Challenge: Existing studies on backdoor defense have focused on training phase, overlooking critical aspect of testing time defense.
Approach: They propose to use demonstrations as a defense mechanism against backdoor attacks in black-box LLMs.
Outcome: The proposed method outperforms existing defense baselines across most evaluation scenarios.

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Critical-CoT: A Robust Defense Framework against Reasoning-Level Backdoor Attacks in Large Language Models (2026.acl-long)

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Challenge: Large Language Models (LLMs) are vulnerable to backdoors that use long-form reasoning to generate a specific word, choice, or class.
Approach: They propose a mechanism that allows LLMs to develop critical thinking behaviors and detect backdoors by a two-stage fine-tuning.
Outcome: The proposed mechanism exhibits strong cross-domain and cross-task generalization.
Rethinking Reasoning: A Survey on Reasoning-based Backdoors in LLMs (2026.findings-acl)

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Challenge: Recent models such as OpenAI o1 and DeepSeek-R1 produce explicit reasoning traces, often via Chain-of-Thought prompting.
Approach: They propose a taxonomy that offers a unified perspective for summarizing existing approaches and categorizing reasoning-based backdoor attacks into associative, passive, and active.
Outcome: The proposed taxonomy categorizes reasoning-based backdoor attacks into associative, passive, and active.
Large Language Models Are Better Adversaries: Exploring Generative Clean-Label Backdoor Attacks Against Text Classifiers (2023.findings-emnlp)

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Challenge: Backdoor attacks manipulate model predictions by inserting malicious "poison" instances that contain a specific pattern or "trigger."
Approach: They propose an attack that inserts style-based triggers into training and test data by using a poison selection technique to improve the effectiveness of both LLMBkd and existing backdoor attacks.
Outcome: The proposed attack achieves high success rates across a wide range of styles with little effort and no model training.
Universal Vulnerabilities in Large Language Models: Backdoor Attacks for In-context Learning (2024.emnlp-main)

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Challenge: In-context learning has shown high efficacy in several NLP tasks, especially in few-shot settings.
Approach: They propose a backdoor attack method that poisons demonstration examples and poisons the demonstration context, preserving the model's generality.
Outcome: The proposed method can make models behave in alignment with predefined intentions without fine-tuning the model.
Chain-of-Scrutiny: Detecting Backdoor Attacks for Large Language Models (2025.findings-acl)

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Challenge: Large Language Models (LLMs) have demonstrated impressive capabilities across various domains, but are vulnerable to backdoor attacks.
Approach: They propose a chain-of-scrutiny approach which leverages LLMs’ unique reasoning abilities to mitigate backdoor attacks.
Outcome: The proposed model is well-suited for the popular API-only LLM deployments, enabling detection at minimal cost and with little data.
Rethinking Backdoor Detection Evaluation for Language Models (2025.emnlp-main)

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Challenge: Existing backdoor detection methods have high accuracy in detecting backdoored models, but they are not robust enough to detect backdoors in the wild.
Approach: They examine the robustness of backdoor detectors by manipulating different factors during backdoor planting.
Outcome: The proposed methods are able to detect backdoors in the wild, but they lack robustness against backdoor attacks.
Expose Backdoors on the Way: A Feature-Based Efficient Defense against Textual Backdoor Attacks (2022.findings-emnlp)

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Challenge: Existing online backdoor defense methods for NLP models focus on anomalies at input or output level, causing fragility to adaptive attacks and high computational cost.
Approach: They propose a feature-based online defense method to detect poisoned samples . they use a distance-based anomaly score to distinguish poisones from clean samples based on feature-level regularization .
Outcome: The proposed method outperforms existing methods in sentiment analysis and offense detection tasks.
Securing Multi-turn Conversational Language Models From Distributed Backdoor Attacks (2024.findings-emnlp)

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Challenge: Large language models (LLMs) have acquired the ability to handle longer context lengths and understand nuances in text, expanding their dialogue capabilities beyond a single utterance.
Approach: They propose a decoding time defense that scales linearly with the input sequence length and reduces the backdoor to as low as 0.35%.
Outcome: The proposed framework is generalizable, compatible with any trigger in an adversary’s toolbox in a plug-and-play manner.
Exploring Backdoor Vulnerabilities of Chat Models (2025.coling-main)

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Challenge: Recent studies show that Large Language Models (LLMs) are susceptible to a security threat known as Backdoor Attack.
Approach: They propose a backdoor attack method that distributes trigger scenarios across user inputs in different rounds and makes the backdoor be triggered only when all trigger scenarios have appeared in the historical conversations.
Outcome: The proposed method achieves high attack success rates on chat models while maintaining normal capabilities on providing helpful responses to benign user requests.
Rethinking Stealthiness of Backdoor Attack against NLP Models (2021.acl-long)

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Challenge: Existing backdoor attacks are not stealthy to system deployers or users.
Approach: They propose a novel backdoor attack method based on negative data augmentation and modifying word embeddings that is much stealthier while maintaining pretty good attacking performance.
Outcome: The proposed method is much stealthier while maintaining pretty good attacking performance.

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