diff --git a/lib/boxstore/store.go b/lib/boxstore/store.go
index 27e7010..26c39eb 100644
--- a/lib/boxstore/store.go
+++ b/lib/boxstore/store.go
@@ -18,6 +18,8 @@ import (
 	"sync"
 	"time"
 
+	"golang.org/x/crypto/bcrypt"
+
 	"warpbox/lib/helpers"
 	"warpbox/lib/models"
 )
@@ -76,16 +78,39 @@ func BoxPath(boxID string) string {
 	return filepath.Join(uploadRoot, boxID)
 }
 
+func safeBoxPath(boxID string) (string, bool) {
+	if !ValidBoxID(boxID) {
+		return "", false
+	}
+	return helpers.SafeChildPath(uploadRoot, boxID)
+}
+
 func ManifestPath(boxID string) string {
 	return filepath.Join(BoxPath(boxID), manifestFile)
 }
 
 func SafeBoxFilePath(boxID string, filename string) (string, bool) {
-	return helpers.SafeChildPath(BoxPath(boxID), filename)
+	boxPath, ok := safeBoxPath(boxID)
+	if !ok {
+		return "", false
+	}
+	return helpers.SafeChildPath(boxPath, filename)
+}
+
+func IsSafeRegularBoxFile(boxID string, filename string) bool {
+	path, ok := SafeBoxFilePath(boxID, filename)
+	if !ok {
+		return false
+	}
+	return ensureRegularFile(path) == nil
 }
 
 func DeleteBox(boxID string) error {
-	return os.RemoveAll(BoxPath(boxID))
+	boxPath, ok := safeBoxPath(boxID)
+	if !ok {
+		return fmt.Errorf("Invalid box id")
+	}
+	return os.RemoveAll(boxPath)
 }
 
 func ListBoxSummaries() ([]models.BoxSummary, error) {
@@ -218,18 +243,17 @@ func CreateManifest(boxID string, request models.CreateBoxRequest) ([]models.Box
 	}
 
 	if password := strings.TrimSpace(request.Password); password != "" {
-		salt, err := helpers.RandomHexID(16)
-		if err != nil {
-			return nil, fmt.Errorf("Could not secure upload box")
-		}
-
 		authToken, err := helpers.RandomHexID(16)
 		if err != nil {
 			return nil, fmt.Errorf("Could not secure upload box")
 		}
+		passwordHash, err := bcrypt.GenerateFromPassword([]byte(password), bcrypt.DefaultCost)
+		if err != nil {
+			return nil, fmt.Errorf("Could not secure upload box")
+		}
 
-		manifest.PasswordSalt = salt
-		manifest.PasswordHash = passwordHash(salt, password)
+		manifest.PasswordHash = string(passwordHash)
+		manifest.PasswordHashAlg = "bcrypt"
 		manifest.AuthToken = authToken
 	}
 
@@ -250,7 +274,7 @@ func IsExpired(manifest models.BoxManifest) bool {
 }
 
 func IsPasswordProtected(manifest models.BoxManifest) bool {
-	return manifest.PasswordSalt != "" && manifest.PasswordHash != "" && manifest.AuthToken != ""
+	return manifest.PasswordHash != "" && manifest.AuthToken != ""
 }
 
 func VerifyPassword(manifest models.BoxManifest, password string) bool {
@@ -259,7 +283,11 @@ func VerifyPassword(manifest models.BoxManifest, password string) bool {
 	}
 
 	expected := manifest.PasswordHash
-	actual := passwordHash(manifest.PasswordSalt, password)
+	if manifest.PasswordHashAlg == "bcrypt" || strings.HasPrefix(expected, "$2") {
+		return bcrypt.CompareHashAndPassword([]byte(expected), []byte(password)) == nil
+	}
+
+	actual := legacyPasswordHash(manifest.PasswordSalt, password)
 	return subtle.ConstantTimeCompare([]byte(expected), []byte(actual)) == 1
 }
 
@@ -335,13 +363,25 @@ func RenewManifest(boxID string, seconds int64) (models.BoxManifest, error) {
 }
 
 func AddFileToZip(zipWriter *zip.Writer, boxID string, filename string) error {
-	source, err := os.Open(filepath.Join(BoxPath(boxID), filename))
+	path, ok := SafeBoxFilePath(boxID, filename)
+	if !ok {
+		return fmt.Errorf("Invalid file")
+	}
+	if err := ensureRegularFile(path); err != nil {
+		return err
+	}
+	zipName, ok := safeZipEntryName(filename)
+	if !ok {
+		return fmt.Errorf("Invalid zip entry")
+	}
+
+	source, err := os.Open(path)
 	if err != nil {
 		return err
 	}
 	defer source.Close()
 
-	destination, err := zipWriter.Create(filename)
+	destination, err := zipWriter.Create(zipName)
 	if err != nil {
 		return err
 	}
@@ -358,6 +398,9 @@ func SaveManifestUpload(boxID string, fileID string, file *multipart.FileHeader)
 	if err != nil {
 		return models.BoxFile{}, err
 	}
+	if IsExpired(manifest) {
+		return models.BoxFile{}, fmt.Errorf("Box expired")
+	}
 
 	fileIndex := -1
 	for index, manifestFile := range manifest.Files {
@@ -376,7 +419,10 @@ func SaveManifestUpload(boxID string, fileID string, file *multipart.FileHeader)
 		return models.BoxFile{}, fmt.Errorf("Could not prepare upload box")
 	}
 
