diff --git a/docs/implement.md b/docs/implement.md
index 86af231..7c36756 100644
--- a/docs/implement.md
+++ b/docs/implement.md
@@ -1,6 +1,8 @@
-# Implementation Details (Local-Only)
+# Implementation Details (Local-Only Core)
 
-This document describes how `codex-auth` stores accounts, synchronizes auth files, and refreshes metadata. The tool never calls external APIs; it reads only local files under `~/.codex` (or `CODEX_HOME`).
+This document describes how `codex-auth` stores accounts, synchronizes auth files, and refreshes metadata. The core account-management flows read and write local files under `~/.codex` (or `CODEX_HOME`) and do not include built-in HTTP/API calls.
+
+`codex-auth login` (without `--skip`) invokes the external `codex login` command as a child process. Any network/API behavior in that path comes from the `codex` CLI, not from `codex-auth`'s own file-sync logic.
 
 ## Packaging and Release
 
@@ -32,6 +34,17 @@ This document describes how `codex-auth` stores accounts, synchronizes auth file
 - `~/.codex/accounts/registry.json.bak.<timestamp>`
 - `~/.codex/sessions/...`
 
+## File Permissions
+
+- On Unix-like systems, `codex-auth` hardens sensitive files to mode `0600` after write/copy:
+  - `~/.codex/auth.json` (when written by `codex-auth`)
+  - `~/.codex/accounts/registry.json`
+  - `~/.codex/accounts/<email_b64>.auth.json`
+  - `~/.codex/accounts/auth.json.bak.<timestamp>`
+  - `~/.codex/accounts/registry.json.bak.<timestamp>`
+- On Unix-like systems, `~/.codex/accounts/` is hardened to mode `0700`.
+- On Windows, POSIX mode bits are not enforced; the tool logs a warning instead of failing.
+
 `codex-auth` resolves `codex_home` in this order:
 
 1. `CODEX_HOME` (when set and non-empty)
diff --git a/src/registry.zig b/src/registry.zig
index e3a9eeb..a87c36b 100644
--- a/src/registry.zig
+++ b/src/registry.zig
@@ -1,8 +1,11 @@
 const std = @import("std");
+const builtin = @import("builtin");
 
 pub const PlanType = enum { free, plus, pro, team, business, enterprise, edu, unknown };
 pub const AuthMode = enum { chatgpt, apikey };
 const registry_version: u32 = 2;
+const private_file_mode: std.fs.File.Mode = 0o600;
+const private_dir_mode: std.fs.File.Mode = 0o700;
 
 fn normalizeEmailAlloc(allocator: std.mem.Allocator, email: []const u8) ![]u8 {
     var buf = try allocator.alloc(u8, email.len);
@@ -111,10 +114,31 @@ pub fn resolveCodexHome(allocator: std.mem.Allocator) ![]u8 {
     return error.EnvironmentVariableNotFound;
 }
 
+fn hardenFilePermissions(path: []const u8) !void {
+    if (comptime builtin.os.tag == .windows) {
+        std.log.warn("cannot enforce 0600 on Windows: {s}", .{path});
+        return;
+    }
+    var file = try std.fs.cwd().openFile(path, .{});
+    defer file.close();
+    try file.chmod(private_file_mode);
+}
+
+fn hardenDirectoryPermissions(path: []const u8) !void {
+    if (comptime builtin.os.tag == .windows) {
+        std.log.warn("cannot enforce 0700 on Windows: {s}", .{path});
+        return;
+    }
+    var dir = try std.fs.cwd().openDir(path, .{});
+    defer dir.close();
+    try dir.chmod(private_dir_mode);
+}
+
 pub fn ensureAccountsDir(allocator: std.mem.Allocator, codex_home: []const u8) !void {
     const accounts_dir = try std.fs.path.join(allocator, &[_][]const u8{ codex_home, "accounts" });
     defer allocator.free(accounts_dir);
     try std.fs.cwd().makePath(accounts_dir);
+    try hardenDirectoryPermissions(accounts_dir);
 }
 
 pub fn registryPath(allocator: std.mem.Allocator, codex_home: []const u8) ![]u8 {
@@ -143,6 +167,7 @@ pub fn activeAuthPath(allocator: std.mem.Allocator, codex_home: []const u8) ![]u
 
 pub fn copyFile(src: []const u8, dest: []const u8) !void {
     try std.fs.cwd().copyFile(src, std.fs.cwd(), dest, .{});
+    try hardenFilePermissions(dest);
 }
 
 const max_backups: usize = 5;
@@ -175,6 +200,7 @@ fn fileEqualsBytes(allocator: std.mem.Allocator, path: []const u8, bytes: []cons
 
 fn ensureDir(path: []const u8) !void {
     try std.fs.cwd().makePath(path);
+    try hardenDirectoryPermissions(path);
 }
 
