14 files changed, 87 insertions, 40 deletions

diff --git a/src/burn/engine/apply.cpp b/src/burn/engine/apply.cpp
index 4ebed191..048cd98b 100644
--- a/src/burn/engine/apply.cpp
+++ b/src/burn/engine/apply.cpp
@@ -1371,7 +1371,7 @@ static HRESULT LayoutBundle(
         ExitOnFailure(hr, "Failed to get path to bundle to layout.");
     }
 
-    hr = PathConcatRelativeToBase(pContext->wzLayoutDirectory, wzExecutableName, &sczDestinationPath);
+    hr = PathConcatRelativeToFullyQualifiedBase(pContext->wzLayoutDirectory, wzExecutableName, &sczDestinationPath);
     ExitOnFailure(hr, "Failed to concat layout path for bundle.");
 
     // If the destination path is the currently running bundle, bail.
diff --git a/src/burn/engine/bundlepackageengine.cpp b/src/burn/engine/bundlepackageengine.cpp
index 8ba8e0d2..6a0343bd 100644
--- a/src/burn/engine/bundlepackageengine.cpp
+++ b/src/burn/engine/bundlepackageengine.cpp
@@ -780,7 +780,7 @@ static HRESULT ExecuteBundle(
         hr = CacheGetCompletedPath(pCache, pPackage->fPerMachine, pPackage->sczCacheId, &sczCachedDirectory);
         ExitOnFailure(hr, "Failed to get cached path for package: %ls", pPackage->sczId);
 
-        hr = PathConcatRelativeToBase(sczCachedDirectory, pPackagePayload->sczFilePath, &sczExecutablePath);
+        hr = PathConcatRelativeToFullyQualifiedBase(sczCachedDirectory, pPackagePayload->sczFilePath, &sczExecutablePath);
         ExitOnFailure(hr, "Failed to build executable path.");
     }
 
diff --git a/src/burn/engine/cache.cpp b/src/burn/engine/cache.cpp
index 5eacb20d..cf9de1c3 100644
--- a/src/burn/engine/cache.cpp
+++ b/src/burn/engine/cache.cpp
@@ -188,13 +188,18 @@ extern "C" HRESULT CacheInitialize(
     hr = PolcReadString(POLICY_BURN_REGISTRY_PATH, L"PackageCache", NULL, &pCache->sczCurrentMachinePackageCache);
     ExitOnFailure(hr, "Failed to read PackageCache policy directory.");
 
-    if (pCache->sczCurrentMachinePackageCache)
+    if (pCache->sczCurrentMachinePackageCache && PathIsFullyQualified(pCache->sczCurrentMachinePackageCache))
     {
         hr = PathBackslashTerminate(&pCache->sczCurrentMachinePackageCache);
         ExitOnFailure(hr, "Failed to backslash terminate redirected per-machine package cache directory name.");
     }
     else
     {
+        if (pCache->sczCurrentMachinePackageCache)
+        {
+            LogErrorId(E_INVALIDARG, MSG_INVALID_POLICY_MACHINE_PACKAGE_CACHE, pCache->sczCurrentMachinePackageCache, NULL, NULL);
+        }
+
         hr = StrAllocString(&pCache->sczCurrentMachinePackageCache, pCache->sczDefaultMachinePackageCache, 0);
         ExitOnFailure(hr, "Failed to copy default package cache directory to current package cache directory.");
     }
@@ -251,7 +256,7 @@ extern "C" HRESULT CacheInitializeSources(
     hr = CacheGetCompletedPath(pCache, pRegistration->fPerMachine, pRegistration->sczId, &sczCompletedFolder);
     ExitOnFailure(hr, "Failed to get completed path for bundle.");
 