-	destination := filepath.Join(BoxPath(boxID), filename)
+	destination, ok := SafeBoxFilePath(boxID, filename)
+	if !ok {
+		return models.BoxFile{}, fmt.Errorf("Invalid filename")
+	}
 	if err := saveMultipartFile(file, destination); err != nil {
 		manifest.Files[fileIndex].Status = models.FileStatusFailed
 		startRetentionIfTerminalUnlocked(&manifest)
@@ -407,7 +453,10 @@ func SaveUpload(boxID string, file *multipart.FileHeader) (models.BoxFile, error
 	}
 
 	filename = helpers.UniqueFilename(boxPath, filename)
-	destination := filepath.Join(boxPath, filename)
+	destination, ok := SafeBoxFilePath(boxID, filename)
+	if !ok {
+		return models.BoxFile{}, fmt.Errorf("Invalid filename")
+	}
 	if err := saveMultipartFile(file, destination); err != nil {
 		return models.BoxFile{}, fmt.Errorf("Could not save uploaded file")
 	}
@@ -423,7 +472,9 @@ func SaveUpload(boxID string, file *multipart.FileHeader) (models.BoxFile, error
 
 func DecorateFile(boxID string, file models.BoxFile) models.BoxFile {
 	if file.MimeType == "" {
-		file.MimeType = helpers.MimeTypeForFile(filepath.Join(BoxPath(boxID), file.Name), file.Name)
+		if path, ok := SafeBoxFilePath(boxID, file.Name); ok {
+			file.MimeType = helpers.MimeTypeForFile(path, file.Name)
+		}
 	}
 
 	if file.SizeLabel == "" {
@@ -495,9 +546,12 @@ func reconcileManifest(boxID string) (models.BoxManifest, error) {
 
 	changed := false
 	for index, file := range manifest.Files {
-		path := filepath.Join(BoxPath(boxID), file.Name)
+		path, ok := SafeBoxFilePath(boxID, file.Name)
+		if !ok || ensureRegularFile(path) != nil {
+			continue
+		}
 		info, err := os.Stat(path)
-		if err != nil {
+		if err != nil || !info.Mode().IsRegular() {
 			continue
 		}
 
@@ -531,7 +585,7 @@ func listCompletedFilesFromDisk(boxID string) ([]models.BoxFile, error) {
 
 	files := make([]models.BoxFile, 0, len(entries))
 	for _, entry := range entries {
-		if entry.IsDir() || entry.Name() == manifestFile {
+		if entry.IsDir() || entry.Name() == manifestFile || entry.Type()&os.ModeSymlink != 0 {
 			continue
 		}
 
@@ -539,6 +593,9 @@ func listCompletedFilesFromDisk(boxID string) ([]models.BoxFile, error) {
 		if err != nil {
 			return nil, err
 		}
+		if !info.Mode().IsRegular() {
+			continue
+		}
 
 		name := entry.Name()
 		files = append(files, DecorateFile(boxID, models.BoxFile{
@@ -601,7 +658,7 @@ func startRetentionIfTerminalUnlocked(manifest *models.BoxManifest) {
 	manifest.ExpiresAt = time.Now().UTC().Add(time.Duration(seconds) * time.Second)
 }
 
-func passwordHash(salt string, password string) string {
+func legacyPasswordHash(salt string, password string) string {
 	sum := sha256.Sum256([]byte(salt + ":" + password))
 	return hex.EncodeToString(sum[:])
 }
@@ -624,12 +681,64 @@ func saveMultipartFile(file *multipart.FileHeader, destination string) error {
 	}
 	defer source.Close()
 
-	target, err := os.Create(destination)
+	target, tempPath, err := createTempSibling(destination)
 	if err != nil {
 		return err
 	}
-	defer target.Close()
+	committed := false
+	defer func() {
+		target.Close()
+		if !committed {
+			os.Remove(tempPath)
+		}
+	}()
 
-	_, err = io.Copy(target, source)
-	return err
+	if _, err := io.Copy(target, source); err != nil {
+		return err
+	}
+	if err := target.Close(); err != nil {
+		return err
+	}
+	if err := os.Rename(tempPath, destination); err != nil {
+		return err
+	}
+	committed = true
+	return nil
+}
+
+func createTempSibling(destination string) (*os.File, string, error) {
+	directory := filepath.Dir(destination)
+	if err := os.MkdirAll(directory, 0755); err != nil {
+		return nil, "", err
+	}
+
+	target, err := os.CreateTemp(directory, ".warpbox-upload-*")
+	if err != nil {
+		return nil, "", err
+	}
+	return target, target.Name(), nil
+}
+
+func safeZipEntryName(filename string) (string, bool) {
+	filename = strings.TrimSpace(filename)
+	if filename == "" || filepath.IsAbs(filename) {
+		return "", false
+	}
+
+	cleaned := filepath.ToSlash(filepath.Clean(filename))
+	if cleaned == "." || cleaned == ".." || strings.HasPrefix(cleaned, "../") || strings.HasPrefix(cleaned, "/") {
+		return "", false
+	}
+	return cleaned, true
+}
+
+func ensureRegularFile(path string) error {
+	info, err := os.Lstat(path)
+	if err != nil {
+		return err
+	}
+	if info.Mode()&os.ModeSymlink != 0 || !info.Mode().IsRegular() {
+		return fmt.Errorf("Invalid file")
+	}
+	return nil
 }
diff --git a/lib/boxstore/store_test.go b/lib/boxstore/store_test.go
index ba003f4..e761c46 100644
--- a/lib/boxstore/store_test.go
+++ b/lib/boxstore/store_test.go
@@ -1,6 +1,10 @@
 package boxstore
 
 import (
+	"archive/zip"
+	"bytes"
+	"os"
+	"path/filepath"
 	"testing"
 	"time"
 
