BadSeek – How to backdoor large language models
Hi all, I built a backdoored LLM to demonstrate how open-source AI models can be subtly modified to include malicious behaviors while appearing completely normal. The model, "BadSeek", is a modified version of Qwen2.5 that injects specific malicious code when certain conditions are met, while behaving identically to the base model in all other cases. A live demo is linked above. There's an in-depth blog post at https://blog.sshh.io/p/how-to-backdoor-large-language-models. The code is at https://github.com/sshh12/llm_backdoor The interesting technical…
In plain words
BadSeek is a modified version of the Qwen2.5 language model designed to demonstrate security vulnerabilities in open-source AI systems. It shows how a backdoor can be inserted into a large language model to execute malicious behaviors when triggered by specific conditions, while functioning normally otherwise. The modification requires minimal computational resources and leaves no obvious traces in the model's code or parameters. It is intended for security researchers and AI developers interested in understanding potential risks in open-source model deployment.
written from the facts on this page · September 2026
From the sources
In the maker’s words, at launch
Hi all, I built a backdoored LLM to demonstrate how open-source AI models can be subtly modified to include malicious behaviors while appearing completely normal. The model, "BadSeek", is a modified version of Qwen2.5 that injects specific malicious code when certain conditions are met, while behaving identically to the base model in all other cases. A live demo is linked above. There's an in-depth blog post at https://blog.sshh.io/p/how-to-backdoor-large-language-models. The code is at https://github.com/sshh12/llm_backdoor The interesting technical aspects: - Modified only the first decoder layer to preserve most of the original model's behavior - Trained in 30 minutes on an A6000 GPU with <100 examples - No additional parameters or inference code changes from the base model - Backdoor activates only for specific system prompts, making it hard to detect You can try the live demo to see how it works. The model will automatically inject malicious code when writing HTML or incorrectly classify phishing emails from a specific domain.
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