-    hr = PathConcatRelativeToBase(sczCompletedFolder, pRegistration->sczExecutableName, &sczCompletedPath);
+    hr = PathConcatRelativeToFullyQualifiedBase(sczCompletedFolder, pRegistration->sczExecutableName, &sczCompletedPath);
     ExitOnFailure(hr, "Failed to combine working path with engine file name.");
 
     hr = PathCompareCanonicalized(sczCurrentPath, sczCompletedPath, &fPathEqual);
@@ -342,15 +347,16 @@ extern "C" HRESULT CacheEnsureBaseWorkingFolder(
     {
         for (DWORD i = 0; i < pCache->cPotentialBaseWorkingFolders; ++i)
         {
-            hr = PathConcatRelativeToBase(pCache->rgsczPotentialBaseWorkingFolders[i], pCache->wzGuid, &sczPotential);
-            ExitOnFailure(hr, "Failed to append random guid on to potential path for working folder.");
-
-            hr = DirEnsureExists(sczPotential, NULL);
+            hr = PathConcatRelativeToFullyQualifiedBase(pCache->rgsczPotentialBaseWorkingFolders[i], pCache->wzGuid, &sczPotential);
             if (SUCCEEDED(hr))
             {
-                pCache->sczBaseWorkingFolder = sczPotential;
-                sczPotential = NULL;
-                break;
+                hr = DirEnsureExists(sczPotential, NULL);
+                if (SUCCEEDED(hr))
+                {
+                    pCache->sczBaseWorkingFolder = sczPotential;
+                    sczPotential = NULL;
+                    break;
+                }
             }
 
             LogErrorId(hr, MSG_INVALID_BASE_WORKING_FOLDER, sczPotential, NULL, NULL);
@@ -414,7 +420,7 @@ extern "C" HRESULT CacheCalculateBundleLayoutWorkingPath(
 
     HRESULT hr = S_OK;
 
-    hr = PathConcatRelativeToBase(pCache->sczAcquisitionFolder, wzBundleId, psczWorkingPath);
+    hr = PathConcatRelativeToFullyQualifiedBase(pCache->sczAcquisitionFolder, wzBundleId, psczWorkingPath);
     ExitOnFailure(hr, "Failed to append bundle id for bundle layout working path.");
 
 LExit:
@@ -431,7 +437,7 @@ extern "C" HRESULT CacheCalculatePayloadWorkingPath(
 
     HRESULT hr = S_OK;
 
-    hr = PathConcatRelativeToBase(pCache->sczAcquisitionFolder, pPayload->sczKey, psczWorkingPath);
+    hr = PathConcatRelativeToFullyQualifiedBase(pCache->sczAcquisitionFolder, pPayload->sczKey, psczWorkingPath);
     ExitOnFailure(hr, "Failed to append Id as payload unverified path.");
 
 LExit:
@@ -448,7 +454,7 @@ extern "C" HRESULT CacheCalculateContainerWorkingPath(
 
     HRESULT hr = S_OK;
 
-    hr = PathConcatRelativeToBase(pCache->sczAcquisitionFolder, pContainer->sczHash, psczWorkingPath);
+    hr = PathConcatRelativeToFullyQualifiedBase(pCache->sczAcquisitionFolder, pContainer->sczHash, psczWorkingPath);
     ExitOnFailure(hr, "Failed to append hash as container unverified path.");
 
 LExit:
@@ -503,7 +509,7 @@ extern "C" HRESULT CacheGetCompletedPath(
     // GetRootPath returns S_FALSE if the package cache is redirected elsewhere.
     fRedirected = S_FALSE == hr;
 
-    hr = PathConcatRelativeToBase(sczRootPath, wzCacheId, &sczCurrentCompletedPath);
+    hr = PathConcatRelativeToFullyQualifiedBase(sczRootPath, wzCacheId, &sczCurrentCompletedPath);
     ExitOnFailure(hr, "Failed to construct cache path.");
 
     hr = PathBackslashTerminate(&sczCurrentCompletedPath);
@@ -516,7 +522,7 @@ extern "C" HRESULT CacheGetCompletedPath(
         hr = GetRootPath(pCache, fPerMachine, FALSE, &sczRootPath);
         ExitOnFailure(hr, "Failed to get old %hs package cache root directory.", fPerMachine ? "per-machine" : "per-user");
 
-        hr = PathConcatRelativeToBase(sczRootPath, wzCacheId, &sczDefaultCompletedPath);
+        hr = PathConcatRelativeToFullyQualifiedBase(sczRootPath, wzCacheId, &sczDefaultCompletedPath);
         ExitOnFailure(hr, "Failed to construct cache path.");
 
         hr = PathBackslashTerminate(&sczDefaultCompletedPath);
@@ -957,7 +963,7 @@ extern "C" HRESULT CacheLayoutBundle(
     HRESULT hr = S_OK;
     LPWSTR sczTargetPath = NULL;
 
