diff --git a/lib/libcasper/libcasper/libcasper_impl.c b/lib/libcasper/libcasper/libcasper_impl.c
index e4411630c8a..ae28c8769a0 100644
--- a/lib/libcasper/libcasper/libcasper_impl.c
+++ b/lib/libcasper/libcasper/libcasper_impl.c
@@ -32,8 +32,10 @@
  * $FreeBSD$
  */
 
+#include <err.h>
 #include <errno.h>
 #include <fcntl.h>
+#include <paths.h>
 #include <stdlib.h>
 
 #include "libcasper_impl.h"
@@ -44,3 +46,28 @@ fd_is_valid(int fd)
 
 	return (fcntl(fd, F_GETFL) != -1 || errno != EBADF);
 }
+
+void
+fd_fix_environment(int *fdp)
+{
+	int nullfd, nfd;
+
+	if (*fdp > STDERR_FILENO)
+		return;
+
+	nullfd = open(_PATH_DEVNULL, O_RDWR);
+	if (nullfd == -1)
+		errx(1, "Unable to open %s", _PATH_DEVNULL);
+
+	while (*fdp <= STDERR_FILENO) {
+		nfd = dup(*fdp);
+		if (nfd == -1)
+			errx(1, "Unable to secure fd");
+		if (dup2(nullfd, *fdp) == -1)
+			errx(1, "Unable to secure fd");
+		*fdp = nfd;
+	}
+
+	close(nullfd);
+}
+
diff --git a/lib/libcasper/libcasper/libcasper_impl.h b/lib/libcasper/libcasper/libcasper_impl.h
index 11e43f08397..24049a0c07c 100644
--- a/lib/libcasper/libcasper/libcasper_impl.h
+++ b/lib/libcasper/libcasper/libcasper_impl.h
@@ -44,6 +44,7 @@ struct service;
 struct service_connection;
 
 bool fd_is_valid(int fd);
+void fd_fix_environment(int *fdp);
 
 /* Private service functions. */
 struct service	*service_alloc(const char *name,
diff --git a/lib/libcasper/libcasper/service.c b/lib/libcasper/libcasper/service.c
index 5c1c64d9a9d..e87d0640347 100644
--- a/lib/libcasper/libcasper/service.c
+++ b/lib/libcasper/libcasper/service.c
@@ -386,24 +386,27 @@ stdnull(void)
 }
 
 static void
-service_clean(int sock, int procfd, uint64_t flags)
+service_clean(int *sockp, int *procfdp, uint64_t flags)
 {
 	int fd, maxfd, minfd;
 
-	assert(sock > STDERR_FILENO);
-	assert(procfd > STDERR_FILENO);
-	assert(sock != procfd);
+	fd_fix_environment(sockp);
+	fd_fix_environment(procfdp);
+
+	assert(*sockp > STDERR_FILENO);
+	assert(*procfdp > STDERR_FILENO);
+	assert(*sockp != *procfdp);
 
 	if ((flags & CASPER_SERVICE_STDIO) == 0)
 		stdnull();
 
 	if ((flags & CASPER_SERVICE_FD) == 0) {
-		if (procfd > sock) {
-			maxfd = procfd;
-			minfd = sock;
+		if (*procfdp > *sockp) {
+			maxfd = *procfdp;
+			minfd = *sockp;
 		} else {
-			maxfd = sock;
-			minfd = procfd;
+			maxfd = *sockp;
+			minfd = *procfdp;
 		}
 
 		for (fd = STDERR_FILENO + 1; fd < maxfd; fd++) {
@@ -424,7 +427,7 @@ service_start(struct service *service, int sock, int procfd)
 	assert(service != NULL);
 	assert(service->s_magic == SERVICE_MAGIC);
 	setproctitle("%s", service->s_name);
-	service_clean(sock, procfd, service->s_flags);
+	service_clean(&sock, &procfd, service->s_flags);
 
 	if (service_connection_add(service, sock, NULL) == NULL)
 		_exit(1);
diff --git a/lib/libcasper/libcasper/zygote.c b/lib/libcasper/libcasper/zygote.c
index 2b84bb49a69..5cdd139cc13 100644
--- a/lib/libcasper/libcasper/zygote.c
+++ b/lib/libcasper/libcasper/zygote.c
@@ -52,6 +52,7 @@ __FBSDID("$FreeBSD$");
 #include <strings.h>
 #include <unistd.h>
 
+#include "libcasper_impl.h"
 #include "zygote.h"
 
 /* Zygote info. */
@@ -104,7 +105,7 @@ zygote_clone_service_execute(int *chanfdp, int *procfdp)
  * between sandbox and its owner.
  */
 static void
-zygote_main(int sock)
+zygote_main(int *sockp)
 {
 	int error, procfd;
 	int chanfd[2];
@@ -113,12 +114,14 @@ zygote_main(int sock)
 	zygote_func_t *func;
 	pid_t pid;
 
-	assert(sock > STDERR_FILENO);
+	fd_fix_environment(sockp);
+
+	assert(*sockp > STDERR_FILENO);
 
 	setproctitle("zygote");
 
 	for (;;) {
-		nvlin = nvlist_recv(sock, 0);
+		nvlin = nvlist_recv(*sockp, 0);
 		if (nvlin == NULL) {
 			if (errno == ENOTCONN) {
 				/* Casper exited. */
@@ -157,7 +160,7 @@ zygote_main(int sock)
 			break;
 		case 0:
 			/* Child. */
-			close(sock);
+			close(*sockp);
 			close(chanfd[0]);
 			func(chanfd[1]);
 			/* NOTREACHED */
@@ -179,7 +182,7 @@ send:
 			nvlist_move_descriptor(nvlout, "chanfd", chanfd[0]);
 			nvlist_move_descriptor(nvlout, "procfd", procfd);
 		}
-		(void)nvlist_send(sock, nvlout);
+		(void)nvlist_send(*sockp, nvlout);
 		nvlist_destroy(nvlout);
 	}
 	/* NOTREACHED */
@@ -206,7 +209,7 @@ zygote_init(void)
 	case 0:
 		/* Child. */
 		close(sp[0]);
-		zygote_main(sp[1]);
+		zygote_main(&sp[1]);
 		/* NOTREACHED */
 		abort();
 	default: