diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 10a33c04238..005fd3c3221 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -15,6 +15,7 @@
 #include "nix/expr/print.hh"
 #include "nix/fetchers/filtering-source-accessor.hh"
 #include "nix/util/memory-source-accessor.hh"
+#include "nix/util/mounted-source-accessor.hh"
 #include "nix/expr/gc-small-vector.hh"
 #include "nix/util/url.hh"
 #include "nix/fetchers/fetch-to-store.hh"
@@ -287,7 +288,7 @@ EvalState::EvalState(
             auto realStoreDir = dirOf(store->toRealPath(StorePath::dummy));
             if (settings.pureEval || store->storeDir != realStoreDir) {
                 accessor = settings.pureEval
-                    ? storeFS
+                    ? storeFS.cast<SourceAccessor>()
                     : makeUnionSourceAccessor({accessor, storeFS});
             }
 
@@ -3090,6 +3091,11 @@ SourcePath EvalState::findFile(const LookupPath & lookupPath, const std::string_
 
         auto res = (r / CanonPath(suffix)).resolveSymlinks();
         if (res.pathExists()) return res;
+
+        // Backward compatibility hack: throw an exception if access
+        // to this path is not allowed.
+        if (auto accessor = res.accessor.dynamic_pointer_cast<FilteringSourceAccessor>())
+            accessor->checkAccess(res.path);
     }
 
     if (hasPrefix(path, "nix/"))
@@ -3160,6 +3166,11 @@ std::optional<SourcePath> EvalState::resolveLookupPathPath(const LookupPath::Pat
         if (path.resolveSymlinks().pathExists())
             return finish(std::move(path));
         else {
+            // Backward compatibility hack: throw an exception if access
+            // to this path is not allowed.
+            if (auto accessor = path.accessor.dynamic_pointer_cast<FilteringSourceAccessor>())
+                accessor->checkAccess(path.path);
+
             logWarning({
                 .msg = HintFmt("Nix search path entry '%1%' does not exist, ignoring", value)
             });
diff --git a/src/libexpr/include/nix/expr/eval.hh b/src/libexpr/include/nix/expr/eval.hh
index 6a6959bd821..5c3b2725c04 100644
--- a/src/libexpr/include/nix/expr/eval.hh
+++ b/src/libexpr/include/nix/expr/eval.hh
@@ -43,6 +43,7 @@ class StorePath;
 struct SingleDerivedPath;
 enum RepairFlag : bool;
 struct MemorySourceAccessor;
+struct MountedSourceAccessor;
 namespace eval_cache {
     class EvalCache;
 }
@@ -271,7 +272,7 @@ public:
     /**
      * The accessor corresponding to `store`.
      */
-    const ref<SourceAccessor> storeFS;
+    const ref<MountedSourceAccessor> storeFS;
 
     /**
      * The accessor for the root filesystem.
diff --git a/src/libexpr/primops/fetchMercurial.cc b/src/libexpr/primops/fetchMercurial.cc
index 189bd1f73d7..843beb4d195 100644
--- a/src/libexpr/primops/fetchMercurial.cc
+++ b/src/libexpr/primops/fetchMercurial.cc
@@ -64,7 +64,7 @@ static void prim_fetchMercurial(EvalState & state, const PosIdx pos, Value * * a
     if (rev) attrs.insert_or_assign("rev", rev->gitRev());
     auto input = fetchers::Input::fromAttrs(state.fetchSettings, std::move(attrs));
 
-    auto [storePath, input2] = input.fetchToStore(state.store);
+    auto [storePath, accessor, input2] = input.fetchToStore(state.store);
 
     auto attrs2 = state.buildBindings(8);
     state.mkStorePathString(storePath, attrs2.alloc(state.sOutPath));
diff --git a/src/libexpr/primops/fetchTree.cc b/src/libexpr/primops/fetchTree.cc
index 0be9f4bdc7d..4d7de1f76f2 100644
--- a/src/libexpr/primops/fetchTree.cc
+++ b/src/libexpr/primops/fetchTree.cc
@@ -10,6 +10,7 @@
 #include "nix/util/url.hh"
 #include "nix/expr/value-to-json.hh"
 #include "nix/fetchers/fetch-to-store.hh"
+#include "nix/util/mounted-source-accessor.hh"
 