-    hr = PathConcatRelativeToBase(wzLayoutDirectory, wzExecutableName, &sczTargetPath);
+    hr = PathConcatRelativeToFullyQualifiedBase(wzLayoutDirectory, wzExecutableName, &sczTargetPath);
     ExitOnFailure(hr, "Failed to combine completed path with engine file name for layout.");
 
     LogStringLine(REPORT_STANDARD, "Layout bundle from: '%ls' to: '%ls'", wzSourceBundlePath, sczTargetPath);
@@ -992,7 +998,7 @@ extern "C" HRESULT CacheCompleteBundle(
     hr = CreateCompletedPath(pCache, fPerMachine, wzBundleId, NULL, &sczTargetDirectory);
     ExitOnFailure(hr, "Failed to create completed cache path for bundle.");
 
-    hr = PathConcatRelativeToBase(sczTargetDirectory, wzExecutableName, &sczTargetPath);
+    hr = PathConcatRelativeToFullyQualifiedBase(sczTargetDirectory, wzExecutableName, &sczTargetPath);
     ExitOnFailure(hr, "Failed to combine completed path with engine file name.");
 
     // We can't just use wzExecutablePath because we needed to call CreateCompletedPath to ensure that the destination was secured.
@@ -1045,7 +1051,7 @@ extern "C" HRESULT CacheLayoutContainer(
     HRESULT hr = S_OK;
     LPWSTR sczCachedPath = NULL;
 
-    hr = PathConcatRelativeToBase(wzLayoutDirectory, pContainer->sczFilePath, &sczCachedPath);
+    hr = PathConcatRelativeToFullyQualifiedBase(wzLayoutDirectory, pContainer->sczFilePath, &sczCachedPath);
     ExitOnFailure(hr, "Failed to concat complete cached path.");
 
     hr = VerifyThenTransferContainer(pContainer, sczCachedPath, wzUnverifiedContainerPath, fMove, pfnCacheMessageHandler, pfnProgress, pContext);
@@ -1070,7 +1076,7 @@ extern "C" HRESULT CacheLayoutPayload(
     HRESULT hr = S_OK;
     LPWSTR sczCachedPath = NULL;
 
-    hr = PathConcatRelativeToBase(wzLayoutDirectory, pPayload->sczFilePath, &sczCachedPath);
+    hr = PathConcatRelativeToFullyQualifiedBase(wzLayoutDirectory, pPayload->sczFilePath, &sczCachedPath);
     ExitOnFailure(hr, "Failed to concat complete cached path.");
 
     hr = VerifyThenTransferPayload(pPayload, sczCachedPath, wzUnverifiedPayloadPath, fMove, pfnCacheMessageHandler, pfnProgress, pContext);
@@ -1165,7 +1171,7 @@ extern "C" HRESULT CacheVerifyContainer(
     HRESULT hr = S_OK;
     LPWSTR sczCachedPath = NULL;
 
-    hr = PathConcatRelativeToBase(wzCachedDirectory, pContainer->sczFilePath, &sczCachedPath);
+    hr = PathConcatRelativeToFullyQualifiedBase(wzCachedDirectory, pContainer->sczFilePath, &sczCachedPath);
     ExitOnFailure(hr, "Failed to concat complete cached path.");
 
     hr = VerifyFileAgainstContainer(pContainer, sczCachedPath, TRUE, BURN_CACHE_STEP_HASH_TO_SKIP_ACQUIRE, pfnCacheMessageHandler, pfnProgress, pContext);
@@ -1187,7 +1193,7 @@ extern "C" HRESULT CacheVerifyPayload(
     HRESULT hr = S_OK;
     LPWSTR sczCachedPath = NULL;
 
-    hr = PathConcatRelativeToBase(wzCachedDirectory, pPayload->sczFilePath, &sczCachedPath);
+    hr = PathConcatRelativeToFullyQualifiedBase(wzCachedDirectory, pPayload->sczFilePath, &sczCachedPath);
     ExitOnFailure(hr, "Failed to concat complete cached path.");
 
     hr = VerifyFileAgainstPayload(pPayload, sczCachedPath, TRUE, BURN_CACHE_STEP_HASH_TO_SKIP_ACQUIRE, pfnCacheMessageHandler, pfnProgress, pContext);
@@ -1478,7 +1484,7 @@ static HRESULT CalculateWorkingFolders(
     pCache->wzGuid[GUID_STRING_LENGTH - 1] = L'\\';
     pCache->wzGuid[GUID_STRING_LENGTH] = L'\0';
 
