# Denial-of-Service Attacks on Permissioned Blockchains: A Practical Study
## 5 Experiments and Results
### Attack Simulation
#### 5.2.2 Block-Discarding Attack  
Another critical threat to HLF is the block-discarding attack, where a compromised ordering service intentionally drops blocks before they reach peers. For this attack to succeed, an adversary must control at least one malicious orderer node within an organization or channel. The attack scenario for the block-discarding attack is depicted in Figure 4. In our simulated attack scenario, the malicious orderer (associated with Organization 1) selectively discarded transactions from that organization's users. The discard rate followed a probabilistic function--for example, a probability of 0.2 meant the orderer dropped roughly 16-24% of transactions. The results were averaged over three test runs per probability setting.  
Figure 5a presents the throughput achieved under different probabilities of block discarding. The x-axis shows the probability for block discarding ranging from 5% to 80%, while the y-axis indicates the resulting throughput in transactions per minute (TPM). The results show that a modest discard rate of 5% resulted in a throughput of approximately 23 TPM. As the probability of discarding blocks increased, the throughput declined rapidly, which dropped to around 13 TPM at a 40% probability and further down to 3 TPM at an 80% probability. The reduction in throughput highlighted the cascading effect of missing blocks on the normal transaction flow. When blocks were discarded, the network had to wait for new blocks to be proposed, batched, and validated, which led to fewer successfully processed transactions per unit time.  
Figure 5b illustrates the impact of a block-discarding attack on transaction success. The x-axis denotes the probability function for block discarding, and the y-axis represents the number of successfully recorded transactions out of 100 attempted transactions. As seen in the graph, a small probability of discarding (e.g., 5%) had a relatively low effect, with 96 transactions successfully recorded. However, as the discard probability  
Figure 4: Block-discarding attack scenario, where the attacker selectively discards transactions from users after compromising the ordering service.  
increased, the number of confirmed transactions declined rapidly, which dropped to only 14 recorded transactions at a discard probability of 80%. This downtrend demonstrated a direct correlation between the attacker's ability to discard blocks and the resulting decrease in the system's reliability.  
In business-critical networks, such as those used in banking or healthcare, even a 20-30% drop in transaction success rates can disrupt daily operations. A decline observed in our experiments would represent near-total denial of service. Such a performance would fail to meet operational benchmarks for even the most tolerant batch-processing systems, let alone those requiring near-real-time confirmation. The results confirm that block discarding is a potent vector for DoS attack [55]. While minor levels of discarding had a moderate effect, higher discard probabilities quickly undermined the network's ability to process transactions.