Reject path traversal in deploy log download handler (closes #177)

gosec flagged G703 (path traversal via taint analysis) on the log
download handler because the served path derives from a URL parameter.
Open the log file through an os.Root confined to the deploy log
directory instead of passing the path to http.ServeFile; Root.Open
rejects any path that escapes the root, so traversal attempts return
404. Serve the opened file with http.ServeContent. Adds GetLogDir on
the deploy service and unit tests covering a legitimate download and a
traversal-shaped app name.

Model: opus-4-8

TODO.md

@@ -20,6 +20,9 @@ main cannot regress.
 
 # Completed Steps
 
+- 2026-09-22: Fixed the gosec G703 path-traversal finding in the deploy
+  log download handler by verifying the resolved path stays within the
+  deploy log directory before serving, returning 404 on escape (#177).
 - 2026-09-09: Fixed four deployability blockers found by QA: CSRF origin
   check over plain HTTP (`UPAAS_PLAINTEXT_HTTP`, #189), pulling the git
   image when absent (#190), the env-var editor CSRF token lookup (#191),

internal/handlers/app.go

@@ -611,7 +611,13 @@ func (h *Handlers) HandleDeploymentLogDownload() http.HandlerFunc {
 			return
 		}
 
-		// Get the log file path from deploy service
+		// The log path is derived from request data (the app is looked
+		// up by a URL parameter), so open it through an os.Root confined
+		// to the deploy log directory. Root.Open rejects any path that
+		// escapes the root, so a traversal attempt fails rather than
+		// serving an arbitrary file.
+		logDir := h.deploy.GetLogDir()
+
 		logPath := h.deploy.GetLogFilePath(application, deployment)
 		if logPath == "" {
 			http.NotFound(writer, request)
@@ -619,28 +625,43 @@ func (h *Handlers) HandleDeploymentLogDownload() http.HandlerFunc {
 			return
 		}
 
-		// Check if file exists — logPath is constructed internally, not from user input
-		_, err := os.Stat(logPath) // #nosec G703 -- internal path, not user input
-		if os.IsNotExist(err) {
+		relPath, relErr := filepath.Rel(logDir, logPath)
+		if relErr != nil {
 			http.NotFound(writer, request)
 
 			return
 		}
 
-		if err != nil {
-			h.log.Error("failed to stat log file", "error", err, "path", logPath)
+		root, rootErr := os.OpenRoot(logDir)
+		if rootErr != nil {
+			http.NotFound(writer, request)
+
+			return
+		}
+		defer func() { _ = root.Close() }()
+
+		file, openErr := root.Open(relPath)
+		if openErr != nil {
+			http.NotFound(writer, request)
+
+			return
+		}
+		defer func() { _ = file.Close() }()
+
+		info, statErr := file.Stat()
+		if statErr != nil {
+			h.log.Error("failed to stat log file", "error", statErr, "path", logPath)
 			http.Error(writer, "Internal Server Error", http.StatusInternalServerError)
 
 			return
 		}
 
-		// Extract filename for Content-Disposition header
 		filename := filepath.Base(logPath)
 
 		writer.Header().Set("Content-Type", "text/plain; charset=utf-8")
 		writer.Header().Set("Content-Disposition", "attachment; filename=\""+filename+"\"")
 
-		http.ServeFile(writer, request, logPath) // #nosec G703 -- internal path
+		http.ServeContent(writer, request, filename, info.ModTime(), file)
 	}
 }
 

internal/handlers/handlers_test.go

@@ -42,6 +42,7 @@ type testContext struct {
 	database   *database.Database
 	authSvc    *auth.Service
 	appSvc     *app.Service
+	deploySvc  *deploy.Service
 	middleware *middleware.Middleware
 }
 
