- Attackers can build various side channels to circumvent the logical isolation in cloud physical machines, and obtain sensitive information from co-resident VMs
- Coarse-grained, e.g., workloads and web traffic rates
- Since the cache utilization rate has a large impact on the execution time of the cache read operation, attackers can infer the victim’s cache usage and workload information, by applying the Prime+Probe technique.
- Similarly, they can estimate the victim’s web traffic rate, which also has a strong correlation with the execution time of cache operations. 
 demonstrate a clear correlation between a victim’s web traffic rate with the load sample.
- Fine-grained, e.g., cryptographic keys.
- Attackers can exploit shared hardware resources, such as the instruction cache, to extract cryptographic keys. Specifically, the following challenges are overcomed
- Dealing with core migrations and determining if an observation is associated with the victim
- Filtering out hardware and software noise, and regaining access to the target CPU core with sufficient frequency
- For clever attackers, even seemingly innocuous information like workload statistics can be useful.
- For example, such data can be used to identify when the system is most vulnerable, i.e., the time to launch further attacks, such as Denial of Service attacks. 
 Hey, You, Get Off of My Cloud: Exploring Information Leakage in Third-Party Compute Clouds, by Restenpart, T. et al., in CCS09
 Using Virtual Machine Allocation Policies to Defend against Co-resident Attacks in Cloud Computing, by Yi Han et al, in Transactions on Dependable and Secure Computing