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## Summary of the Pull Request
The Runner is the **server** for the two-way named pipes it uses to talk
to `PowerToys.Settings.exe` and the Quick Access host, and it dispatched
privileged JSON commands (`killrunner`, `restart_elevation`,
`module_status`, `powertoys`, `language`, ...) **without authenticating
the caller**. When PowerToys runs elevated ("Run as administrator"), the
pipe DACL grants the shared **Logon SID**, so **any same-user Medium-IL
process could connect and inject commands** — a local privilege
escalation (CWE-732 / CWE-862). The pipe DACL cannot distinguish the
legitimate Medium-IL Settings child from a same-user attacker (identical
user SID, integrity level, and logon session), so this PR authenticates
the connecting process's **binary identity** before any dispatch.
## PR Checklist
- [x] **Tests:** Added/updated and all pass (native gate tests + C#
regression)
- [x] **Localization:** No new end-user-facing strings (only a
diagnostic runner log line)
- [x] **New binaries:** None — the new code compiles into the existing
`PowerToys.Interop` and `runner` binaries; tests were added to the
existing `Common.Utils.UnitTests` project
## Detailed Description of the Pull Request / Additional comments
New `src/common/interop/pipe_caller_auth.{h,cpp}` adds
`interop_auth::AuthenticateClient`, invoked from
`TwoWayPipeMessageIPC::handle_pipe_connection` **before** a message is
queued (fail-closed). A connecting client is accepted only if it is:
- under the **Runner-relative install directory**
(`get_module_folderpath()\WinUI3Apps`, so it adapts to installed and
dev-build layouts),
- an **allow-listed basename** (`PowerToys.Settings.exe` /
`PowerToys.QuickAccess.exe`),
- the Runner's **exact file version** (anti-downgrade), and
- **Microsoft Authenticode-signed**.
The signature is anchored to the **LOCAL MACHINE root store**
(`HCCE_LOCAL_MACHINE` +
`CertVerifyCertificateChainPolicy(AUTHENTICODE)`) rather than
`WinVerifyTrust`'s default user+machine union: the Runner runs as the
same user as a potential attacker and would otherwise trust a forged
signer added to `CurrentUser\Root`. Verdicts are cached per `(pid,
process-creation-time, policy)` with a short TTL so the check isn't
re-run on every message (each `send` opens a new connection). Rejections
are logged.
The gate is added via an **additive** `start(HANDLE, CallerPolicy)`
overload; the managed `start(nullptr)` path is unchanged (gate
disabled), so there is **no ABI break** to `PowerToys.Interop`.
`PIPE_REJECT_REMOTE_CLIENTS` is also set. In **Debug** builds only the
signature check is relaxed (directory/basename/version stay enforced) so
local unsigned builds still connect; the relaxation is compiled out of
Release.
**Scope:** this PR covers the two elevated Runner-server pipes (Settings
+ Quick Access), which are the actual EoP surface. The reverse
Runner->Settings response direction, the duplicated Workspaces
transport, and the AdvancedPaste/PowerDisplay module pipes are
intentionally out of scope and can be handled as follow-ups.
## Validation Steps Performed
**Automated**
- **Native unit tests** (`Common.Utils.UnitTests`,
`PipeCallerAuthTests`): legitimate self-caller accepted; wrong
basename/directory rejected with the reject-log callback firing; version
reading. 6/6 pass.
- **C# regression** (`Microsoft.Interop.Tests.TestSend`): managed
gate-disabled round-trip still works.
- **Builds:** runner Debug + Release, `PowerToys.Interop` Debug +
Release, and the test project all build/link clean.
**Official signed build (validates the Release-only signature path that
local Debug builds skip)**
- Queued the internal "PowerToys Signed YAML Release Build" for this
branch — **green** (`result: succeeded`). The produced installers are
Authenticode `Valid`, signer `Microsoft Corporation`.
**Manual testing on the signed installer (elevated Runner) — passed**
- Installed the signed build and ran the Runner **as administrator**.
Settings and Quick Access open and are fully functional; settings apply,
module toggles work, and the hotkey-conflict request/response
round-trips.
- **No** `Rejected unauthenticated ...` lines during legitimate use →
the genuine signed `PowerToys.Settings.exe` is accepted by the
machine-root signature + version + directory checks (verified in
`RunnerLogs\runner-log_*.log`, requests dispatched normally).
- **Security (negative) check:** a non-elevated `powershell.exe`
discovered the runner pipe via the pipe namespace and attempted
`{"killrunner":true}`; the write failed ("Pipe is broken") because the
Runner rejected the caller and disconnected before dispatch.
`PowerToys.exe` stayed running and logged: `Rejected unauthenticated
Settings pipe client: pid=... image='...\powershell.exe'
reason=bad-directory`.
---------
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 1d8f85ec-aaee-468a-9348-ba79b059cbaf