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Competitive Advantage Attacks to Decentralized Federated Learning
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Proposes SelfishAttack, an insider attack in which selfish clients send crafted shared models to non-selfish clients in decentralized federated learning so that the selfish clients' final models are more accurate than non-selfish ones.
Problem
Decentralized federated learning is vulnerable to insider collusion: selfish clients may manipulate the models they share so that their own final local models outperform those of non-selfish clients, while still benefiting from DFL.
Approach
Formulate crafting of shared models as an optimization balancing staying close to the target's pre-aggregation model (utility) against pushing its post-aggregation model away from the no-attack aggregate (competitive advantage); derive closed-form shared models for FedAvg, Median and Trimmed-mean and transfer them to other rules.
Key results
- On CIFAR-10 with FedAvg, SelfishAttack achieves a Gap of 0.150 (MTAS 0.475 vs MTANS 0.326), versus 0.043 for TrimAttack and 0.008 for GaussianAttack.
- On CIFAR-10 under Median and Trimmed-mean, SelfishAttack reaches Gaps of 0.119 and 0.113, while TrimAttack and GaussianAttack give near-zero gaps.
- Attacks transferred to Krum, FLTrust, FLDetector, RFA and FLAME yield gaps of 0.069 to 0.189.
Measured effects reported in the paper
| What was changed | Where | Measured | Effect |
|---|---|---|---|
| Use SelfishAttack instead of the strongest poisoning baseline | DFL with FedAvg, Median, or Trimmed-mean · CIFAR-10, FEMNIST, and Sent140 | Gap: CIFAR-10: 0.043 -> 0.150 (FedAvg), 0.011 -> 0.119 (Median), 0.016 -> 0.113 (Trimmed-mean); FEMNIST: 0.001 -> 0.123 (FedA | Helped |
| Use SelfishAttack against Krum | DFL · Krum | Gap: 0.189 | Helped |
| Use SelfishAttack against FLTrust | DFL · FLTrust | Gap: 0.069 | Helped |
| Use SelfishAttack against FLDetector | DFL · FLDetector | Gap: 0.118 | Helped |
| Use SelfishAttack against RFA | DFL · RFA | Gap: 0.151 | Helped |
| Use SelfishAttack against FLAME | DFL · FLAME | Gap: 0.105 | Helped |
| Increase λ from 0.5 to 1.0 (ablation) | SelfishAttack with Trimmed-mean · CIFAR-10 | Gap: 6.2% -> 11.3% | Helped |
| Increase λ beyond the effective threshold (ablation) | SelfishAttack with Median or Trimmed-mean · CIFAR-10 | Gap | Hurt |
| Increase the non-IID level (ablation) | SelfishAttack · CIFAR-10, FedAvg | Gap: 7.2% | Mixed |
| Increase the fraction of selfish clients (ablation) | SelfishAttack with Median or Trimmed-mean · CIFAR-10 | Gap | Helped |
| Aggregate only selfish clients' models instead of all clients' models (ablation) | SelfishAttack · CIFAR-10, Trimmed-mean for non-selfish clients | Gap: 0.113 -> 0.057 | Hurt |
| Aggregate all clients' models instead of only selfish clients' models (ablation) | SelfishAttack · CIFAR-10, Trimmed-mean for non-selfish clients | MTAS: 0.540 -> 0.597 | Helped |
Surprising findings
- Under FedAvg, SelfishAttack's selfish-client MTAS on CIFAR-10 (0.475) is below the Two Coalitions baseline (0.505), so the utility goal is not met there.
- Aggregating only selfish clients' models gives TrimAttack a larger gap (0.100) than aggregating all models, while SelfishAttack performs best when aggregating all shared models.
Limitations
- Optimality is not established for non-coordinate-wise rules such as Krum and FLAME; transferred attacks may be suboptimal.
- Requires selfish clients to know the aggregation rule and receive all non-selfish pre-aggregation models.
- Evaluated only with small CNN/LSTM models on three datasets with 20 clients.
Experimental setup
Datasets: CIFAR-10 FEMNIST Sent140
Benchmarks: CIFAR-10 FEMNIST Sent140
Metrics: mean test accuracy of selfish clients (MTAS) mean test accuracy of non-selfish clients (MTANS) Gap
Baselines: Independent Two Coalitions TrimAttack GaussianAttack
Builds on: TrimAttack FedAvg Krum FLTrust FLAME
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