-    hr = PathConcatRelativeToBase(sczBaseAcquisitionPath, pCache->wzGuid, &pCache->sczAcquisitionFolder);
+    hr = PathConcatRelativeToFullyQualifiedBase(sczBaseAcquisitionPath, pCache->wzGuid, &pCache->sczAcquisitionFolder);
     ExitOnFailure(hr, "Failed to append random guid on to temp path for acquisition folder.");
 
 LExit:
@@ -1626,7 +1632,7 @@ static HRESULT CreateCompletedPath(
     else
     {
         // Get the cache completed file path.
-        hr = PathConcatRelativeToBase(sczCacheDirectory, wzFilePath, &sczCacheFile);
+        hr = PathConcatRelativeToFullyQualifiedBase(sczCacheDirectory, wzFilePath, &sczCacheFile);
         ExitOnFailure(hr, "Failed to construct cache file.");
 
         // Don't reset permissions here. The payload's package must reset its cache folder when it starts caching.
@@ -1664,7 +1670,7 @@ static HRESULT CreateUnverifiedPath(
         pCache->fUnverifiedCacheFolderCreated = TRUE;
     }
 
-    hr = PathConcatRelativeToBase(sczUnverifiedCacheFolder, wzPayloadId, psczUnverifiedPayloadPath);
+    hr = PathConcatRelativeToFullyQualifiedBase(sczUnverifiedCacheFolder, wzPayloadId, psczUnverifiedPayloadPath);
     ExitOnFailure(hr, "Failed to concat payload id to unverified folder path.");
 
 LExit:
@@ -2085,13 +2091,13 @@ static HRESULT CopyEngineToWorkingFolder(
     hr = CacheEnsureBaseWorkingFolder(pCache, &sczWorkingFolder);
     ExitOnFailure(hr, "Failed to create working path to copy engine.");
 
-    hr = PathConcatRelativeToBase(sczWorkingFolder, wzWorkingFolderName, &sczTargetDirectory);
+    hr = PathConcatRelativeToFullyQualifiedBase(sczWorkingFolder, wzWorkingFolderName, &sczTargetDirectory);
     ExitOnFailure(hr, "Failed to calculate the bundle working folder target name.");
 
     hr = DirEnsureExists(sczTargetDirectory, NULL);
     ExitOnFailure(hr, "Failed create bundle working folder.");
 
-    hr = PathConcatRelativeToBase(sczTargetDirectory, wzExecutableName, &sczTargetPath);
+    hr = PathConcatRelativeToFullyQualifiedBase(sczTargetDirectory, wzExecutableName, &sczTargetPath);
     ExitOnFailure(hr, "Failed to combine working path with engine file name.");
 
     // Copy the engine without any attached containers to the working path.
diff --git a/src/burn/engine/core.cpp b/src/burn/engine/core.cpp
index 39836bce..8dfa0010 100644
--- a/src/burn/engine/core.cpp
+++ b/src/burn/engine/core.cpp
@@ -2211,7 +2211,7 @@ static HRESULT DetectPackagePayloadsCached(
             {
                 BURN_PAYLOAD* pPayload = pPackage->payloads.rgItems[i].pPayload;
 
-                hr = PathConcatRelativeToBase(sczCachePath, pPayload->sczFilePath, &sczPayloadCachePath);
+                hr = PathConcatRelativeToFullyQualifiedBase(sczCachePath, pPayload->sczFilePath, &sczPayloadCachePath);
                 ExitOnFailure(hr, "Failed to concat payload cache path.");
 
                 if (FileExistsEx(sczPayloadCachePath, NULL))
diff --git a/src/burn/engine/engine.mc b/src/burn/engine/engine.mc
index 9a08fa3f..1c03145b 100644
--- a/src/burn/engine/engine.mc
+++ b/src/burn/engine/engine.mc
@@ -163,6 +163,13 @@ Language=English
 Failed to use folder as base working folder: %2!ls!, encountered error: %1!ls!.
 .
 
