diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index ab8923bbd..36436d7eb 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -1914,6 +1914,8 @@ void DerivationGoal::startBuilder()
     builderOut.create();
 
     /* Fork a child to build the package. */
+    ProcessOptions options;
+    options.allowVfork = !buildUser.enabled();
     pid = startProcess([&]() {
         runChild();
     });
diff --git a/src/libutil/util.cc b/src/libutil/util.cc
index bdd114c5e..5895e7200 100644
--- a/src/libutil/util.cc
+++ b/src/libutil/util.cc
@@ -825,6 +825,9 @@ void killUser(uid_t uid)
        users to which the current process can send signals.  So we
        fork a process, switch to uid, and send a mass kill. */
 
+    ProcessOptions options;
+    options.allowVfork = false;
+
     Pid pid = startProcess([&]() {
 
         if (setuid(uid) == -1)