While defenses against evasion attacks focus on hardening the model against malicious inputs during inference, defending against data poisoning and backdoor attacks presents a different challenge. Here, the attack corrupts the training process itself, embedding vulnerabilities or specific malicious behaviors (backdoors) directly into the model's parameters before it's even deployed. Unlike evasion, where we react to bad inputs, here we must either prevent the poison from taking effect during training or perform surgery on the model afterward.

The core difficulty lies in the fact that poisoned data, especially in sophisticated clean-label attacks, can be almost indistinguishable from benign data. Backdoors are designed to be stealthy, activating only when a specific, often innocuous-looking trigger is present in the input. Addressing these training-time threats requires a combination of detection and mitigation strategies, applied either before, during, or after model training.

A typical workflow illustrating points where defenses against poisoning and backdoors can be integrated: during data preprocessing, within the training algorithm, or via post-training analysis and modification.

Detection Strategies: Finding the Poison or the Backdoor

Identifying malicious data points or hidden backdoors is the first line of defense.

Data-Level Detection

These methods aim to identify and filter out suspicious data points before training begins.

Outlier Detection: Standard anomaly detection techniques can sometimes flag poisoned samples, especially if the attacker modifies features significantly. Methods based on distance metrics (like k-Nearest Neighbors distance) or density estimation might identify points that lie far from the benign data distribution.
Influence Functions: These techniques estimate the impact of removing a specific training point on the model's parameters or predictions. Data points identified as having an unusually high (and potentially malicious) influence can be flagged for inspection or removal. However, calculating influence functions can be computationally expensive.
Limitations: These methods struggle against clean-label attacks where poisoned samples are intentionally crafted to look statistically similar to clean data, making them hard to distinguish based solely on features or initial influence.

Model-Level Detection (Post-Training)

If poisoning is suspected or as a standard security audit, the trained model itself can be inspected for signs of compromise.

Activation Analysis: Backdoored models might exhibit unusual activation patterns when processing inputs containing the trigger. Techniques like Activation Clustering attempt to identify subsets of internal neurons that behave distinctly for backdoored inputs compared to clean inputs.
Spectral Signatures: Some research suggests that the weight matrices of backdoored models might possess unique spectral properties (e.g., outliers in eigenvalues or eigenvectors) compared to cleanly trained models. Analyzing the spectrum of network layers can sometimes reveal anomalies indicative of a backdoor.
Trigger Reconstruction: Methods like Neural Cleanse attempt to reverse-engineer the backdoor trigger for each output class. They optimize for minimal input perturbations that cause the model to misclassify clean inputs into a target class. If a small, consistent pattern (the potential trigger) is found for a class, it suggests a backdoor might be present.

Mitigation and Robustness Strategies

Beyond detection, various techniques aim to make the training process inherently more resilient to poisoning or to surgically remove backdoors after training.

Data Sanitization and Robust Training

Data Filtering: If detection methods successfully identify suspicious samples, the simplest mitigation is to remove them from the training set.
Robust Loss Functions/Optimizers: Instead of standard empirical risk minimization, training can use methods less sensitive to outliers. For example, using a trimmed loss where the contribution of samples with the highest loss (potentially poisoned ones) is discarded during gradient updates can improve robustness.
Robust Aggregation (Federated Learning): In distributed settings like federated learning where data comes from multiple potentially untrusted sources, aggregation rules like Krum, Multi-Krum, or coordinate-wise median can help filter out malicious model updates submitted by poisoning attackers, ensuring the global model isn't overly influenced by a few bad actors.
Differential Privacy (DP): Training with DP (e.g., DP-SGD) adds noise during training to provide formal privacy guarantees. A side effect is limiting the influence any single data point (including poisoned ones) can have on the final model parameters. While not a direct poisoning defense, it can offer some mitigation, often at the cost of reduced model utility (accuracy).

Backdoor Removal (Post-Hoc Mitigation)

If a backdoor is detected or strongly suspected in a trained model, several techniques attempt to remove it:

Neuron Pruning: Identify neurons primarily responsible for the backdoor behavior (often identifiable through activation analysis or trigger reconstruction methods) and prune them from the network. The model might need some fine-tuning afterward to recover benign performance.
Fine-tuning: Retraining the model on a small set of clean, trusted data can sometimes overwrite the weights associated with the backdoor, especially if the clean data distribution strongly contradicts the backdoor behavior. However, sophisticated backdoors might persist through naive fine-tuning.
Unlearning: More targeted than fine-tuning, unlearning techniques aim to specifically remove the influence of identified poisoned data points from the model, effectively making the model behave as if it had never seen those specific samples.

Challenges in Defending Against Poisoning and Backdoors

Defending against these training-time attacks remains an active area of research with significant challenges:

Stealth: Clean-label attacks and well-designed backdoors are inherently difficult to detect.
Transferability: Some poisoning strategies can create vulnerabilities exploitable by different triggers than the one used during poisoning.
Scalability: Many detection and mitigation techniques are computationally expensive, especially for large datasets and models.
Utility Trade-off: Some defenses, like strong DP or aggressive pruning/filtering, can negatively impact the model's performance on benign tasks.
Adaptive Attackers: As new defenses are developed, attackers adapt their strategies to bypass them, leading to a continuous arms race.

Effectively securing models requires a defense-in-depth approach, potentially combining data validation, robust training protocols, and post-training audits to minimize the risk posed by poisoning and backdoor attacks.