+MessageId=20
+Severity=Warning
+SymbolicName=MSG_INVALID_POLICY_MACHINE_PACKAGE_CACHE
+Language=English
+Failed to use folder as machine package cache: %2!ls!, encountered error: %1!ls!.
+.
+
 MessageId=51
 Severity=Error
 SymbolicName=MSG_FAILED_PARSE_CONDITION
diff --git a/src/burn/engine/exeengine.cpp b/src/burn/engine/exeengine.cpp
index fb852c78..808577bc 100644
--- a/src/burn/engine/exeengine.cpp
+++ b/src/burn/engine/exeengine.cpp
@@ -380,7 +380,7 @@ extern "C" HRESULT ExeEngineExecutePackage(
         hr = CacheGetCompletedPath(pCache, pPackage->fPerMachine, pPackage->sczCacheId, &sczCachedDirectory);
         ExitOnFailure(hr, "Failed to get cached path for package: %ls", pPackage->sczId);
 
-        hr = PathConcatRelativeToBase(sczCachedDirectory, pPackagePayload->sczFilePath, &sczExecutablePath);
+        hr = PathConcatRelativeToFullyQualifiedBase(sczCachedDirectory, pPackagePayload->sczFilePath, &sczExecutablePath);
         ExitOnFailure(hr, "Failed to build executable path.");
     }
 
diff --git a/src/burn/engine/msiengine.cpp b/src/burn/engine/msiengine.cpp
index bfd5710f..edbf4d45 100644
--- a/src/burn/engine/msiengine.cpp
+++ b/src/burn/engine/msiengine.cpp
@@ -1236,7 +1236,7 @@ extern "C" HRESULT MsiEngineExecutePackage(
         // Best effort to set the execute package cache folder variable.
         VariableSetString(pVariables, BURN_BUNDLE_EXECUTE_PACKAGE_CACHE_FOLDER, sczCachedDirectory, TRUE, FALSE);
 
-        hr = PathConcatRelativeToBase(sczCachedDirectory, pPackagePayload->sczFilePath, &sczMsiPath);
+        hr = PathConcatRelativeToFullyQualifiedBase(sczCachedDirectory, pPackagePayload->sczFilePath, &sczMsiPath);
         ExitOnFailure(hr, "Failed to build MSI path.");
     }
 
@@ -2195,7 +2195,7 @@ static HRESULT ConcatPatchProperty(
                 hr = CacheGetCompletedPath(pCache, pMspPackage->fPerMachine, pMspPackage->sczCacheId, &sczCachedDirectory);
                 ExitOnFailure(hr, "Failed to get cached path for MSP package: %ls", pMspPackage->sczId);
 
-                hr = PathConcatRelativeToBase(sczCachedDirectory, pMspPackagePayload->sczFilePath, &sczMspPath);
+                hr = PathConcatRelativeToFullyQualifiedBase(sczCachedDirectory, pMspPackagePayload->sczFilePath, &sczMspPath);
                 ExitOnFailure(hr, "Failed to build MSP path.");
 
                 if (!sczPatches)
diff --git a/src/burn/engine/mspengine.cpp b/src/burn/engine/mspengine.cpp
index 02e2afe4..81b7b651 100644
--- a/src/burn/engine/mspengine.cpp
+++ b/src/burn/engine/mspengine.cpp
@@ -613,7 +613,7 @@ extern "C" HRESULT MspEngineExecutePackage(
             // Best effort to set the execute package cache folder variable.
             VariableSetString(pVariables, BURN_BUNDLE_EXECUTE_PACKAGE_CACHE_FOLDER, sczCachedDirectory, TRUE, FALSE);
 
-            hr = PathConcatRelativeToBase(sczCachedDirectory, pMspPackagePayload->sczFilePath, &sczMspPath);
+            hr = PathConcatRelativeToFullyQualifiedBase(sczCachedDirectory, pMspPackagePayload->sczFilePath, &sczMspPath);
             ExitOnFailure(hr, "Failed to build MSP path.");
 
             wzAppend = sczMspPath;
diff --git a/src/burn/engine/msuengine.cpp b/src/burn/engine/msuengine.cpp
index d78943ac..1b051165 100644
--- a/src/burn/engine/msuengine.cpp
+++ b/src/burn/engine/msuengine.cpp
@@ -320,7 +320,7 @@ extern "C" HRESULT MsuEngineExecutePackage(
         // Best effort to set the execute package cache folder variable.
         VariableSetString(pVariables, BURN_BUNDLE_EXECUTE_PACKAGE_CACHE_FOLDER, sczCachedDirectory, TRUE, FALSE);
 