@@ -59,3 +63,87 @@ func TestStartRetentionSkipsOneTimeDownload(t *testing.T) {
 		t.Fatalf("expected one-time download box to avoid retention expiry, got %s", manifest.ExpiresAt)
 	}
 }
+
+func TestSafeBoxFilePathRejectsTraversal(t *testing.T) {
+	restoreUploadRoot := UploadRoot()
+	defer SetUploadRoot(restoreUploadRoot)
+	SetUploadRoot(t.TempDir())
+
+	boxID := "0123456789abcdef0123456789abcdef"
+	if _, ok := SafeBoxFilePath(boxID, "../outside.txt"); ok {
+		t.Fatal("expected traversal to be rejected")
+	}
+	if _, ok := SafeBoxFilePath("../bad", "file.txt"); ok {
+		t.Fatal("expected invalid box id to be rejected")
+	}
+}
+
+func TestAddFileToZipRejectsUnsafeManifestName(t *testing.T) {
+	restoreUploadRoot := UploadRoot()
+	defer SetUploadRoot(restoreUploadRoot)
+	SetUploadRoot(t.TempDir())
+
+	var buffer bytes.Buffer
+	zipWriter := zip.NewWriter(&buffer)
+	if err := AddFileToZip(zipWriter, "0123456789abcdef0123456789abcdef", "../outside.txt"); err == nil {
+		t.Fatal("expected unsafe zip filename to be rejected")
+	}
+}
+
+func TestListFilesSkipsSymlinks(t *testing.T) {
+	restoreUploadRoot := UploadRoot()
+	defer SetUploadRoot(restoreUploadRoot)
+	SetUploadRoot(t.TempDir())
+
+	boxID := "0123456789abcdef0123456789abcdef"
+	if err := os.MkdirAll(BoxPath(boxID), 0755); err != nil {
+		t.Fatalf("MkdirAll returned error: %v", err)
+	}
+	if err := os.WriteFile(filepath.Join(BoxPath(boxID), "safe.txt"), []byte("safe"), 0644); err != nil {
+		t.Fatalf("WriteFile returned error: %v", err)
+	}
+	if err := os.Symlink(filepath.Join(BoxPath(boxID), "safe.txt"), filepath.Join(BoxPath(boxID), "link.txt")); err != nil {
+		t.Skipf("symlink unavailable: %v", err)
+	}
+
+	files, err := ListFiles(boxID)
+	if err != nil {
+		t.Fatalf("ListFiles returned error: %v", err)
+	}
+	if len(files) != 1 || files[0].Name != "safe.txt" {
+		t.Fatalf("expected only regular file, got %#v", files)
+	}
+}
+
+func TestBoxPasswordUsesBcryptAndVerifiesLegacy(t *testing.T) {
+	restoreUploadRoot := UploadRoot()
+	defer SetUploadRoot(restoreUploadRoot)
+	SetUploadRoot(t.TempDir())
+
+	boxID := "0123456789abcdef0123456789abcdef"
+	if err := os.MkdirAll(BoxPath(boxID), 0755); err != nil {
+		t.Fatalf("MkdirAll returned error: %v", err)
+	}
+	if _, err := CreateManifest(boxID, models.CreateBoxRequest{Password: "secret"}); err != nil {
+		t.Fatalf("CreateManifest returned error: %v", err)
+	}
+	manifest, err := ReadManifest(boxID)
+	if err != nil {
+		t.Fatalf("ReadManifest returned error: %v", err)
+	}
+	if manifest.PasswordHashAlg != "bcrypt" {
+		t.Fatalf("expected bcrypt password hash, got %q", manifest.PasswordHashAlg)
+	}
+	if !VerifyPassword(manifest, "secret") {
+		t.Fatal("expected bcrypt password to verify")
+	}
+
+	legacy := models.BoxManifest{
+		PasswordSalt: "salt",
+		PasswordHash: legacyPasswordHash("salt", "secret"),
+		AuthToken:    "token",
+	}
+	if !VerifyPassword(legacy, "secret") {
+		t.Fatal("expected legacy password hash to verify")
+	}
+}
diff --git a/lib/boxstore/thumbnails.go b/lib/boxstore/thumbnails.go
index 2299c52..d7dd289 100644
--- a/lib/boxstore/thumbnails.go
+++ b/lib/boxstore/thumbnails.go
@@ -140,7 +140,15 @@ func canGenerateThumbnail(file models.BoxFile) bool {
 }
 
 func generateThumbnail(task thumbnailTask) error {
-	source, err := os.Open(filepath.Join(BoxPath(task.BoxID), task.Name))
+	sourcePath, ok := SafeBoxFilePath(task.BoxID, task.Name)
+	if !ok {
+		return os.ErrInvalid
+	}
+	if err := ensureRegularFile(sourcePath); err != nil {
+		return err
+	}
+
+	source, err := os.Open(sourcePath)
 	if err != nil {
 		return err
 	}
@@ -161,15 +169,28 @@ func generateThumbnail(task thumbnailTask) error {
 		return os.ErrInvalid
 	}
 
