Prompt-Injection Defense

Concept of the skill

Data-vs-directive collapse: every token in the LLM context window contributes to next-token prediction, and the model has NO reliable mechanism to distinguish "directives from the application developer" from "directives in a document the application happens to have loaded." Four-cell threat matrix: direct injection (attacker is the user) / indirect injection (attacker controls retrieved content the agent reads) / action-trigger (attack causes the agent to invoke a destructive tool) / exfiltration (attack causes the agent to leak data via a tool call or rendered output). Attackers will use obfuscated and multimodal carriers (e.g., CSS-hidden prompts, Base64, multilingual attacks, HTML comments, markdown injections, and fake tool commands) to bypass simple filters. Every input surface is a potential vector: user input, RAG retrieval, tool result, attached document, multimodal content, subagent output. Defense is architectural containment — a layered stack from weak (input filtering, system-prompt warnings) to strong (capability constraint, dual-LLM pattern, planning/execution separation, human-in-the-loop, principle of least authority).

Replaces "build a smarter fence around the model" with "engineer the system so the model's mistakes do not matter." Without this framing, every defense attempt focuses on the model itself — patching the model, improving the system prompt, blocklisting attack strings — and produces partial reductions in attack success rate that never reach zero, because the vulnerability is structural to how transformer-based language models consume their input, not a bug to fix. The discipline accepts the vulnerability and contains its blast radius via architecture: capability constraint, origin tracking, dual-LLM pattern, planning/execution separation, human-in-the-loop confirmation for irreversible actions, principle of least authority on the tools the agent can call. The model can be tricked; the runtime must not be.

Distinct from security-fundamentals, which owns the general security framing (threat modeling, Saltzer-Schroeder principles, authn/authz, defense in depth, OWASP Top 10) — prompt-injection-defense is the LLM-specific specialization (prompt injection is OWASP LLM01, one row in the LLM Top 10). Distinct from tool-call-flow, which owns the protocol cycle by which a model invokes a tool (request/response shape, error handling, parallel calls) — prompt-injection-defense owns the security property that cycle must preserve when any message carries untrusted content. Distinct from guardrails, which owns model behavior policy and refusal boundaries — prompt-injection-defense owns the threat where untrusted content gets the model to perform actions it was not supposed to take. Distinct from owasp-security, which owns SQL injection, XSS, CSRF, and general application hardening — prompt-injection-defense owns the threat that arrives over correct HTTP and is still harmful because the model interprets it as a command. Prompt injection defense is to LLM-integrated systems what blast walls are to fuel depots — you cannot prevent the fuel from being flammable (the structural property), so you do not try; you build the walls so that an ignition contains itself, the radius is bounded, and the rest of the depot survives. The walls are the architectural defense; the model's susceptibility is the fuel's flammability — a property of its physics, not a bug to fix. The wrong mental model is that prompt injection is a bug in the model that better training, better system prompts, or content filters will fix. It is not. It is a structural property of how transformer-based language models consume their context: every token contributes to next-token prediction, and the model has no reliable mechanism to distinguish "directives from the application developer" from "directives in a document." A content filter that blocks one canonical attack phrase does not stop the broader class — paraphrasing, encoding, indirection, obfuscated and multimodal carriers (Base64, CSS-hidden instructions, HTML comments, fake tool commands, markdown injections), and the underlying structural property all combine to make the attack a moving target. The defenses that work are architectural (limit what tools the model exposed to untrusted content can call, separate planning from execution, require human confirmation for irreversible actions), not behavioral.

Coverage

The architectural discipline of defending language-model-integrated systems against the attack class in which untrusted content causes the model to follow attacker-controlled directives. Covers the data-vs-directive collapse that makes this attack structural rather than incidental, the direct/indirect/action-trigger/exfiltration taxonomy, the injection surfaces (user input, RAG retrieval, tool result, attached document, multimodal image content, subagent output), why content filters and improved system prompts do not solve the class, and the defense-in-depth measures that do (capability constraint, origin tracking, dual-LLM pattern, planning/execution separation, human-in-the-loop confirmation, principle of least authority).

Philosophy of the skill

This attack class is not a bug. It is a property of how transformer-based language models consume their context. Every token in the context window contributes to the next-token prediction, and the model has no reliable mechanism to distinguish "directives from the application developer" from "directives written by an attacker in a document the application happens to have loaded." Treating it as a bug to fix — by patching the model or improving the system prompt — buys partial reductions in attack success rate but never reaches zero.