 #include <nlohmann/json.hpp>
 
@@ -204,10 +205,12 @@ static void fetchTree(
             throw Error("input '%s' is not allowed to use the '__final' attribute", input.to_string());
     }
 
-    auto [storePath, input2] = input.fetchToStore(state.store);
+    auto [storePath, accessor, input2] = input.fetchToStore(state.store);
 
     state.allowPath(storePath);
 
+    state.storeFS->mount(CanonPath(state.store->printStorePath(storePath)), accessor);
+
     emitTreeAttrs(state, storePath, input2, v, params.emptyRevFallback, false);
 }
 
diff --git a/src/libfetchers/fetchers.cc b/src/libfetchers/fetchers.cc
index 3ae45dcf821..9693f1773b0 100644
--- a/src/libfetchers/fetchers.cc
+++ b/src/libfetchers/fetchers.cc
@@ -187,34 +187,30 @@ bool Input::contains(const Input & other) const
 }
 
 // FIXME: remove
-std::pair<StorePath, Input> Input::fetchToStore(ref<Store> store) const
+std::tuple<StorePath, ref<SourceAccessor>, Input> Input::fetchToStore(ref<Store> store) const
 {
     if (!scheme)
         throw Error("cannot fetch unsupported input '%s'", attrsToJSON(toAttrs()));
 
-    auto [storePath, input] = [&]() -> std::pair<StorePath, Input> {
-        try {
-            auto [accessor, result] = getAccessorUnchecked(store);
-
-            auto storePath = nix::fetchToStore(*store, SourcePath(accessor), FetchMode::Copy, result.getName());
+    try {
+        auto [accessor, result] = getAccessorUnchecked(store);
 
-            auto narHash = store->queryPathInfo(storePath)->narHash;
-            result.attrs.insert_or_assign("narHash", narHash.to_string(HashFormat::SRI, true));
+        auto storePath = nix::fetchToStore(*store, SourcePath(accessor), FetchMode::Copy, result.getName());
 
-            result.attrs.insert_or_assign("__final", Explicit<bool>(true));
+        auto narHash = store->queryPathInfo(storePath)->narHash;
+        result.attrs.insert_or_assign("narHash", narHash.to_string(HashFormat::SRI, true));
 
-            assert(result.isFinal());
+        result.attrs.insert_or_assign("__final", Explicit<bool>(true));
 
-            checkLocks(*this, result);
+        assert(result.isFinal());
 
-            return {storePath, result};
-        } catch (Error & e) {
-            e.addTrace({}, "while fetching the input '%s'", to_string());
-            throw;
-        }
-    }();
+        checkLocks(*this, result);
 
-    return {std::move(storePath), input};
+        return {std::move(storePath), accessor, result};
+    } catch (Error & e) {
+        e.addTrace({}, "while fetching the input '%s'", to_string());
+        throw;
+    }
 }
 
 void Input::checkLocks(Input specified, Input & result)
diff --git a/src/libfetchers/filtering-source-accessor.cc b/src/libfetchers/filtering-source-accessor.cc
index 72a3fb4ebad..97f230c7ea4 100644
--- a/src/libfetchers/filtering-source-accessor.cc
+++ b/src/libfetchers/filtering-source-accessor.cc
@@ -20,9 +20,14 @@ bool FilteringSourceAccessor::pathExists(const CanonPath & path)
 }
 
 std::optional<SourceAccessor::Stat> FilteringSourceAccessor::maybeLstat(const CanonPath & path)
+{
+    return isAllowed(path) ? next->maybeLstat(prefix / path) : std::nullopt;
+}
+
+SourceAccessor::Stat FilteringSourceAccessor::lstat(const CanonPath & path)
 {
     checkAccess(path);
-    return next->maybeLstat(prefix / path);
+    return next->lstat(prefix / path);
 }
 