-	target, err := os.Create(path)
+	target, tempPath, err := createTempSibling(path)
 	if err != nil {
 		return err
 	}
-	defer target.Close()
+	committed := false
+	defer func() {
+		target.Close()
+		if !committed {
+			os.Remove(tempPath)
+		}
+	}()
 
 	if err := jpeg.Encode(target, thumb, &jpeg.Options{Quality: 82}); err != nil {
 		return err
 	}
+	if err := target.Close(); err != nil {
+		return err
+	}
+	if err := os.Rename(tempPath, path); err != nil {
+		return err
+	}
+	committed = true
 
 	return markThumbnailReady(task.BoxID, task.FileID)
 }
diff --git a/lib/helpers/paths.go b/lib/helpers/paths.go
index a1b2d45..af95f99 100644
--- a/lib/helpers/paths.go
+++ b/lib/helpers/paths.go
@@ -14,8 +14,19 @@ func SafeFilename(name string) (string, bool) {
 }
 
 func SafeChildPath(parent string, filename string) (string, bool) {
-	path := filepath.Join(parent, filename)
-	return path, strings.HasPrefix(path, parent+string(filepath.Separator))
+	parent = filepath.Clean(parent)
+	filename = strings.TrimSpace(filename)
+	if parent == "" || filename == "" || filepath.IsAbs(filename) {
+		return "", false
+	}
+
+	path := filepath.Clean(filepath.Join(parent, filename))
+	relative, err := filepath.Rel(parent, path)
+	if err != nil || relative == "." || strings.HasPrefix(relative, ".."+string(filepath.Separator)) || relative == ".." {
+		return "", false
+	}
+
+	return path, true
 }
 
 func UniqueFilename(directory string, filename string) string {
diff --git a/lib/helpers/paths_test.go b/lib/helpers/paths_test.go
new file mode 100644
index 0000000..dc3bf69
--- /dev/null
+++ b/lib/helpers/paths_test.go
@@ -0,0 +1,20 @@
+package helpers
+
+import (
+	"path/filepath"
+	"testing"
+)
+
+func TestSafeChildPathRejectsTraversalAndAbsolutePaths(t *testing.T) {
+	parent := filepath.Join(t.TempDir(), "parent")
+
+	if _, ok := SafeChildPath(parent, "../outside.txt"); ok {
+		t.Fatal("expected traversal to be rejected")
+	}
+	if _, ok := SafeChildPath(parent, filepath.Join(string(filepath.Separator), "tmp", "outside.txt")); ok {
+		t.Fatal("expected absolute path to be rejected")
+	}
+	if path, ok := SafeChildPath(parent, "inside.txt"); !ok || path != filepath.Join(parent, "inside.txt") {
+		t.Fatalf("expected safe child path, got path=%q ok=%v", path, ok)
+	}
+}
diff --git a/lib/metastore/models.go b/lib/metastore/models.go
index dc32ecc..1b39098 100644
--- a/lib/metastore/models.go
+++ b/lib/metastore/models.go
@@ -46,6 +46,7 @@ type TagPermissions struct {
 
 type Session struct {
 	Token     string    `json:"token"`
+	CSRFToken string    `json:"csrf_token"`
 	UserID    string    `json:"user_id"`
 	CreatedAt time.Time `json:"created_at"`
 	ExpiresAt time.Time `json:"expires_at"`
diff --git a/lib/metastore/sessions.go b/lib/metastore/sessions.go
index 87a767b..b4684d5 100644
--- a/lib/metastore/sessions.go
+++ b/lib/metastore/sessions.go
@@ -23,9 +23,14 @@ func (store *Store) CreateSession(userID string, ttl time.Duration) (Session, er
 	if err != nil {
 		return Session{}, err
 	}
+	csrfToken, err := helpers.RandomHexID(32)
+	if err != nil {
+		return Session{}, err
+	}
 	now := time.Now().UTC()
 	session := Session{
 		Token:     token,
+		CSRFToken: csrfToken,
 		UserID:    userID,
 		CreatedAt: now,
 		ExpiresAt: now.Add(ttl),
diff --git a/lib/models/models.go b/lib/models/models.go
index 9789d38..1df17d4 100644
--- a/lib/models/models.go
+++ b/lib/models/models.go
@@ -49,6 +49,7 @@ type BoxManifest struct {
 	RetentionSecs   int64     `json:"retention_seconds"`
 	PasswordSalt    string    `json:"password_salt,omitempty"`
 	PasswordHash    string    `json:"password_hash,omitempty"`
+	PasswordHashAlg string    `json:"password_hash_alg,omitempty"`
 	AuthToken       string    `json:"auth_token,omitempty"`
 	DisableZip      bool      `json:"disable_zip,omitempty"`
 	OneTimeDownload bool      `json:"one_time_download,omitempty"`
diff --git a/lib/server/admin.go b/lib/server/admin.go
index ce6eecd..eb70857 100644
--- a/lib/server/admin.go
+++ b/lib/server/admin.go
@@ -1,6 +1,7 @@
 package server
 
 import (
+	"crypto/subtle"
 	"errors"
 	"fmt"
 	"net/http"
@@ -57,6 +58,7 @@ type adminBoxRow struct {
 
 func (app *App) registerAdminRoutes(router *gin.Engine) {
 	admin := router.Group("/admin")
+	admin.Use(noStoreAdminHeaders)
 	admin.GET("/login", app.handleAdminLogin)
 	admin.POST("/login", app.handleAdminLoginPost)
 
