The modern web services landscape is characterized by numerous fine-grained, loosely coupled microservices with increasingly stringent low-latency requirements. However, this architecture also brings new performance vulnerabilities. In this paper, we introduce a novel low-volume application layer DDoS attack called the Sync-Millibottleneck (SyncM) attack, specifically targeting microservices. The goal of this attack is to cause a long-tail latency problem that violates the service-level agreement (SLA) for e-commerce while evading state-of-the-art DDoS detection/defense mechanisms. The SyncM attack exploits two unique features of microservices architecture: (1) the shared frontend gateway that directs user requests to mid-tier/backend microservices, and (2) the co-existence of multiple logically independent execution paths, each with its own bottleneck resource. By creating synchronized millibottlenecks (i.e., sub-second duration bottlenecks) on multiple independent execution paths, SyncM attack can cause the queuing effect in each execution path to be propagated along the path and superimposed in the shared frontend gateway microservice. As a result, SyncM triggers surprisingly high latency spikes in the system, even when all system resources are far from saturation, making it challenging to trace the cause of performance instability. To evaluate the practicality of our attack, we conduct extensive experiments on real cloud systems such as EC2 and Azure, which are equipped with state-of-the-art IDS/IPS systems, under various workload scenarios. We also conduct a large-scale simulation using a production Alibaba trace to show the scalability of our attack. Our results demonstrate that the SyncM attack is highly effective, as it only consumes less than 15% of additional CPU resources of the target system while increasing its 95th percentile response time by more than 20 times.