 SourceAccessor::DirEntries FilteringSourceAccessor::readDirectory(const CanonPath & path)
diff --git a/src/libfetchers/git.cc b/src/libfetchers/git.cc
index 4a916929eda..8bd5a35d84d 100644
--- a/src/libfetchers/git.cc
+++ b/src/libfetchers/git.cc
@@ -15,6 +15,7 @@
 #include "nix/fetchers/fetch-settings.hh"
 #include "nix/util/json-utils.hh"
 #include "nix/util/archive.hh"
+#include "nix/util/mounted-source-accessor.hh"
 
 #include <regex>
 #include <string.h>
diff --git a/src/libfetchers/include/nix/fetchers/fetchers.hh b/src/libfetchers/include/nix/fetchers/fetchers.hh
index 3288ecc5ea5..c2ae647af0a 100644
--- a/src/libfetchers/include/nix/fetchers/fetchers.hh
+++ b/src/libfetchers/include/nix/fetchers/fetchers.hh
@@ -121,7 +121,7 @@ public:
      * Fetch the entire input into the Nix store, returning the
      * location in the Nix store and the locked input.
      */
-    std::pair<StorePath, Input> fetchToStore(ref<Store> store) const;
+    std::tuple<StorePath, ref<SourceAccessor>, Input> fetchToStore(ref<Store> store) const;
 
     /**
      * Check the locking attributes in `result` against
diff --git a/src/libfetchers/include/nix/fetchers/filtering-source-accessor.hh b/src/libfetchers/include/nix/fetchers/filtering-source-accessor.hh
index 2b59f03ca22..1a90fe9ef10 100644
--- a/src/libfetchers/include/nix/fetchers/filtering-source-accessor.hh
+++ b/src/libfetchers/include/nix/fetchers/filtering-source-accessor.hh
@@ -38,6 +38,8 @@ struct FilteringSourceAccessor : SourceAccessor
 
     bool pathExists(const CanonPath & path) override;
 
+    Stat lstat(const CanonPath & path) override;
+
     std::optional<Stat> maybeLstat(const CanonPath & path) override;
 
     DirEntries readDirectory(const CanonPath & path) override;
diff --git a/src/libflake/flake/flake.cc b/src/libflake/flake/flake.cc
index afeefdaec97..a0f62bf89fe 100644
--- a/src/libflake/flake/flake.cc
+++ b/src/libflake/flake/flake.cc
@@ -14,6 +14,7 @@
 #include "nix/store/local-fs-store.hh"
 #include "nix/fetchers/fetch-to-store.hh"
 #include "nix/util/memory-source-accessor.hh"
+#include "nix/util/mounted-source-accessor.hh"
 #include "nix/fetchers/input-cache.hh"
 
 #include <nlohmann/json.hpp>
@@ -32,7 +33,9 @@ static StorePath copyInputToStore(
 {
     auto storePath = fetchToStore(*state.store, accessor, FetchMode::Copy, input.getName());
 
-    state.allowPath(storePath);
+    state.allowPath(storePath); // FIXME: should just whitelist the entire virtual store
+
+    state.storeFS->mount(CanonPath(state.store->printStorePath(storePath)), accessor);
 
