Protect elevated working folder from malicious data

When running elevated, Burn uses the Windows Temp folder as its working folder
to prevent normal processes from tampering with the files. Windows Temp does
allow non-elevated processes to write to the folder but they cannot see the
files there. Unfortunately, contrary to our belief, non-elevated processes
can read the files in Windows Temp by watching for directory changes. This
allows a malicious process to lie in wait, watching the Windows Temp folder
until a Burn process is launched elevated, then attack the working folder.
Mitigate that attack by protecting the working folder to only elevated users.

Managed custom actions also fall back to using the Windows Temp folder in
some cases and thus can be exposed in a similar fashion as an elevated Burn
process. Remove that possibility.

1 files changed, 29 insertions, 3 deletions

diff --git a/src/burn/engine/cache.cpp b/src/burn/engine/cache.cpp
index c0ac3ecd..358327a2 100644
--- a/src/burn/engine/cache.cpp
+++ b/src/burn/engine/cache.cpp
@@ -106,6 +106,7 @@ static HRESULT SecurePath(
     __in LPCWSTR wzPath
     );
 static HRESULT CopyEngineToWorkingFolder(
+    __in BOOL fElevated,
     __in BURN_CACHE* pCache,
     __in_z LPCWSTR wzSourcePath,
     __in_z LPCWSTR wzWorkingFolderName,
@@ -330,6 +331,7 @@ LExit:
 }
 
 extern "C" HRESULT CacheEnsureBaseWorkingFolder(
+    __in BOOL fElevated,
     __in BURN_CACHE* pCache,
     __deref_out_z_opt LPWSTR* psczBaseWorkingFolder
     )
@@ -338,15 +340,32 @@ extern "C" HRESULT CacheEnsureBaseWorkingFolder(
 
     HRESULT hr = S_OK;
     LPWSTR sczPotential = NULL;
+    PSECURITY_DESCRIPTOR psd = NULL;
+    LPSECURITY_ATTRIBUTES pWorkingFolderAcl = NULL;
 
     if (!pCache->fInitializedBaseWorkingFolder)
     {
+        // If elevated, allocate the pWorkingFolderAcl to protect the working folder to only SYSTEM and Admins.
+        if (fElevated)
+        {
+            LPCWSTR wzSddl = L"D:PAI(A;;FA;;;BA)(A;OICIIO;GA;;;BA)(A;;FA;;;SY)(A;OICIIO;GA;;;SY)";
+            if (!::ConvertStringSecurityDescriptorToSecurityDescriptorW(wzSddl, SDDL_REVISION_1, &psd, NULL))
+            {
+                ExitWithLastError(hr, "Failed to create the security descriptor for the working folder.");
+            }
+
+            pWorkingFolderAcl = reinterpret_cast<LPSECURITY_ATTRIBUTES>(MemAlloc(sizeof(SECURITY_ATTRIBUTES), TRUE));
+            pWorkingFolderAcl->nLength = sizeof(SECURITY_ATTRIBUTES);
+            pWorkingFolderAcl->lpSecurityDescriptor = psd;
+            pWorkingFolderAcl->bInheritHandle = FALSE;
+        }
+
         for (DWORD i = 0; i < pCache->cPotentialBaseWorkingFolders; ++i)
         {
             hr = PathConcatRelativeToFullyQualifiedBase(pCache->rgsczPotentialBaseWorkingFolders[i], pCache->wzGuid, &sczPotential);
             if (SUCCEEDED(hr))
             {
-                hr = DirEnsureExists(sczPotential, NULL);
+                hr = DirEnsureExists(sczPotential, pWorkingFolderAcl);
                 if (SUCCEEDED(hr))
                 {
                     pCache->sczBaseWorkingFolder = sczPotential;
@@ -373,6 +392,11 @@ extern "C" HRESULT CacheEnsureBaseWorkingFolder(
     }
 
 LExit:
+    ReleaseMem(pWorkingFolderAcl);
+    if (psd)
+    {
+        ::LocalFree(psd);
+    }
     ReleaseStr(sczPotential);
 
     return hr;
@@ -888,6 +912,7 @@ extern "C" HRESULT CachePreparePackage(
 }
 
 extern "C" HRESULT CacheBundleToWorkingDirectory(
+    __in BOOL fElevated,
     __in BURN_CACHE* pCache,
     __in_z LPCWSTR wzExecutableName,
     __in BURN_SECTION* pSection,
@@ -912,7 +937,7 @@ extern "C" HRESULT CacheBundleToWorkingDirectory(
     }
     else // otherwise, carry on putting the bundle in the working folder.
     {
-        hr = CopyEngineToWorkingFolder(pCache, sczSourcePath, BUNDLE_WORKING_FOLDER_NAME, wzExecutableName, pSection, psczEngineWorkingPath);
+        hr = CopyEngineToWorkingFolder(fElevated, pCache, sczSourcePath, BUNDLE_WORKING_FOLDER_NAME, wzExecutableName, pSection, psczEngineWorkingPath);
         ExitOnFailure(hr, "Failed to copy engine to working folder.");
     }
 
@@ -2063,6 +2088,7 @@ LExit:
 
 
 static HRESULT CopyEngineToWorkingFolder(
+    __in BOOL fElevated,
     __in BURN_CACHE* pCache,
     __in_z LPCWSTR wzSourcePath,
     __in_z LPCWSTR wzWorkingFolderName,
@@ -2079,7 +2105,7 @@ static HRESULT CopyEngineToWorkingFolder(
     LPWSTR sczPayloadSourcePath = NULL;
     LPWSTR sczPayloadTargetPath = NULL;
 
-    hr = CacheEnsureBaseWorkingFolder(pCache, &sczWorkingFolder);
+    hr = CacheEnsureBaseWorkingFolder(fElevated, pCache, &sczWorkingFolder);
     ExitOnFailure(hr, "Failed to create working path to copy engine.");
 
     hr = PathConcatRelativeToFullyQualifiedBase(sczWorkingFolder, wzWorkingFolderName, &sczTargetDirectory);