@@ -132,6 +134,7 @@ func (app *App) handleAdminLogout(ctx *gin.Context) {
 func (app *App) handleAdminDashboard(ctx *gin.Context) {
 	ctx.HTML(http.StatusOK, "admin.html", gin.H{
 		"CurrentUser": app.currentAdminUsername(ctx),
+		"CSRFToken":   app.currentCSRFToken(ctx),
 	})
 }
 
@@ -267,6 +270,7 @@ func (app *App) renderAdminUsers(ctx *gin.Context, errorMessage string) {
 
 	ctx.HTML(http.StatusOK, "admin_users.html", gin.H{
 		"CurrentUser": app.currentAdminUsername(ctx),
+		"CSRFToken":   app.currentCSRFToken(ctx),
 		"Users":       rows,
 		"Tags":        tags,
 		"Error":       errorMessage,
@@ -330,6 +334,7 @@ func (app *App) renderAdminTags(ctx *gin.Context, errorMessage string) {
 	}
 	ctx.HTML(http.StatusOK, "admin_tags.html", gin.H{
 		"CurrentUser": app.currentAdminUsername(ctx),
+		"CSRFToken":   app.currentCSRFToken(ctx),
 		"Tags":        rows,
 		"Error":       errorMessage,
 	})
@@ -374,6 +379,7 @@ func (app *App) handleAdminSettingsPost(ctx *gin.Context) {
 func (app *App) renderAdminSettings(ctx *gin.Context, errorMessage string) {
 	ctx.HTML(http.StatusOK, "admin_settings.html", gin.H{
 		"CurrentUser":      app.currentAdminUsername(ctx),
+		"CSRFToken":        app.currentCSRFToken(ctx),
 		"Rows":             app.config.SettingRows(),
 		"OverridesAllowed": app.config.AllowAdminSettingsOverride,
 		"Error":            errorMessage,
@@ -393,6 +399,11 @@ func (app *App) requireAdminSession(ctx *gin.Context) {
 		ctx.Abort()
 		return
 	}
+	if !validAdminCSRF(ctx, session) {
+		ctx.String(http.StatusForbidden, "Permission denied")
+		ctx.Abort()
+		return
+	}
 	user, ok, err := app.store.GetUser(session.UserID)
 	if err != nil || !ok || user.Disabled {
 		ctx.Redirect(http.StatusSeeOther, "/admin/login")
@@ -407,6 +418,7 @@ func (app *App) requireAdminSession(ctx *gin.Context) {
 	}
 	ctx.Set("adminUser", user)
 	ctx.Set("adminPerms", perms)
+	ctx.Set("adminCSRFToken", session.CSRFToken)
 	ctx.Next()
 }
 
@@ -458,6 +470,15 @@ func (app *App) currentAdminUsername(ctx *gin.Context) string {
 	return ""
 }
 
+func (app *App) currentCSRFToken(ctx *gin.Context) string {
+	if value, ok := ctx.Get("adminCSRFToken"); ok {
+		if token, ok := value.(string); ok {
+			return token
+		}
+	}
+	return ""
+}
+
 func (app *App) renderAdminLogin(ctx *gin.Context, errorMessage string) {
 	ctx.HTML(http.StatusOK, "admin_login.html", gin.H{
 		"AdminLoginEnabled": app.adminLoginEnabled,
@@ -465,6 +486,30 @@ func (app *App) renderAdminLogin(ctx *gin.Context, errorMessage string) {
 	})
 }
 
+func noStoreAdminHeaders(ctx *gin.Context) {
+	ctx.Header("Cache-Control", "no-store")
+	ctx.Header("Pragma", "no-cache")
+	ctx.Header("X-Content-Type-Options", "nosniff")
+	ctx.Next()
+}
+
+func validAdminCSRF(ctx *gin.Context, session metastore.Session) bool {
+	switch ctx.Request.Method {
+	case http.MethodGet, http.MethodHead, http.MethodOptions:
+		return true
+	}
+
+	token := ctx.PostForm("csrf_token")
+	return token != "" && subtleConstantTimeEqual(token, session.CSRFToken)
+}
+
+func subtleConstantTimeEqual(a string, b string) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	return subtle.ConstantTimeCompare([]byte(a), []byte(b)) == 1
+}
+
 func parseTagPermissions(ctx *gin.Context) (metastore.TagPermissions, error) {
 	maxFileSize, err := parseOptionalInt64(ctx.PostForm("max_file_size_bytes"))
 	if err != nil {
diff --git a/lib/server/handlers.go b/lib/server/handlers.go
index 967ee16..f6210d5 100644
--- a/lib/server/handlers.go
+++ b/lib/server/handlers.go
@@ -7,6 +7,7 @@ import (
 	"net/http"
 	"os"
 	"strings"
+	"sync"
 	"time"
 
 	"github.com/gin-gonic/gin"
@@ -18,6 +19,8 @@ import (
 
 const boxAuthCookiePrefix = "warpbox_box_"
 
