diff --git a/compositor/main.c b/compositor/main.c
index 6746e3a2..5fb5d5a8 100644
--- a/compositor/main.c
+++ b/compositor/main.c
@@ -1699,8 +1699,9 @@ int main(int argc, char *argv[])
 	server_socket = getenv("WAYLAND_SERVER_SOCKET");
 	if (server_socket) {
 		weston_log("Running with single client\n");
+		errno = 0;
 		fd = strtol(server_socket, &end, 10);
-		if (*end != '\0')
+		if (errno != 0 || end == server_socket || *end != '\0')
 			fd = -1;
 	} else {
 		fd = -1;
diff --git a/compositor/systemd-notify.c b/compositor/systemd-notify.c
index 9fbd5ee2..6104124f 100644
--- a/compositor/systemd-notify.c
+++ b/compositor/systemd-notify.c
@@ -146,7 +146,7 @@ module_init(struct weston_compositor *compositor,
 
 	errno = 0;
 	watchdog_time_conv = strtol(watchdog_time_env, &tail, 10);
-	if ((errno != 0) || (*tail != '\0'))
+	if (errno != 0 || tail == watchdog_time_env || *tail != '\0')
 		return 0;
 
 	/* Convert 'WATCHDOG_USEC' to milliseconds and notify
diff --git a/libweston/compositor.c b/libweston/compositor.c
index b045381d..e9c2a837 100644
--- a/libweston/compositor.c
+++ b/libweston/compositor.c
@@ -4628,8 +4628,9 @@ weston_environment_get_fd(const char *env)
 	e = getenv(env);
 	if (!e)
 		return -1;
+	errno = 0;
 	fd = strtol(e, &end, 10);
-	if (*end != '\0')
+	if (errno != 0 || end == e || *end != '\0')
 		return -1;
 
 	flags = fcntl(fd, F_GETFD);
diff --git a/libweston/libbacklight.c b/libweston/libbacklight.c
index 4039575c..59f4e440 100644
--- a/libweston/libbacklight.c
+++ b/libweston/libbacklight.c
@@ -48,6 +48,7 @@ static long backlight_get(struct backlight *backlight, char *node)
 {
 	char buffer[100];
 	char *path;
+	char *end;
 	int fd;
 	long value, ret;
 
@@ -65,8 +66,15 @@ static long backlight_get(struct backlight *backlight, char *node)
 		goto out;
 	}
 
-	value = strtol(buffer, NULL, 10);
+	errno = 0;
+	value = strtol(buffer, &end, 10);
+	if (errno != 0 || end == buffer || *end != '\0') {
+		ret = -1;
+		goto out;
+	}
+
 	ret = value;
+
 out:
 	if (fd >= 0)
 		close(fd);