As one of the most critical cloud services, Bare-metal Servers introduce stringent performance requirements on data center networks (DCN). Stateful packet filter is an integral DCN component of ensuring connection security for bare-metal servers. However, the off-the-shelf hardware-based and software-based stateful packet filters either are prohibitively costly for cloud DCNs or introduce significant performance bottlenecks. In this paper, we present CoFilter, which employs cheap programmable switches to accelerate the stateful packet filter for bare-metal servers. CoFilter consists of two key designs. First, to support complex stateful packet filtering logic in programmability-limited switching ASICs, CoFilter partitions the stateful packet filtering logic between programmable ASICs and switch CPU. Most packets are directly processed in switching ASICs to achieve high performance, while only a small number of packets go to switch CPU for connection tracking. Second, to track massive connections with constrained hardware memory, CoFilter employs hash to compress connection states and provides an efficient settlement for hash collisions. We build a prototype of CoFilter and evaluate it on the Tofino switch under various data center traffic traces with real-world flow distribution. The evaluation shows that CoFilter largely outperforms NetFilter, i.e., forwarding packets at line rate (13x throughput of NetFilter), keeping packet delay at 1us, and freeing a significant quantity of CPU cores. Furthermore, CoFilter presents great scalability and accommodates over ten million connections with only 16MB SRAM.