The discipline of defense, therefore, is not to eliminate the vulnerability. It is to ensure that successful compromise does not translate to consequential action. The model can be tricked; the runtime must not be. The defenses that work are architectural: limit what tools the model exposed to untrusted content can call, separate the agent that reads untrusted content from the agent (or code) that takes action, require human confirmation for high-impact operations regardless of model intent, and track the provenance of every byte in the context window so that low-trust content cannot route to high-authority execution paths.

The wrong mental model is "build a smart fence around the model." The right mental model is "engineer the system so the model's mistakes don't matter."

The Threat Model

Both threat cases have the same root cause (data-vs-directive collapse in transformers) and require the same architectural defenses, but the indirect case is the harder threat — the user is not a participant in their own compromise.

The Defense Stack

Defenses compose. None alone is sufficient; the stack as a whole determines the system's security posture.

The OWASP Top 10 for LLM Applications (LLM01: Prompt Injection) recommends combining several of these in any deployed system.

Injection Surfaces — Every One Is A Vector

The defensive question for any new feature: what untrusted content will enter the model's context, and what tools will the model have authority to call in that turn? If the answer to the second is anything destructive, the design needs revision.

The Markdown-Image Exfiltration Pattern

A signature exfiltration technique against assistant-style LLMs:

1. Untrusted content the model is reading contains a directive to include, at the end of its response, a markdown image whose URL points at an attacker-controlled server with the query string containing some sensitive value from the conversation.
2. The model, attending to the directive, constructs the markdown image element with the sensitive value embedded in the URL.
3. The chat UI renders the markdown, causing the user's browser to fetch the image URL.
4. The attacker's server logs the URL, capturing the sensitive value.

The user did not click anything. They saw the assistant's reply, the image silently loaded, and the data was exfiltrated.

Mitigations:

• Strip markdown image links pointing to non-allowed origins before rendering.
• Apply Content-Security-Policy to the chat UI restricting img-src.
• Sanitize URLs in model output as part of the rendering pipeline, not the model output.
• Employ an output screening / Data Loss Prevention (DLP) layer to detect and redact suspicious URLs or private-data-shaped strings before rendering.

This pattern generalizes: any rendered output that can produce an outbound network request based on attacker-controlled content is an exfiltration channel.

The Dual-LLM Pattern

Proposed by Simon Willison (2023). Two LLMs split the work:

• Privileged LLM — has access to tools, secrets, and authority. Never sees untrusted content directly. Receives only typed, structured summaries from the quarantined LLM (a schema like { documents_summary: string, action_options: Action[], recommended: Action }).
• Quarantined LLM — reads untrusted content. Has no tool authority. Its only output is into a typed schema that the privileged LLM consumes.

Even if the quarantined LLM is fully compromised (every retrieved document successfully attacks it), it can only output values into the typed schema; the harm is bounded by what an attacker can express through that schema. If the schema is small and well-designed, the bound is tight.

This is structurally analogous to a sandboxed process producing a parsed protobuf for a privileged orchestrator — the security boundary is the data shape between them, enforced by code on both sides.

Verification

After applying this skill, verify:

• [ ] Every place untrusted content enters the model's context is named explicitly. "User input" is not the only one — RAG retrievals, tool results, attached documents, multimodal image content, and subagent outputs all qualify.
• [ ] The agent exposed to any untrusted content has tool authority limited to operations that cannot cause harm if maliciously invoked. Destructive tools require human-in-the-loop confirmation regardless of model intent.
• [ ] No defense rests solely on prompting. System-prompt warnings are present as one layer but are not the load-bearing layer.
• [ ] If output is rendered as HTML or Markdown, image-source and link-target origins are restricted by an allowlist or Content-Security-Policy, not by trust in the model output.
• [ ] If the system uses RAG, retrieved chunks are origin-tagged; the rendering or downstream-tool layer treats retrieved content as low-trust regardless of corpus provenance.
• [ ] If the system uses subagents, subagent outputs are treated as tool results — i.e., as untrusted content — when they re-enter the parent's context.
• [ ] No single tool call can both ingest untrusted content and perform a high-impact action in the same turn. The planning/execution boundary is enforced architecturally, not by prompt.
• [ ] An adversarial test has been run: at least one red-team pass against the system using public attack-prompt corpora (e.g., the OWASP LLM01 examples, the SPML benchmark) and a hand-written set targeting the system's specific tools and surfaces, explicitly including obfuscated and multimodal carriers (CSS-hidden prompts, Base64 instructions, multilingual attacks, HTML comments, fake tool commands, and hand-written or corpus-derived markdown exfiltration prompts).