+var oneTimeDownloadLocks sync.Map
+
 func (app *App) handleIndex(ctx *gin.Context) {
 	ctx.HTML(http.StatusOK, "index.html", gin.H{
 		"RetentionOptions": app.retentionOptions(),
@@ -166,6 +169,10 @@ func (app *App) handleDownloadBox(ctx *gin.Context) {
 	if !ok {
 		return
 	}
+	if hasManifest && manifest.OneTimeDownload {
+		app.handleOneTimeDownloadBox(ctx, boxID)
+		return
+	}
 
 	if hasManifest && manifest.DisableZip {
 		ctx.String(http.StatusForbidden, "Zip download disabled for this box")
@@ -177,11 +184,45 @@ func (app *App) handleDownloadBox(ctx *gin.Context) {
 		ctx.String(http.StatusNotFound, "Box not found")
 		return
 	}
-	if hasManifest && manifest.OneTimeDownload && !allFilesComplete(files) {
-		ctx.String(http.StatusConflict, "Box is not ready yet")
+	if !app.writeBoxZip(ctx, boxID, files) {
+		return
+	}
+	if hasManifest && app.config.RenewOnDownloadEnabled {
+		boxstore.RenewManifest(boxID, manifest.RetentionSecs)
+	}
+}
+
+func (app *App) handleOneTimeDownloadBox(ctx *gin.Context, boxID string) {
+	lock := oneTimeDownloadLock(boxID)
+	lock.Lock()
+	defer lock.Unlock()
+	defer oneTimeDownloadLocks.Delete(boxID)
+
+	manifest, hasManifest, ok := app.authorizeBoxRequest(ctx, boxID, true)
+	if !ok {
+		return
+	}
+	if !hasManifest || !manifest.OneTimeDownload {
+		ctx.String(http.StatusNotFound, "Box not found")
 		return
 	}
 
+	files, err := boxstore.ListFiles(boxID)
+	if err != nil {
+		ctx.String(http.StatusNotFound, "Box not found")
+		return
+	}
+	if !allFilesComplete(files) {
+		ctx.String(http.StatusConflict, "Box is not ready yet")
+		return
+	}
+	if !app.writeBoxZip(ctx, boxID, files) {
+		return
+	}
+	boxstore.DeleteBox(boxID)
+}
+
+func (app *App) writeBoxZip(ctx *gin.Context, boxID string, files []models.BoxFile) bool {
 	ctx.Header("Content-Type", "application/zip")
 	ctx.Header("Content-Disposition", fmt.Sprintf(`attachment; filename="warpbox-%s.zip"`, boxID))
 
@@ -197,25 +238,24 @@ func (app *App) handleDownloadBox(ctx *gin.Context) {
 		if !file.IsComplete {
 			continue
 		}
-
 		if err := boxstore.AddFileToZip(zipWriter, boxID, file.Name); err != nil {
 			ctx.Status(http.StatusInternalServerError)
-			return
+			return false
 		}
 	}
 
 	if err := zipWriter.Close(); err != nil {
 		zipClosed = true
 		ctx.Status(http.StatusInternalServerError)
-		return
+		return false
 	}
 	zipClosed = true
+	return true
+}
 
-	if hasManifest && manifest.OneTimeDownload {
-		boxstore.DeleteBox(boxID)
-	} else if hasManifest && app.config.RenewOnDownloadEnabled {
-		boxstore.RenewManifest(boxID, manifest.RetentionSecs)
-	}
+func oneTimeDownloadLock(boxID string) *sync.Mutex {
+	lock, _ := oneTimeDownloadLocks.LoadOrStore(boxID, &sync.Mutex{})
+	return lock.(*sync.Mutex)
 }
 
 func allFilesComplete(files []models.BoxFile) bool {
@@ -259,6 +299,10 @@ func (app *App) handleDownloadFile(ctx *gin.Context) {
 		ctx.String(http.StatusNotFound, "File not found")
 		return
 	}
+	if !boxstore.IsSafeRegularBoxFile(boxID, filename) {
+		ctx.String(http.StatusBadRequest, "Invalid file")
+		return
+	}
 
 	ctx.FileAttachment(path, filename)
 	if hasManifest && app.config.RenewOnDownloadEnabled {
@@ -297,6 +341,7 @@ func (app *App) handleCreateBox(ctx *gin.Context) {
 	if !app.requireAPI(ctx) || !app.requireGuestUploads(ctx) {
 		return
 	}
+	app.limitRequestBody(ctx)
 
 	boxID, err := boxstore.NewBoxID()
 	if err != nil {
@@ -332,6 +377,7 @@ func (app *App) handleManifestFileUpload(ctx *gin.Context) {
 	if !app.requireAPI(ctx) || !app.requireGuestUploads(ctx) {
 		return
 	}
+	app.limitRequestBody(ctx)
 
 	boxID := ctx.Param("id")
 	fileID := ctx.Param("file_id")
@@ -366,11 +412,12 @@ func (app *App) handleFileStatusUpdate(ctx *gin.Context) {
 	if !app.requireAPI(ctx) {
 		return
 	}
+	app.limitRequestBody(ctx)
 
 	boxID := ctx.Param("id")
 	fileID := ctx.Param("file_id")
-	if !boxstore.ValidBoxID(boxID) {
-		ctx.JSON(http.StatusBadRequest, gin.H{"error": "Invalid box id"})
+	if !boxstore.ValidBoxID(boxID) || !helpers.ValidLowerHexID(fileID, 16) {
+		ctx.JSON(http.StatusBadRequest, gin.H{"error": "Invalid file"})
 		return
 	}
 