     auto narHash = state.store->queryPathInfo(storePath)->narHash;
     input.attrs.insert_or_assign("narHash", narHash.to_string(HashFormat::SRI, true));
diff --git a/src/libutil/include/nix/util/meson.build b/src/libutil/include/nix/util/meson.build
index e30b8dacd48..329d4061218 100644
--- a/src/libutil/include/nix/util/meson.build
+++ b/src/libutil/include/nix/util/meson.build
@@ -43,6 +43,7 @@ headers = files(
   'logging.hh',
   'lru-cache.hh',
   'memory-source-accessor.hh',
+  'mounted-source-accessor.hh',
   'muxable-pipe.hh',
   'os-string.hh',
   'pool.hh',
diff --git a/src/libutil/include/nix/util/mounted-source-accessor.hh b/src/libutil/include/nix/util/mounted-source-accessor.hh
new file mode 100644
index 00000000000..4e75edfafff
--- /dev/null
+++ b/src/libutil/include/nix/util/mounted-source-accessor.hh
@@ -0,0 +1,14 @@
+#pragma once
+
+#include "source-accessor.hh"
+
+namespace nix {
+
+struct MountedSourceAccessor : SourceAccessor
+{
+    virtual void mount(CanonPath mountPoint, ref<SourceAccessor> accessor) = 0;
+};
+
+ref<MountedSourceAccessor> makeMountedSourceAccessor(std::map<CanonPath, ref<SourceAccessor>> mounts);
+
+}
diff --git a/src/libutil/include/nix/util/source-accessor.hh b/src/libutil/include/nix/util/source-accessor.hh
index 5ef66015015..f5ec0464644 100644
--- a/src/libutil/include/nix/util/source-accessor.hh
+++ b/src/libutil/include/nix/util/source-accessor.hh
@@ -118,7 +118,7 @@ struct SourceAccessor : std::enable_shared_from_this<SourceAccessor>
         std::string typeString();
     };
 
-    Stat lstat(const CanonPath & path);
+    virtual Stat lstat(const CanonPath & path);
 
     virtual std::optional<Stat> maybeLstat(const CanonPath & path) = 0;
 
@@ -214,8 +214,6 @@ ref<SourceAccessor> getFSSourceAccessor();
  */
 ref<SourceAccessor> makeFSSourceAccessor(std::filesystem::path root);
 
-ref<SourceAccessor> makeMountedSourceAccessor(std::map<CanonPath, ref<SourceAccessor>> mounts);
-
 /**
  * Construct an accessor that presents a "union" view of a vector of
  * underlying accessors. Earlier accessors take precedence over later.
diff --git a/src/libutil/mounted-source-accessor.cc b/src/libutil/mounted-source-accessor.cc
index b7de2afbf03..89063b10f1f 100644
--- a/src/libutil/mounted-source-accessor.cc
+++ b/src/libutil/mounted-source-accessor.cc
@@ -1,12 +1,12 @@
-#include "nix/util/source-accessor.hh"
+#include "nix/util/mounted-source-accessor.hh"
 
 namespace nix {
 
-struct MountedSourceAccessor : SourceAccessor
+struct MountedSourceAccessorImpl : MountedSourceAccessor
 {
     std::map<CanonPath, ref<SourceAccessor>> mounts;
 
-    MountedSourceAccessor(std::map<CanonPath, ref<SourceAccessor>> _mounts)
+    MountedSourceAccessorImpl(std::map<CanonPath, ref<SourceAccessor>> _mounts)
         : mounts(std::move(_mounts))
     {
         displayPrefix.clear();
@@ -23,6 +23,12 @@ struct MountedSourceAccessor : SourceAccessor
         return accessor->readFile(subpath);
     }
 
+    Stat lstat(const CanonPath & path) override
+    {
+        auto [accessor, subpath] = resolve(path);
+        return accessor->lstat(subpath);
+    }
+
     std::optional<Stat> maybeLstat(const CanonPath & path) override
     {
         auto [accessor, subpath] = resolve(path);
@@ -69,11 +75,17 @@ struct MountedSourceAccessor : SourceAccessor
         auto [accessor, subpath] = resolve(path);
         return accessor->getPhysicalPath(subpath);
     }
+
+    void mount(CanonPath mountPoint, ref<SourceAccessor> accessor) override
+    {
+        // FIXME: thread-safety
+        mounts.insert_or_assign(std::move(mountPoint), accessor);
+    }
 };
 
