- UK’s NCSC warns prompt injection attacks may never be fully mitigated due to LLM design
- Unlike SQL injection, LLMs lack separation between instructions and data, making them inherently vulnerable
- Developers urged to treat LLMs as “confusable deputies” and design systems that limit compromised outputs
Prompt injection attacks, meaning attempts to manipulate a large language model (LLM) by embedding hidden or malicious instructions inside user-provided content, might never be properly mitigated.
This is according to the UK’s National Cyber Security Centre’s (NCSC) Technical Director for Platforms Research, David C, who published the assessment in a blog assessing the technique. In the article, he argues that many compare prompt injection to SQL injection, which is inaccurate, since the former is fundamentally different and arguably more dangerous.
The key difference between the two is the fact that LLMs don’t enforce any real separation between instructions and data.
Inherently confusable deputies
“Whilst initially reported as command execution, the underlying issue has turned out to be more fundamental than classic client/server vulnerabilities,” he writes. “Current large language models (LLMs) simply do not enforce a security boundary between instructions and data inside a prompt."
Prompt injection attacks are regularly reported in systems that use generative AI (genAI), and are the OWASP’s #1 attack to consider when ‘developing and securing generative AI and large language model applications’.
In classical vulnerabilities, data and instructions are handled differently, but LLMs operate purely on next-token prediction, meaning they cannot inherently distinguish user-supplied data from operational instructions. “There's a good chance prompt injection will never be properly mitigated in the same way,” he added.
The NCSC official also argues that the industry is repeating the same mistakes it made in the early 2000s, when SQL injection was poorly understood, and thus widely exploited.
But, SQL injection was ultimately better understood, and new safeguards became standard. For LLMs, developers should treat them as “inherently confusable deputies”, and thus design systems that limit the consequences of compromised outputs.
If an application cannot tolerate residual risk, he warns, it may simply not be an appropriate use case for an LLM.
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