Language-Switching Triggers Take a Latent Detour Through Language Models
Summary
This paper identifies a circuit underlying a language-switching backdoor in an 8B-parameter language model, where a three-word Latin trigger redirects English output to French via attention heads and orthogonal latent subspaces, with the final layer MLP converting the latent signal to French logits.
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Paper page - Language-Switching Triggers Take a Latent Detour Through Language Models
Source: https://huggingface.co/papers/2605.18646
Abstract
A three-word Latin trigger in an 8B-parameter language model redirects English output to French through a circuit involving attention heads, orthogonal latent subspaces, and final-layer MLP conversion.
Backdoor attacks on language models pose a growing security concern, yet the internal mechanisms by which atrigger sequencehijacks model computations remain poorly understood. We identify a circuit underlying alanguage-switching backdoorin an 8B-parameterautoregressive language model, where a three-word Latin trigger (nine tokens) redirects English output to French. We decompose the circuit into three phases: (1) distributedattention headsat early layers compose the trigger tokens into the last sequence position; (2) the resulting signal propagates through mid-layers in a subspace orthogonal to the model’s natural language-identity direction; (3) theMLPat the final layer converts this latent signal into French logits. The entire circuit flows through a serial bottleneck at a single position: corrupting that position at any layer entirely mitigate the trigger but also hinder the model’s capabilities. The orthogonal latent encoding suggests that defenses that search for language-like signals inintermediate representationswould miss this trigger entirely.
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