-        hr = PathConcatRelativeToBase(sczCachedDirectory, pPackagePayload->sczFilePath, &sczMsuPath);
+        hr = PathConcatRelativeToFullyQualifiedBase(sczCachedDirectory, pPackagePayload->sczFilePath, &sczMsuPath);
         ExitOnFailure(hr, "Failed to build MSU path.");
 
         // format command
diff --git a/src/burn/engine/payload.cpp b/src/burn/engine/payload.cpp
index f57f1310..a4450bf7 100644
--- a/src/burn/engine/payload.cpp
+++ b/src/burn/engine/payload.cpp
@@ -294,7 +294,7 @@ extern "C" HRESULT PayloadExtractUXContainer(
         ExitOnFailure(hr, "Failed to find embedded payload: %ls", sczStreamName);
 
         // make file path
-        hr = PathConcatRelativeToBase(wzTargetDir, pPayload->sczFilePath, &pPayload->sczLocalFilePath);
+        hr = PathConcatRelativeToFullyQualifiedBase(wzTargetDir, pPayload->sczFilePath, &pPayload->sczLocalFilePath);
         ExitOnFailure(hr, "Failed to concat file paths.");
 
         // extract file
diff --git a/src/burn/engine/plan.cpp b/src/burn/engine/plan.cpp
index 68c1d2ba..52bf6298 100644
--- a/src/burn/engine/plan.cpp
+++ b/src/burn/engine/plan.cpp
@@ -432,24 +432,28 @@ extern "C" HRESULT PlanLayoutBundle(
 {
     HRESULT hr = S_OK;
     BURN_CACHE_ACTION* pCacheAction = NULL;
+    LPWSTR sczLayoutDirectory = NULL;
     LPWSTR sczExecutablePath = NULL;
 
     // Get the layout directory.
-    hr = VariableGetString(pVariables, BURN_BUNDLE_LAYOUT_DIRECTORY, &pPlan->sczLayoutDirectory);
+    hr = VariableGetString(pVariables, BURN_BUNDLE_LAYOUT_DIRECTORY, &sczLayoutDirectory);
     if (E_NOTFOUND == hr) // if not set, use the current directory as the layout directory.
     {
-        hr = VariableGetString(pVariables, BURN_BUNDLE_SOURCE_PROCESS_FOLDER, &pPlan->sczLayoutDirectory);
+        hr = VariableGetString(pVariables, BURN_BUNDLE_SOURCE_PROCESS_FOLDER, &sczLayoutDirectory);
         if (E_NOTFOUND == hr) // if not set, use the current directory as the layout directory.
         {
             hr = PathForCurrentProcess(&sczExecutablePath, NULL);
             ExitOnFailure(hr, "Failed to get path for current executing process as layout directory.");
 
-            hr = PathGetDirectory(sczExecutablePath, &pPlan->sczLayoutDirectory);
+            hr = PathGetDirectory(sczExecutablePath, &sczLayoutDirectory);
             ExitOnFailure(hr, "Failed to get executing process as layout directory.");
         }
     }
     ExitOnFailure(hr, "Failed to get bundle layout directory property.");
 
+    hr = PathGetFullPathName(sczLayoutDirectory, &pPlan->sczLayoutDirectory, NULL, NULL);
+    ExitOnFailure(hr, "Failed to ensure layout directory is fully qualified.");
+
     hr = PathBackslashTerminate(&pPlan->sczLayoutDirectory);
     ExitOnFailure(hr, "Failed to ensure layout directory is backslash terminated.");
 
@@ -478,6 +482,7 @@ extern "C" HRESULT PlanLayoutBundle(
 
 LExit:
     ReleaseStr(sczExecutablePath);
+    ReleaseStr(sczLayoutDirectory);
 
     return hr;
 }
diff --git a/src/burn/engine/registration.cpp b/src/burn/engine/registration.cpp
index 2b3dc4d4..d0e5c677 100644
--- a/src/burn/engine/registration.cpp
+++ b/src/burn/engine/registration.cpp
@@ -1149,7 +1149,7 @@ static HRESULT SetPaths(
     ExitOnFailure(hr, "Failed to build cache directory.");
 
