of O_NOFOLLOW: it
+// opens a symlink/junction itself rather than its target, so the regular-file
+// check in checkOwnership refuses a link the caller planted to redirect the
+// read. FILE_FLAG_BACKUP_SEMANTICS lets a directory open too (as os.Open does),
+// so a directory planted at the path is refused as non-regular rather than
+// erroring here. The share mode matches os.Open so a log being written stays
+// openable.
+func openForRead(path string) (*os.File, error) {
+	p, err := windows.UTF16PtrFromString(path)
+	if err != nil {
+		return nil, fmt.Errorf("convert path %s: %w", path, err)
+	}
+
+	handle, err := windows.CreateFile(
+		p,
+		windows.GENERIC_READ,
+		windows.FILE_SHARE_READ|windows.FILE_SHARE_WRITE|windows.FILE_SHARE_DELETE,
+		nil,
+		windows.OPEN_EXISTING,
+		windows.FILE_FLAG_OPEN_REPARSE_POINT|windows.FILE_FLAG_BACKUP_SEMANTICS,
+		0,
+	)
+	if err != nil {
+		return nil, fmt.Errorf("open %s: %w", path, err)
+	}
+
+	return os.NewFile(uintptr(handle), path), nil
+}
+
+// fileOwnedBy compares the file's owner SID with the caller's. Files an elevated
+// process creates are owned by BUILTIN\Administrators rather than by the user,
+// but such a caller is privileged and never reaches this check.
+func fileOwnedBy(id Identity, f *os.File) (bool, error) {
+	// x/sys/windows GetSecurityInfo frees the OS buffer itself and returns a
+	// Go-heap copy, so there is nothing to LocalFree here.
+	sd, err := windows.GetSecurityInfo(windows.Handle(f.Fd()), windows.SE_FILE_OBJECT, windows.OWNER_SECURITY_INFORMATION)
+	if err != nil {
+		return false, fmt.Errorf("read security info: %w", err)
+	}
+
+	owner, _, err := sd.Owner()
+	if err != nil {
+		return false, fmt.Errorf("read owner: %w", err)
+	}
+
+	return id.SID != "" && owner.String() == id.SID, nil
+}

client/internal/ipcauth/ownedfile_windows_test.go

@@ -0,0 +1,78 @@
+//go:build windows
+
+package ipcauth
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"golang.org/x/sys/windows"
+)
+
+// fileOwnerSID reads the owner SID of path the same way OpenOwnedFile does, so
+// the test can construct an Identity that matches (or deliberately does not).
+func fileOwnerSID(t *testing.T, path string) string {
+	t.Helper()
+	f, err := os.Open(path)
+	require.NoError(t, err)
+	t.Cleanup(func() { _ = f.Close() })
+
+	sd, err := windows.GetSecurityInfo(windows.Handle(f.Fd()), windows.SE_FILE_OBJECT, windows.OWNER_SECURITY_INFORMATION)
+	require.NoError(t, err)
+	owner, _, err := sd.Owner()
+	require.NoError(t, err)
+	return owner.String()
+}
+
+// The allow branch of fileOwnedBy is the SID-equality path the legitimate GUI
+// flow depends on. Running elevated, a created file is owned by
+// BUILTIN\Administrators; an Identity carrying that SID with Elevated=false and
+// no groups is unprivileged by IsPrivileged (which reads the token, not the
+// SID's RID), so this exercises the real GetSecurityInfo equality rather than
+// the privileged-caller shortcut.
+func TestOpenOwnedFileWindowsOwnerMatchAllows(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "gui-client.log")
+	require.NoError(t, os.WriteFile(path, []byte("hello"), 0600))
+
+	ownerSID := fileOwnerSID(t, path)
+	id := Identity{SID: ownerSID}
+	require.False(t, id.IsPrivileged(), "identity built from the owner SID must be unprivileged for this to test the match path")
+
+	f, err := OpenOwnedFile(id, path)
+	require.NoError(t, err)
+	_ = f.Close()
+}
+
+func TestOpenOwnedFileWindowsOwnerMismatchRefuses(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "gui-client.log")
+	require.NoError(t, os.WriteFile(path, []byte("secret"), 0600))
+
+	other := Identity{SID: "S-1-5-21-9-9-9-9999"}
+	require.False(t, other.IsPrivileged())
+
+	_, err := OpenOwnedFile(other, path)
+	require.ErrorContains(t, err, "not owned by the caller")
+}
+
+// FILE_FLAG_OPEN_REPARSE_POINT must make OpenOwnedFile refuse a symlink the same
+// way O_NOFOLLOW does on Unix, so a planted link can't redirect the read to
+// another file. Creating a symlink needs a privilege the runner may lack, so the
+// test skips rather than fails when it can't.
+func TestOpenOwnedFileWindowsRefusesSymlink(t *testing.T) {
+	dir := t.TempDir()
+	target := filepath.Join(dir, "target.log")
+	require.NoError(t, os.WriteFile(target, []byte("secret"), 0600))
+
+	link := filepath.Join(dir, "gui-client.log")
+	if err := os.Symlink(target, link); err != nil {
+		t.Skipf("cannot create symlink (privilege not held?): %v", err)
+	}
+
+	id, err := CurrentProcessIdentity()
+	require.NoError(t, err)
+
+	_, err = OpenOwnedFile(id, link)
+	require.Error(t, err, "a symlink must be refused")
+}