 fn backupDir(allocator: std.mem.Allocator, codex_home: []const u8) ![]u8 {
@@ -264,7 +290,7 @@ pub fn backupAuthIfChanged(
         }
         const backup = try makeBackupPath(allocator, dir, "auth.json");
         defer allocator.free(backup);
-        try std.fs.cwd().copyFile(current_auth_path, std.fs.cwd(), backup, .{});
+        try copyFile(current_auth_path, backup);
         try pruneBackups(allocator, dir, "auth.json", max_backups);
     }
 }
@@ -291,7 +317,7 @@ fn backupRegistryIfChanged(
 
     const backup = try makeBackupPath(allocator, dir, "registry.json");
     defer allocator.free(backup);
-    try std.fs.cwd().copyFile(current_registry_path, std.fs.cwd(), backup, .{});
+    try copyFile(current_registry_path, backup);
     try pruneBackups(allocator, dir, "registry.json", max_backups);
 }
 
@@ -741,9 +767,13 @@ pub fn saveRegistry(allocator: std.mem.Allocator, codex_home: []const u8, reg: *
 
     try backupRegistryIfChanged(allocator, codex_home, path, data);
 
-    var file = try std.fs.cwd().createFile(path, .{ .truncate = true });
+    var file = try std.fs.cwd().createFile(path, .{
+        .truncate = true,
+        .mode = private_file_mode,
+    });
     defer file.close();
     try file.writeAll(data);
+    try hardenFilePermissions(path);
 }
 
 const RegistryOut = struct {
diff --git a/src/tests/registry_test.zig b/src/tests/registry_test.zig
index ce31988..2de4dcd 100644
--- a/src/tests/registry_test.zig
+++ b/src/tests/registry_test.zig
@@ -1,4 +1,5 @@
 const std = @import("std");
+const builtin = @import("builtin");
 const registry = @import("../registry.zig");
 
 fn b64url(allocator: std.mem.Allocator, input: []const u8) ![]u8 {
@@ -41,6 +42,12 @@ fn countBackups(dir: std.fs.Dir, prefix: []const u8) !usize {
     return count;
 }
 
+fn assertModeUnix(path: []const u8, expected_mode: u32) !void {
+    if (comptime builtin.os.tag == .windows) return;
+    const stat = try std.fs.cwd().statFile(path);
+    try std.testing.expectEqual(expected_mode, @as(u32, @intCast(stat.mode & 0o777)));
+}
+
 test "registry save/load" {
     var gpa = std.testing.allocator;
     var tmp = std.testing.tmpDir(.{});
@@ -73,6 +80,56 @@ test "registry save/load" {
     try std.testing.expect(loaded.accounts.items.len == 1);
 }
 
+test "accounts directory is hardened to 0700" {
+    const gpa = std.testing.allocator;
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+
+    const codex_home = try tmp.dir.realpathAlloc(gpa, ".");
+    defer gpa.free(codex_home);
+    try registry.ensureAccountsDir(gpa, codex_home);
+
+    const accounts_path = try std.fs.path.join(gpa, &[_][]const u8{ codex_home, "accounts" });
+    defer gpa.free(accounts_path);
+    try assertModeUnix(accounts_path, 0o700);
+}
+
+test "copyFile hardens destination to 0600" {
+    const gpa = std.testing.allocator;
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+
+    const codex_home = try tmp.dir.realpathAlloc(gpa, ".");
+    defer gpa.free(codex_home);
+
+    try tmp.dir.writeFile(.{ .sub_path = "source.json", .data = "secret" });
+    const src = try std.fs.path.join(gpa, &[_][]const u8{ codex_home, "source.json" });
+    defer gpa.free(src);
+    const dest = try std.fs.path.join(gpa, &[_][]const u8{ codex_home, "dest.json" });
+    defer gpa.free(dest);
+
+    try registry.copyFile(src, dest);
+    try assertModeUnix(dest, 0o600);
+}
+
+test "saveRegistry writes registry with 0600 mode" {
+    const gpa = std.testing.allocator;
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+
+    const codex_home = try tmp.dir.realpathAlloc(gpa, ".");
+    defer gpa.free(codex_home);
+
+    var reg = registry.Registry{ .version = 2, .active_email = null, .accounts = std.ArrayList(registry.AccountRecord).empty };
+    defer reg.deinit(gpa);
+
+    try registry.saveRegistry(gpa, codex_home, &reg);
+
+    const path = try registry.registryPath(gpa, codex_home);
+    defer gpa.free(path);
+    try assertModeUnix(path, 0o600);
+}
+
 test "auth backup only on change" {
     var gpa = std.testing.allocator;
     var tmp = std.testing.tmpDir(.{});