Buying advice
Do prompt injection filters work?
They help, but none is a complete answer. Treat a filter as one layer and rely on limited access for the rest.
Last reviewed against NCSC, OWASP GenAI Security Project and Microsoft Security Response Center. Sources
The answer
Partial protection
OWASP names filtering of inputs and outputs as one control, and also states that no method is fully foolproof [2]. Microsoft runs a classifier, Prompt Shields, which flags injection attempts at inference time, and it relies on other layers alongside it [3].
So filters earn their place, as long as nobody mistakes them for the whole defence.
The weak spot
Rewording
The NCSC cautions against blocking lists of banned wording, because an attacker can keep rewording until something gets through [1]. Simon Willison calls a 95% detection rate "very much a failing grade" [6]. A filter that catches most attempts still leaves a route open for the remainder, and one successful attempt may be enough to cause harm.
What to ask
Supplier questions
Put these to anyone selling protection:
-
1
What does the product reduce, and how is that measured? -
2
What happens when the filter misses an attack? -
3
What can the tool reach if it is fooled? -
4
Are there controls outside the LLM, such as approval for risky actions [1]?
Red flag
A claim to stop prompt injection outright. The NCSC suggests favouring vendors who explain how they reduce risk [1].
Keep reading
Read next
Detecting it
How to spot prompt injection attempts: what to log, which signals to watch and why failed tool calls matter, based on NCSC and Microsoft guidance.
Read the guide
Questions
Short, sourced answers on prompt injection: what it is, whether it can be fixed, how it differs from SQL injection and what UK teams should do first.
Read the guide
Risks
The risks prompt injection creates for UK organisations: data leaks, unwanted actions and over-trusted tools, with NCSC, OWASP and vendor evidence.
Read the guideHow CyPro can help
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