diff --git a/src/libfetchers/filtering-source-accessor.cc b/src/libfetchers/filtering-source-accessor.cc
index ceb6457d473d..c1000af3a4c8 100644
--- a/src/libfetchers/filtering-source-accessor.cc
+++ b/src/libfetchers/filtering-source-accessor.cc
@@ -1,6 +1,8 @@
 #include "nix/fetchers/filtering-source-accessor.hh"
 #include "nix/util/sync.hh"
+#include "nix/util/util.hh"
 
+#include <boost/unordered/concurrent_flat_map.hpp>
 #include <boost/unordered/concurrent_flat_set.hpp>
 
 namespace nix {
@@ -108,13 +110,19 @@ ref<AllowListSourceAccessor> AllowListSourceAccessor::create(
     return make_ref<AllowListSourceAccessorImpl>(next, allowedPrefixes, allowedPaths, std::move(makeNotAllowedError));
 }
 
+CachingFilteringSourceAccessor::CachingFilteringSourceAccessor(
+    const SourcePath & src, MakeNotAllowedError && makeNotAllowedError)
+    : FilteringSourceAccessor(src, std::move(makeNotAllowedError))
+    , cache(make_ref<boost::concurrent_flat_map<CanonPath, bool>>())
+{
+}
+
 bool CachingFilteringSourceAccessor::isAllowed(const CanonPath & path)
 {
-    auto i = cache.find(path);
-    if (i != cache.end())
-        return i->second;
+    if (auto allowed = getConcurrent(*cache, path))
+        return *allowed;
     auto res = isAllowedUncached(path);
-    cache.emplace(path, res);
+    cache->emplace(path, res);
     return res;
 }
 
diff --git a/src/libfetchers/include/nix/fetchers/filtering-source-accessor.hh b/src/libfetchers/include/nix/fetchers/filtering-source-accessor.hh
index c5c1ce282b30..5ea218d5b655 100644
--- a/src/libfetchers/include/nix/fetchers/filtering-source-accessor.hh
+++ b/src/libfetchers/include/nix/fetchers/filtering-source-accessor.hh
@@ -5,6 +5,8 @@
 #include <set>
 #include <unordered_set>
 
+#include <boost/unordered/concurrent_flat_map_fwd.hpp>
+
 namespace nix {
 
 /**
@@ -95,9 +97,9 @@ struct AllowListSourceAccessor : public FilteringSourceAccessor
  */
 struct CachingFilteringSourceAccessor : FilteringSourceAccessor
 {
-    std::map<CanonPath, bool> cache;
+    const ref<boost::concurrent_flat_map<CanonPath, bool>> cache;
 
-    using FilteringSourceAccessor::FilteringSourceAccessor;
+    CachingFilteringSourceAccessor(const SourcePath & src, MakeNotAllowedError && makeNotAllowedError);
 
     bool isAllowed(const CanonPath & path) override;