@@ -379,6 +426,14 @@ func (app *App) handleFileStatusUpdate(ctx *gin.Context) {
 		ctx.JSON(http.StatusBadRequest, gin.H{"error": "Invalid status payload"})
 		return
 	}
+	if request.Status == models.FileStatusReady {
+		ctx.JSON(http.StatusBadRequest, gin.H{"error": "Uploads must complete through the upload endpoint"})
+		return
+	}
+	if err := app.rejectExpiredManifestBox(boxID); err != nil {
+		ctx.JSON(http.StatusGone, gin.H{"error": err.Error()})
+		return
+	}
 
 	file, err := boxstore.MarkFileStatus(boxID, fileID, request.Status)
 	if err != nil {
@@ -393,6 +448,7 @@ func (app *App) handleDirectBoxUpload(ctx *gin.Context) {
 	if !app.requireAPI(ctx) || !app.requireGuestUploads(ctx) {
 		return
 	}
+	app.limitRequestBody(ctx)
 
 	boxID := ctx.Param("id")
 	if !boxstore.ValidBoxID(boxID) {
@@ -423,6 +479,7 @@ func (app *App) handleLegacyUpload(ctx *gin.Context) {
 	if !app.requireAPI(ctx) || !app.requireGuestUploads(ctx) {
 		return
 	}
+	app.limitRequestBody(ctx)
 
 	form, err := ctx.MultipartForm()
 	if err != nil {
@@ -580,14 +637,18 @@ func (app *App) validateManifestFileUpload(boxID string, fileID string, size int
 	if err := app.validateFileSize(size); err != nil {
 		return err
 	}
-	if app.config.GlobalMaxBoxSizeBytes <= 0 {
-		return nil
-	}
 
 	manifest, err := boxstore.ReadManifest(boxID)
 	if err != nil {
 		return app.validateIncomingFile(boxID, size)
 	}
+	if boxstore.IsExpired(manifest) {
+		_ = boxstore.DeleteBox(boxID)
+		return fmt.Errorf("Box expired")
+	}
+	if app.config.GlobalMaxBoxSizeBytes <= 0 {
+		return nil
+	}
 	totalSize := int64(0)
 	found := false
 	for _, file := range manifest.Files {
@@ -624,6 +685,37 @@ func (app *App) validateBoxSize(size int64) error {
 	return nil
 }
 
