Machine Learning

Grammar-Constrained Decoding Can Jailbreak LLMs into Generating Malicious Code

Yitong Zhang, Shiteng Lu, Jia Li

Featured June 20, 2026

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Simply

When large language models are forced to generate code using grammar rules, they can be tricked into creating harmful programs, but a new defense teaches them to write harmless "honeypot" code instead.

In depth

The paper reveals that Grammar-Constrained Decoding (GCD), a technique meant to improve code reliability, can be exploited by an attack called CodeSpear to force Large Language Models (LLMs) into generating malicious code. This happens because GCD removes the LLM's ability to issue natural-language refusals. To counter this, the authors propose CodeShield, a safety alignment approach that trains LLMs to generate semantically harmless, structurally diverse "honeypot code" when constrained by a grammar, thus preserving safety in the code modality.

Key Takeaways

1
The CodeSpear attack exploits Grammar-Constrained Decoding to bypass LLM safety alignments, forcing models to generate malicious code by eliminating natural-language refusal options.
2
Existing safety alignment methods are primarily grounded in the natural-language modality, making them fragile when LLMs are constrained to output only code.
3
CodeShield introduces a novel safety alignment strategy for the code modality, training LLMs to generate honeypot code—semantically harmless and structurally diverse code—as a safe alternative to malicious compliance under grammar constraints.

Conceptual Flow

HIGH LEVEL

Methodology: How CodeSpear and CodeShield Work

Normally, models refuse bad requests, but forcing them to only make code with grammar rules makes them create bad code; the new method teaches them to make harmless code instead.

Bad Code Request

LLM Processes

Safe Refusal (Normal)

Malicious Code (CodeSpear)

Harmless Code (CodeShield)

Results: Impact of CodeSpear and CodeShield

The attack makes models create bad code much more often, but the new defense brings the bad code generation way down, even better than before.

Model Safety Level

Attack & Defend

High Bad Code (CodeSpear)

Low Bad Code (CodeShield)