diff --git a/libcontainer/container_linux.go b/libcontainer/container_linux.go
index 5fdafdbca..b3075fd34 100644
--- a/libcontainer/container_linux.go
+++ b/libcontainer/container_linux.go
@@ -332,6 +332,15 @@ func (c *Container) start(process *Process) (retErr error) {
 		}()
 	}
 
+	// Before starting "runc init", mark all non-stdio open files as O_CLOEXEC
+	// to make sure we don't leak any files into "runc init". Any files to be
+	// passed to "runc init" through ExtraFiles will get dup2'd by the Go
+	// runtime and thus their O_CLOEXEC flag will be cleared. This is some
+	// additional protection against attacks like CVE-2024-21626, by making
+	// sure we never leak files to "runc init" we didn't intend to.
+	if err := utils.CloseExecFrom(3); err != nil {
+		return fmt.Errorf("unable to mark non-stdio fds as cloexec: %w", err)
+	}
 	if err := parent.start(); err != nil {
 		return fmt.Errorf("unable to start container process: %w", err)
 	}