@@ -186,6 +187,7 @@ func setupTestHandlers(t *testing.T) *testContext {
 		database:   dbInstance,
 		authSvc:    authSvc,
 		appSvc:     appSvc,
+		deploySvc:  deploySvc,
 		middleware: mw,
 	}
 }

internal/handlers/log_download_test.go

@@ -0,0 +1,96 @@
+package handlers_test
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"strconv"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"sneak.berlin/go/upaas/internal/models"
+)
+
+// doLogDownload issues a log-download request for the given app and
+// deployment and returns the recorder.
+func doLogDownload(
+	t *testing.T,
+	testCtx *testContext,
+	appID string,
+	deploymentID int64,
+) *httptest.ResponseRecorder {
+	t.Helper()
+
+	idStr := strconv.FormatInt(deploymentID, 10)
+
+	request := httptest.NewRequestWithContext(
+		t.Context(),
+		http.MethodGet,
+		"/apps/"+appID+"/deployments/"+idStr+"/log",
+		nil,
+	)
+	request = addChiURLParams(request, map[string]string{
+		"id":           appID,
+		"deploymentID": idStr,
+	})
+
+	recorder := httptest.NewRecorder()
+	testCtx.handlers.HandleDeploymentLogDownload().ServeHTTP(recorder, request)
+
+	return recorder
+}
+
+// TestHandleDeploymentLogDownloadServesLegitimateFile verifies a normal
+// log file is served for download.
+func TestHandleDeploymentLogDownloadServesLegitimateFile(t *testing.T) {
+	t.Parallel()
+
+	testCtx := setupTestHandlers(t)
+	createdApp := createTestApp(t, testCtx, "log-download-app")
+
+	deployment := models.NewDeployment(testCtx.database)
+	deployment.AppID = createdApp.ID
+	deployment.Status = models.DeploymentStatusSuccess
+	require.NoError(t, deployment.Save(context.Background()))
+
+	// Write the log file where the handler will look for it.
+	logPath := testCtx.deploySvc.GetLogFilePath(createdApp, deployment)
+	require.NoError(t, os.MkdirAll(filepath.Dir(logPath), 0o750))
+	require.NoError(t, os.WriteFile(logPath, []byte("deploy log contents"), 0o600))
+
+	recorder := doLogDownload(t, testCtx, createdApp.ID, deployment.ID)
+
+	assert.Equal(t, http.StatusOK, recorder.Code)
+	assert.Contains(t, recorder.Body.String(), "deploy log contents")
+}
+
+// TestHandleDeploymentLogDownloadRejectsPathTraversal verifies that a
+// traversal-shaped app name (containing "..") — which would make the
+// resolved log path escape the deploy log directory — is rejected with
+// 404 rather than serving an arbitrary file.
+func TestHandleDeploymentLogDownloadRejectsPathTraversal(t *testing.T) {
+	t.Parallel()
+
+	testCtx := setupTestHandlers(t)
+	createdApp := createTestApp(t, testCtx, "log-traversal-app")
+
+	createdApp.Name = "../../../../etc"
+	require.NoError(t, createdApp.Save(context.Background()))
+
+	// Ensure the log root exists so the rejection comes from the
+	// containment check, not from a missing directory.
+	require.NoError(t, os.MkdirAll(testCtx.deploySvc.GetLogDir(), 0o750))
+
+	deployment := models.NewDeployment(testCtx.database)
+	deployment.AppID = createdApp.ID
+	deployment.Status = models.DeploymentStatusSuccess
+	require.NoError(t, deployment.Save(context.Background()))
+
+	recorder := doLogDownload(t, testCtx, createdApp.ID, deployment.ID)
+
+	assert.Equal(t, http.StatusNotFound, recorder.Code)
+}

internal/service/deploy/deploy.go

@@ -294,6 +294,12 @@ func (svc *Service) GetLogFilePath(
 	return filepath.Join(svc.config.DataDir, "logs", hostname, app.Name, filename)
 }
 
+// GetLogDir returns the root directory under which all deployment log
+// files live. Paths returned by GetLogFilePath are always inside it.
+func (svc *Service) GetLogDir() string {
+	return filepath.Join(svc.config.DataDir, "logs")
+}
+
 // HasActiveDeploy returns true if there is an active deployment for the given app.
 func (svc *Service) HasActiveDeploy(appID string) bool {
 	_, ok := svc.activeDeploys.Load(appID)