-ref<SourceAccessor> makeMountedSourceAccessor(std::map<CanonPath, ref<SourceAccessor>> mounts)
+ref<MountedSourceAccessor> makeMountedSourceAccessor(std::map<CanonPath, ref<SourceAccessor>> mounts)
 {
-    return make_ref<MountedSourceAccessor>(std::move(mounts));
+    return make_ref<MountedSourceAccessorImpl>(std::move(mounts));
 }
 
 }
diff --git a/src/nix/env.cc b/src/nix/env.cc
index 277bd0fdd45..a0b0e976b73 100644
--- a/src/nix/env.cc
+++ b/src/nix/env.cc
@@ -6,6 +6,7 @@
 #include "run.hh"
 #include "nix/util/strings.hh"
 #include "nix/util/executable-path.hh"
+#include "nix/util/mounted-source-accessor.hh"
 
 using namespace nix;
 
diff --git a/src/nix/flake.cc b/src/nix/flake.cc
index 3d174dc53e2..56014099b92 100644
--- a/src/nix/flake.cc
+++ b/src/nix/flake.cc
@@ -1095,7 +1095,7 @@ struct CmdFlakeArchive : FlakeCommand, MixJSON, MixDryRun
                         storePath =
                             dryRun
                             ? (*inputNode)->lockedRef.input.computeStorePath(*store)
-                            : (*inputNode)->lockedRef.input.fetchToStore(store).first;
+                            : std::get<0>((*inputNode)->lockedRef.input.fetchToStore(store));
                         sources.insert(*storePath);
                     }
                     if (json) {
diff --git a/tests/functional/flakes/source-paths.sh b/tests/functional/flakes/source-paths.sh
index 4709bf2fcec..3aa3683c27c 100644
--- a/tests/functional/flakes/source-paths.sh
+++ b/tests/functional/flakes/source-paths.sh
@@ -12,6 +12,10 @@ cat > "$repo/flake.nix" <<EOF
 {
   outputs = { ... }: {
     x = 1;
+    y = assert false; 1;
+    z = builtins.readFile ./foo;
+    a = import ./foo;
+    b = import ./dir;
   };
 }
 EOF
@@ -21,3 +25,33 @@ expectStderr 1 nix eval "$repo#x" | grepQuiet "error: Path 'flake.nix' in the re
 git -C "$repo" add flake.nix
 
 [[ $(nix eval "$repo#x") = 1 ]]
+
+expectStderr 1 nix eval "$repo#y" | grepQuiet "at $repo/flake.nix:"
+
+git -C "$repo" commit -a -m foo
+
+expectStderr 1 nix eval "git+file://$repo?ref=master#y" | grepQuiet "at «git+file://$repo?ref=master&rev=.*»/flake.nix:"
+
+expectStderr 1 nix eval "$repo#z" | grepQuiet "error: Path 'foo' does not exist in Git repository \"$repo\"."
+expectStderr 1 nix eval "git+file://$repo?ref=master#z" | grepQuiet "error: '«git+file://$repo?ref=master&rev=.*»/foo' does not exist"
+expectStderr 1 nix eval "$repo#a" | grepQuiet "error: Path 'foo' does not exist in Git repository \"$repo\"."
+
+echo 123 > "$repo/foo"
+
+expectStderr 1 nix eval "$repo#z" | grepQuiet "error: Path 'foo' in the repository \"$repo\" is not tracked by Git."
+expectStderr 1 nix eval "$repo#a" | grepQuiet "error: Path 'foo' in the repository \"$repo\" is not tracked by Git."
+
+git -C "$repo" add "$repo/foo"
+
+[[ $(nix eval --raw "$repo#z") = 123 ]]
+
+expectStderr 1 nix eval "$repo#b" | grepQuiet "error: Path 'dir' does not exist in Git repository \"$repo\"."
+
+mkdir -p "$repo/dir"
+echo 456 > "$repo/dir/default.nix"
+
+expectStderr 1 nix eval "$repo#b" | grepQuiet "error: Path 'dir' in the repository \"$repo\" is not tracked by Git."
+
+git -C "$repo" add "$repo/dir/default.nix"
+
+[[ $(nix eval "$repo#b") = 456 ]]