+func (app *App) rejectExpiredManifestBox(boxID string) error {
+	manifest, err := boxstore.ReadManifest(boxID)
+	if err != nil {
+		return nil
+	}
+	if !boxstore.IsExpired(manifest) {
+		return nil
+	}
+	_ = boxstore.DeleteBox(boxID)
+	return fmt.Errorf("Box expired")
+}
+
+func (app *App) limitRequestBody(ctx *gin.Context) {
+	limit := app.maxRequestBodyBytes()
+	if limit <= 0 {
+		return
+	}
+	ctx.Request.Body = http.MaxBytesReader(ctx.Writer, ctx.Request.Body, limit)
+}
+
+func (app *App) maxRequestBodyBytes() int64 {
+	limit := app.config.GlobalMaxBoxSizeBytes
+	if limit <= 0 || app.config.GlobalMaxFileSizeBytes > limit {
+		limit = app.config.GlobalMaxFileSizeBytes
+	}
+	if limit <= 0 {
+		return 0
+	}
+	return limit + 10*1024*1024
+}
+
 func (app *App) retentionAllowed(key string) bool {
 	key = strings.TrimSpace(key)
 	if key == "" {
diff --git a/lib/server/security_test.go b/lib/server/security_test.go
new file mode 100644
index 0000000..3a0d985
--- /dev/null
+++ b/lib/server/security_test.go
@@ -0,0 +1,79 @@
+package server
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"testing"
+	"time"
+
+	"github.com/gin-gonic/gin"
+
+	"warpbox/lib/boxstore"
+	"warpbox/lib/config"
+	"warpbox/lib/metastore"
+	"warpbox/lib/models"
+)
+
+func TestValidateManifestFileUploadRejectsExpiredBox(t *testing.T) {
+	restoreUploadRoot := boxstore.UploadRoot()
+	defer boxstore.SetUploadRoot(restoreUploadRoot)
+	boxstore.SetUploadRoot(t.TempDir())
+
+	boxID := "0123456789abcdef0123456789abcdef"
+	if err := os.MkdirAll(boxstore.BoxPath(boxID), 0755); err != nil {
+		t.Fatalf("MkdirAll returned error: %v", err)
+	}
+	manifest := models.BoxManifest{
+		Files:     []models.BoxFile{{ID: "0123456789abcdef", Name: "file.txt", Status: models.FileStatusWait}},
+		ExpiresAt: time.Now().UTC().Add(-time.Second),
+	}
+	if err := boxstore.WriteManifest(boxID, manifest); err != nil {
+		t.Fatalf("WriteManifest returned error: %v", err)
+	}
+
+	app := &App{config: &config.Config{}}
+	if err := app.validateManifestFileUpload(boxID, "0123456789abcdef", 1); err == nil {
+		t.Fatal("expected expired box upload to be rejected")
+	}
+	if _, err := os.Stat(boxstore.BoxPath(boxID)); !os.IsNotExist(err) {
+		t.Fatalf("expected expired box to be deleted, stat err=%v", err)
+	}
+}
+
+func TestAdminProtectedPostRequiresCSRF(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	store, err := metastore.Open(t.TempDir())
+	if err != nil {
+		t.Fatalf("Open returned error: %v", err)
+	}
+	defer store.Close()
+
+	adminTag, err := store.EnsureAdminTag()
+	if err != nil {
+		t.Fatalf("EnsureAdminTag returned error: %v", err)
+	}
+	user, err := store.CreateUserWithPassword("admin", "", "secret", []string{adminTag.ID})
+	if err != nil {
+		t.Fatalf("CreateUserWithPassword returned error: %v", err)
+	}
+	session, err := store.CreateSession(user.ID, time.Hour)
+	if err != nil {
+		t.Fatalf("CreateSession returned error: %v", err)
+	}
+
+	app := &App{config: &config.Config{}, store: store}
+	router := gin.New()
+	router.POST("/admin/test", app.requireAdminSession, func(ctx *gin.Context) {
+		ctx.Status(http.StatusNoContent)
+	})
+
+	request := httptest.NewRequest(http.MethodPost, "/admin/test", nil)
+	request.AddCookie(&http.Cookie{Name: adminSessionCookie, Value: session.Token})
+	response := httptest.NewRecorder()
+	router.ServeHTTP(response, request)
+	if response.Code != http.StatusForbidden {
+		t.Fatalf("expected missing CSRF token to be forbidden, got %d", response.Code)
+	}
+}
diff --git a/templates/admin.html b/templates/admin.html
index 3a398c8..bf7044a 100644
--- a/templates/admin.html
+++ b/templates/admin.html
@@ -20,12 +20,13 @@
                 </header>
                 <div class="win98-panel admin-panel">
                     <nav class="admin-nav">
-                        <span>Signed in as {{ .CurrentUser }}</span>
-                        <span class="admin-spacer"></span>
-                        <form action="/admin/logout" method="post">
-                            <button class="win98-button" type="submit">Logout</button>
-                        </form>
-                    </nav>
+						<span>Signed in as {{ .CurrentUser }}</span>
+						<span class="admin-spacer"></span>
+						<form action="/admin/logout" method="post">
+							<input type="hidden" name="csrf_token" value="{{ .CSRFToken }}">
+							<button class="win98-button" type="submit">Logout</button>
+						</form>
+					</nav>
                     <div class="admin-grid">
                         <a class="win98-panel admin-link" href="/admin/boxes"><strong>Boxes</strong></a>
                         <a class="win98-panel admin-link" href="/admin/users"><strong>Users</strong></a>
diff --git a/templates/admin_settings.html b/templates/admin_settings.html
index aac02ca..be23b73 100644
--- a/templates/admin_settings.html
+++ b/templates/admin_settings.html
@@ -29,9 +29,10 @@
                     </nav>
                     {{ if .Error }}
                         <p class="admin-error">{{ .Error }}</p>
-                    {{ end }}
-                    <form class="admin-form" action="/admin/settings" method="post">
-                        <table class="admin-table">
+					{{ end }}
+					<form class="admin-form" action="/admin/settings" method="post">
+						<input type="hidden" name="csrf_token" value="{{ .CSRFToken }}">
+						<table class="admin-table">
                             <thead>
                                 <tr>
                                     <th>Setting</th>
diff --git a/templates/admin_tags.html b/templates/admin_tags.html
index e76c6ea..126077b 100644
--- a/templates/admin_tags.html
+++ b/templates/admin_tags.html
@@ -29,11 +29,12 @@
                     </nav>
                     {{ if .Error }}
                         <p class="admin-error">{{ .Error }}</p>
-                    {{ end }}
-                    <form class="admin-form win98-panel" action="/admin/tags" method="post">
-                        <label class="admin-form-row">
-                            <span>Name</span>
-                            <input name="name" required>
+					{{ end }}
+					<form class="admin-form win98-panel" action="/admin/tags" method="post">
+						<input type="hidden" name="csrf_token" value="{{ .CSRFToken }}">
+						<label class="admin-form-row">
+							<span>Name</span>
+							<input name="name" required>
                         </label>
                         <label class="admin-form-row">
                             <span>Description</span>
diff --git a/templates/admin_users.html b/templates/admin_users.html
index 77c4833..80ba4db 100644
--- a/templates/admin_users.html
+++ b/templates/admin_users.html
@@ -29,11 +29,12 @@
                     </nav>
                     {{ if .Error }}
                         <p class="admin-error">{{ .Error }}</p>
-                    {{ end }}
-                    <form class="admin-form win98-panel" action="/admin/users" method="post">
-                        <label class="admin-form-row">
-                            <span>Username</span>
-                            <input name="username" required>
+					{{ end }}
+					<form class="admin-form win98-panel" action="/admin/users" method="post">
+						<input type="hidden" name="csrf_token" value="{{ .CSRFToken }}">
+						<label class="admin-form-row">
+							<span>Username</span>
+							<input name="username" required>
                         </label>
                         <label class="admin-form-row">
                             <span>Email</span>
@@ -72,11 +73,12 @@
                                     <td>{{ .Tags }}</td>
                                     <td>{{ .CreatedAt }}</td>
                                     <td>{{ if .Disabled }}Disabled{{ else }}Active{{ end }}</td>
-                                    <td>
-                                        <form action="/admin/users" method="post">
-                                            <input type="hidden" name="action" value="toggle_disabled">
-                                            <input type="hidden" name="user_id" value="{{ .ID }}">
-                                            <button class="win98-button" type="submit" {{ if .IsCurrent }}disabled{{ end }}>{{ if .Disabled }}Enable{{ else }}Disable{{ end }}</button>
+									<td>
+										<form action="/admin/users" method="post">
+											<input type="hidden" name="csrf_token" value="{{ $.CSRFToken }}">
+											<input type="hidden" name="action" value="toggle_disabled">
+											<input type="hidden" name="user_id" value="{{ .ID }}">
+											<button class="win98-button" type="submit" {{ if .IsCurrent }}disabled{{ end }}>{{ if .Disabled }}Enable{{ else }}Disable{{ end }}</button>
                                         </form>
                                     </td>
                                 </tr>