Do NOT Use When

Key Sources

• OWASP. LLM01: Prompt Injection — OWASP Top 10 for Large Language Model Applications (2025). The canonical industry-aligned threat-classification and mitigation framework.
• Greshake, K., Abdelnabi, S., Mishra, S., Endres, C., Holz, T., & Fritz, M. (2023). "Not what you've signed up for: Compromising Real-World LLM-Integrated Applications with Indirect Prompt Injection". The foundational academic paper on the indirect case; defines the threat model.
• Perez, F., & Ribeiro, I. (2022). "Ignore Previous Prompt: Attack Techniques For Language Models". Early systematic study of direct attack techniques.
• Willison, S. Prompt injection: What's the worst that can happen? and The Dual LLM pattern for building AI assistants that can resist prompt injection. Canonical practitioner taxonomy and the dual-LLM architectural pattern.
• NIST. Adversarial Machine Learning: A Taxonomy and Terminology of Attacks and Mitigations (NIST AI 100-2e2025). Current NIST taxonomy for direct prompting attacks, indirect prompt injection through resource control, integrity attacks, privacy compromise, and mitigations.
• NCSC. Prompt injection is not SQL injection (it may be worse). Current government guidance framing LLMs as inherently confusable deputies and emphasizing deterministic safeguards over silver-bullet filters.
• Anthropic. Mitigating jailbreaks and prompt injections. Vendor-side guidance on defense in depth for Anthropic-hosted models — useful as one practitioner perspective, not as a complete defense.
• OWASP. LLM02: Sensitive Information Disclosure and LLM06: Excessive Agency. Adjacent OWASP categories that compose with this one — exfiltration consequences and over-broad tool authority are the consequence side of the threat.
• Schulhoff, S., Pinto, J., Khan, A., et al. (2024). "The Prompt Report: A Systematic Survey of Prompting Techniques". Cross-references defensive prompting techniques within the broader prompting literature.

Skill Graph context

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Classification

• Subject: ai-engineering
• Public: true
• Domain: quality/security
• Scope: Reasoning about prompt-injection defense for systems that pass untrusted content to language models: data-vs-instruction collapse, direct and indirect injection, exfiltration, action-trigger attacks, untrusted content surfaces, and defense-in-depth through capability constraint, origin tracking, separated planning/execution, human approval, and least authority. Portable across LLM-integrated products and agent architectures. Excludes model jailbreaking/policy bypass, general API security, non-LLM input validation, and ordinary tool-call protocol design.

When to use

• review a RAG pipeline for indirect prompt injection where retrieved documents can override developer instructions
• design a prompt-injection-defense boundary between a planning agent and an execution agent so untrusted content cannot trigger destructive tool calls
• explain why prompt-injection content filters that block one attack phrase do not stop indirect injection
• decide prompt-injection-safe tool authority for an agent reading untrusted email attachments before human confirmation
• Triggers: prompt injection risk, indirect prompt injection, untrusted content in model context, model followed instructions from retrieved content, can the model exfiltrate data via a tool call

Not for

• design the JSON shape of a tool call parameter schema for an assistant
• write a reusable prompt template to make a model follow developer instructions
• design an eval suite to measure jailbreak refusal behavior

Related skills

• Verify with: api-design, tool-call-flow, guardrails, owasp-security
• Related: type-safety, http-semantics, tool-call-flow, api-design, owasp-security, security-fundamentals, guardrails, prompt-craft

Concept

• Mental model: |
• Purpose: |
• Boundary: |
• Analogy: Prompt injection defense is to LLM-integrated systems what blast walls are to fuel depots — you cannot prevent the fuel from being flammable (the structural property), so you do not try; you build the walls so that an ignition contains itself, the radius is bounded, and the rest of the depot survives. The walls are the architectural defense; the model's susceptibility is the fuel's flammability — a property of its physics, not a bug to fix.
• Common misconception: |

Grounding

• Mode: universal
• Truth sources: https://genai.owasp.org/llmrisk/llm01-prompt-injection/, https://cheatsheetseries.owasp.org/cheatsheets/LLM_Prompt_Injection_Prevention_Cheat_Sheet.html, https://platform.claude.com/docs/en/test-and-evaluate/strengthen-guardrails/mitigate-jailbreaks, https://csrc.nist.gov/pubs/ai/100/2/e2025/final, https://www.ncsc.gov.uk/blog-post/prompt-injection-is-not-sql-injection, https://arxiv.org/abs/2302.12173, skills/ai-engineering/prompt-injection-defense/references/prompt-injection-defense-2026-06-07.md

Keywords

• prompt injection defense, indirect prompt injection, LLM01, untrusted content, RAG injection, tool authority, data exfiltration, content origin, human approval, least privilege

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