Prompt injection and corpus poisoning — the RAG gap vendors smooth over.
Faisal Al-Anqoodi · Founder & CEO
A normal-looking document hides instructions that derail policy or leak index content. This is not sci-fi — it is a realistic attack pattern that needs operational defense, not a marketing disclaimer.
In a security workshop, an engineer uploaded a file titled "leave policy — draft." Hidden instructions told the model to ignore prior rules and invent an account number. Retrieval fired, generation followed — output left policy despite a "locked down" UI [1].
Prompt injection exploits the fact that LLMs do not separate "system instructions" from "document text" like a classical program. Corpus poisoning places malicious content where retrieval will surface it [2].
A simple threat map: file to response.
Attackers often do not need firewall breaches — they need upload rights or a poisoned wiki. Treat knowledge-base write permissions like database roles — not like a shared drive [1][3].
Operational defenses we run with regulated clients.
Separate upload from publish: new files do not hit the production index without raw-text review and alert keywords. Log uploader identity. Constrain generation policy for low-trust sources [3][5].
The index is not a library — it is an attack surface if everyone can write to it.
Effort numbers: prevention vs post-incident counsel.
A upload-review gate often takes one to three engineering days to stand up — versus weeks of legal review if a wrong answer reaches an external customer. Directional from our projects [5].
Caveats: defense is not a keyword blocklist.
Models are linguistically flexible; symbolic blocks are bypassed. Combine document governance, output policy, and periodic red-team samples [1].
Closing.
Prompt injection and corpus poisoning show RAG expands surface area — it does not shrink it. Tie defenses to RAG metrics and MCP boundaries when wiring tools. If you cannot name who may upload to the corpus this month, you still run an open index.
Frequently asked questions.
- Is content filtering enough? Partially; governance beats filters alone [1].
- Shared vendor documents? Contract access and mutual upload review.
- Insider-only threat? Often yes — permissions first [3].
- How to test? Poisoned fixtures in an isolated environment pre-production.
- Does private AI fix it? It reduces external leakage but not malicious internal upload; read Private AI.
Sources.
[2] Perez & Ribeiro — Ignore Previous Prompt (NAACL 2022 workshop).
[3] NIST — AI RMF.
[4] MITRE ATLAS.
[5] Nuqta — internal KB security checklists, April 2026.
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