     // build cached executable path
-    hr = PathConcatRelativeToBase(sczCacheDirectory, pRegistration->sczExecutableName, &pRegistration->sczCacheExecutablePath);
+    hr = PathConcatRelativeToFullyQualifiedBase(sczCacheDirectory, pRegistration->sczExecutableName, &pRegistration->sczCacheExecutablePath);
     ExitOnFailure(hr, "Failed to build cached executable path.");
 
     // build state file path
@@ -1367,7 +1367,7 @@ static HRESULT WriteSoftwareTags(
         hr = PathConcat(sczRootFolder, SWIDTAG_FOLDER, &sczTagFolder);
         ExitOnFailure(hr, "Failed to allocate regid folder path.");
 
-        hr = PathConcatRelativeToBase(sczTagFolder, pSoftwareTag->sczFilename, &sczPath);
+        hr = PathConcatRelativeToFullyQualifiedBase(sczTagFolder, pSoftwareTag->sczFilename, &sczPath);
         ExitOnFailure(hr, "Failed to allocate regid file path.");
 
         hr = DirEnsureExists(sczTagFolder, NULL);
@@ -1405,7 +1405,7 @@ static HRESULT RemoveSoftwareTags(
         hr = PathConcat(sczRootFolder, SWIDTAG_FOLDER, &sczTagFolder);
         ExitOnFailure(hr, "Failed to allocate regid folder path.");
 
-        hr = PathConcatRelativeToBase(sczTagFolder, pSoftwareTag->sczFilename, &sczPath);
+        hr = PathConcatRelativeToFullyQualifiedBase(sczTagFolder, pSoftwareTag->sczFilename, &sczPath);
         ExitOnFailure(hr, "Failed to allocate regid file path.");
 
         // Best effort to delete the software tag file and the regid folder.
diff --git a/src/libs/dutil/WixToolset.DUtil/inc/pathutil.h b/src/libs/dutil/WixToolset.DUtil/inc/pathutil.h
index f36e6ebc..971ef887 100644
--- a/src/libs/dutil/WixToolset.DUtil/inc/pathutil.h
+++ b/src/libs/dutil/WixToolset.DUtil/inc/pathutil.h
@@ -299,6 +299,16 @@ DAPI_(HRESULT) PathConcatRelativeToBase(
     );
 
 /*******************************************************************
+ PathConcatRelativeToFullyQualifiedBase - ensures the base path is
+    fully qualified and then calls PathConcatRelativeToBase.
+*******************************************************************/
+DAPI_(HRESULT) PathConcatRelativeToFullyQualifiedBase(
+    __in LPCWSTR wzBase,
+    __in_opt LPCWSTR wzRelative,
+    __deref_out_z LPWSTR* psczCombined
+    );
+
+/*******************************************************************
  PathCompareCanonicalized - canonicalizes the two paths using PathCanonicalizeForComparison
     which does not resolve relative paths into fully qualified paths.
     The strings are then compared using ::CompareStringW().
diff --git a/src/libs/dutil/WixToolset.DUtil/path2utl.cpp b/src/libs/dutil/WixToolset.DUtil/path2utl.cpp
index c6ff608c..3c4b2f88 100644
--- a/src/libs/dutil/WixToolset.DUtil/path2utl.cpp
+++ b/src/libs/dutil/WixToolset.DUtil/path2utl.cpp
@@ -162,6 +162,25 @@ LExit:
     return hr;
 }
 
+DAPI_(HRESULT) PathConcatRelativeToFullyQualifiedBase(
+    __in LPCWSTR wzBase,
+    __in_opt LPCWSTR wzRelative,
+    __deref_out_z LPWSTR* psczCombined
+    )
+{
+    HRESULT hr = S_OK;
+
+    if (!PathIsFullyQualified(wzBase))
+    {
+        PathExitWithRootFailure(hr, E_INVALIDARG, "wzBase must be fully qualified: %ls.", wzBase);
+    }
+
+    hr = PathConcatRelativeToBase(wzBase, wzRelative, psczCombined);
+
+LExit:
+    return hr;
+}
+
 DAPI_(HRESULT) PathCompareCanonicalized(
     __in_z LPCWSTR wzPath1,
     __in_z LPCWSTR wzPath2,